1use super::super::{
2 ArrayChunks, ByRefSized, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, FlatMap,
3 Flatten, Fuse, Inspect, Intersperse, IntersperseWith, Map, MapWhile, MapWindows, Peekable,
4 Product, Rev, Scan, Skip, SkipWhile, StepBy, Sum, Take, TakeWhile, TrustedRandomAccessNoCoerce,
5 Zip, try_process,
6};
7use crate::array;
8use crate::cmp::{self, Ordering};
9use crate::num::NonZero;
10use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
11
12fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
13
14/// A trait for dealing with iterators.
15///
16/// This is the main iterator trait. For more about the concept of iterators
17/// generally, please see the [module-level documentation]. In particular, you
18/// may want to know how to [implement `Iterator`][impl].
19///
20/// [module-level documentation]: crate::iter
21/// [impl]: crate::iter#implementing-iterator
22#[stable(feature = "rust1", since = "1.0.0")]
23#[rustc_on_unimplemented(
24 on(
25 _Self = "core::ops::range::RangeTo<Idx>",
26 note = "you might have meant to use a bounded `Range`"
27 ),
28 on(
29 _Self = "core::ops::range::RangeToInclusive<Idx>",
30 note = "you might have meant to use a bounded `RangeInclusive`"
31 ),
32 label = "`{Self}` is not an iterator",
33 message = "`{Self}` is not an iterator"
34)]
35#[doc(notable_trait)]
36#[lang = "iterator"]
37#[rustc_diagnostic_item = "Iterator"]
38#[must_use = "iterators are lazy and do nothing unless consumed"]
39pub trait Iterator {
40 /// The type of the elements being iterated over.
41 #[rustc_diagnostic_item = "IteratorItem"]
42 #[stable(feature = "rust1", since = "1.0.0")]
43 type Item;
44
45 /// Advances the iterator and returns the next value.
46 ///
47 /// Returns [`None`] when iteration is finished. Individual iterator
48 /// implementations may choose to resume iteration, and so calling `next()`
49 /// again may or may not eventually start returning [`Some(Item)`] again at some
50 /// point.
51 ///
52 /// [`Some(Item)`]: Some
53 ///
54 /// # Examples
55 ///
56 /// ```
57 /// let a = [1, 2, 3];
58 ///
59 /// let mut iter = a.iter();
60 ///
61 /// // A call to next() returns the next value...
62 /// assert_eq!(Some(&1), iter.next());
63 /// assert_eq!(Some(&2), iter.next());
64 /// assert_eq!(Some(&3), iter.next());
65 ///
66 /// // ... and then None once it's over.
67 /// assert_eq!(None, iter.next());
68 ///
69 /// // More calls may or may not return `None`. Here, they always will.
70 /// assert_eq!(None, iter.next());
71 /// assert_eq!(None, iter.next());
72 /// ```
73 #[lang = "next"]
74 #[stable(feature = "rust1", since = "1.0.0")]
75 fn next(&mut self) -> Option<Self::Item>;
76
77 /// Advances the iterator and returns an array containing the next `N` values.
78 ///
79 /// If there are not enough elements to fill the array then `Err` is returned
80 /// containing an iterator over the remaining elements.
81 ///
82 /// # Examples
83 ///
84 /// Basic usage:
85 ///
86 /// ```
87 /// #![feature(iter_next_chunk)]
88 ///
89 /// let mut iter = "lorem".chars();
90 ///
91 /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2
92 /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3
93 /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
94 /// ```
95 ///
96 /// Split a string and get the first three items.
97 ///
98 /// ```
99 /// #![feature(iter_next_chunk)]
100 ///
101 /// let quote = "not all those who wander are lost";
102 /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
103 /// assert_eq!(first, "not");
104 /// assert_eq!(second, "all");
105 /// assert_eq!(third, "those");
106 /// ```
107 #[inline]
108 #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
109 fn next_chunk<const N: usize>(
110 &mut self,
111 ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
112 where
113 Self: Sized,
114 {
115 array::iter_next_chunk(self)
116 }
117
118 /// Returns the bounds on the remaining length of the iterator.
119 ///
120 /// Specifically, `size_hint()` returns a tuple where the first element
121 /// is the lower bound, and the second element is the upper bound.
122 ///
123 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
124 /// A [`None`] here means that either there is no known upper bound, or the
125 /// upper bound is larger than [`usize`].
126 ///
127 /// # Implementation notes
128 ///
129 /// It is not enforced that an iterator implementation yields the declared
130 /// number of elements. A buggy iterator may yield less than the lower bound
131 /// or more than the upper bound of elements.
132 ///
133 /// `size_hint()` is primarily intended to be used for optimizations such as
134 /// reserving space for the elements of the iterator, but must not be
135 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
136 /// implementation of `size_hint()` should not lead to memory safety
137 /// violations.
138 ///
139 /// That said, the implementation should provide a correct estimation,
140 /// because otherwise it would be a violation of the trait's protocol.
141 ///
142 /// The default implementation returns <code>(0, [None])</code> which is correct for any
143 /// iterator.
144 ///
145 /// # Examples
146 ///
147 /// Basic usage:
148 ///
149 /// ```
150 /// let a = [1, 2, 3];
151 /// let mut iter = a.iter();
152 ///
153 /// assert_eq!((3, Some(3)), iter.size_hint());
154 /// let _ = iter.next();
155 /// assert_eq!((2, Some(2)), iter.size_hint());
156 /// ```
157 ///
158 /// A more complex example:
159 ///
160 /// ```
161 /// // The even numbers in the range of zero to nine.
162 /// let iter = (0..10).filter(|x| x % 2 == 0);
163 ///
164 /// // We might iterate from zero to ten times. Knowing that it's five
165 /// // exactly wouldn't be possible without executing filter().
166 /// assert_eq!((0, Some(10)), iter.size_hint());
167 ///
168 /// // Let's add five more numbers with chain()
169 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
170 ///
171 /// // now both bounds are increased by five
172 /// assert_eq!((5, Some(15)), iter.size_hint());
173 /// ```
174 ///
175 /// Returning `None` for an upper bound:
176 ///
177 /// ```
178 /// // an infinite iterator has no upper bound
179 /// // and the maximum possible lower bound
180 /// let iter = 0..;
181 ///
182 /// assert_eq!((usize::MAX, None), iter.size_hint());
183 /// ```
184 #[inline]
185 #[stable(feature = "rust1", since = "1.0.0")]
186 fn size_hint(&self) -> (usize, Option<usize>) {
187 (0, None)
188 }
189
190 /// Consumes the iterator, counting the number of iterations and returning it.
191 ///
192 /// This method will call [`next`] repeatedly until [`None`] is encountered,
193 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
194 /// called at least once even if the iterator does not have any elements.
195 ///
196 /// [`next`]: Iterator::next
197 ///
198 /// # Overflow Behavior
199 ///
200 /// The method does no guarding against overflows, so counting elements of
201 /// an iterator with more than [`usize::MAX`] elements either produces the
202 /// wrong result or panics. If debug assertions are enabled, a panic is
203 /// guaranteed.
204 ///
205 /// # Panics
206 ///
207 /// This function might panic if the iterator has more than [`usize::MAX`]
208 /// elements.
209 ///
210 /// # Examples
211 ///
212 /// ```
213 /// let a = [1, 2, 3];
214 /// assert_eq!(a.iter().count(), 3);
215 ///
216 /// let a = [1, 2, 3, 4, 5];
217 /// assert_eq!(a.iter().count(), 5);
218 /// ```
219 #[inline]
220 #[stable(feature = "rust1", since = "1.0.0")]
221 fn count(self) -> usize
222 where
223 Self: Sized,
224 {
225 self.fold(
226 0,
227 #[rustc_inherit_overflow_checks]
228 |count, _| count + 1,
229 )
230 }
231
232 /// Consumes the iterator, returning the last element.
233 ///
234 /// This method will evaluate the iterator until it returns [`None`]. While
235 /// doing so, it keeps track of the current element. After [`None`] is
236 /// returned, `last()` will then return the last element it saw.
237 ///
238 /// # Examples
239 ///
240 /// ```
241 /// let a = [1, 2, 3];
242 /// assert_eq!(a.iter().last(), Some(&3));
243 ///
244 /// let a = [1, 2, 3, 4, 5];
245 /// assert_eq!(a.iter().last(), Some(&5));
246 /// ```
247 #[inline]
248 #[stable(feature = "rust1", since = "1.0.0")]
249 fn last(self) -> Option<Self::Item>
250 where
251 Self: Sized,
252 {
253 #[inline]
254 fn some<T>(_: Option<T>, x: T) -> Option<T> {
255 Some(x)
256 }
257
258 self.fold(None, some)
259 }
260
261 /// Advances the iterator by `n` elements.
262 ///
263 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
264 /// times until [`None`] is encountered.
265 ///
266 /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
267 /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered,
268 /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
269 /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
270 /// Otherwise, `k` is always less than `n`.
271 ///
272 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
273 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
274 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
275 ///
276 /// [`Flatten`]: crate::iter::Flatten
277 /// [`next`]: Iterator::next
278 ///
279 /// # Examples
280 ///
281 /// ```
282 /// #![feature(iter_advance_by)]
283 ///
284 /// use std::num::NonZero;
285 ///
286 /// let a = [1, 2, 3, 4];
287 /// let mut iter = a.iter();
288 ///
289 /// assert_eq!(iter.advance_by(2), Ok(()));
290 /// assert_eq!(iter.next(), Some(&3));
291 /// assert_eq!(iter.advance_by(0), Ok(()));
292 /// assert_eq!(iter.advance_by(100), Err(NonZero::new(99).unwrap())); // only `&4` was skipped
293 /// ```
294 #[inline]
295 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
296 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
297 for i in 0..n {
298 if self.next().is_none() {
299 // SAFETY: `i` is always less than `n`.
300 return Err(unsafe { NonZero::new_unchecked(n - i) });
301 }
302 }
303 Ok(())
304 }
305
306 /// Returns the `n`th element of the iterator.
307 ///
308 /// Like most indexing operations, the count starts from zero, so `nth(0)`
309 /// returns the first value, `nth(1)` the second, and so on.
310 ///
311 /// Note that all preceding elements, as well as the returned element, will be
312 /// consumed from the iterator. That means that the preceding elements will be
313 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
314 /// will return different elements.
315 ///
316 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
317 /// iterator.
318 ///
319 /// # Examples
320 ///
321 /// Basic usage:
322 ///
323 /// ```
324 /// let a = [1, 2, 3];
325 /// assert_eq!(a.iter().nth(1), Some(&2));
326 /// ```
327 ///
328 /// Calling `nth()` multiple times doesn't rewind the iterator:
329 ///
330 /// ```
331 /// let a = [1, 2, 3];
332 ///
333 /// let mut iter = a.iter();
334 ///
335 /// assert_eq!(iter.nth(1), Some(&2));
336 /// assert_eq!(iter.nth(1), None);
337 /// ```
338 ///
339 /// Returning `None` if there are less than `n + 1` elements:
340 ///
341 /// ```
342 /// let a = [1, 2, 3];
343 /// assert_eq!(a.iter().nth(10), None);
344 /// ```
345 #[inline]
346 #[stable(feature = "rust1", since = "1.0.0")]
347 fn nth(&mut self, n: usize) -> Option<Self::Item> {
348 self.advance_by(n).ok()?;
349 self.next()
350 }
351
352 /// Creates an iterator starting at the same point, but stepping by
353 /// the given amount at each iteration.
354 ///
355 /// Note 1: The first element of the iterator will always be returned,
356 /// regardless of the step given.
357 ///
358 /// Note 2: The time at which ignored elements are pulled is not fixed.
359 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
360 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
361 /// `advance_n_and_return_first(&mut self, step)`,
362 /// `advance_n_and_return_first(&mut self, step)`, …
363 /// Which way is used may change for some iterators for performance reasons.
364 /// The second way will advance the iterator earlier and may consume more items.
365 ///
366 /// `advance_n_and_return_first` is the equivalent of:
367 /// ```
368 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
369 /// where
370 /// I: Iterator,
371 /// {
372 /// let next = iter.next();
373 /// if n > 1 {
374 /// iter.nth(n - 2);
375 /// }
376 /// next
377 /// }
378 /// ```
379 ///
380 /// # Panics
381 ///
382 /// The method will panic if the given step is `0`.
383 ///
384 /// # Examples
385 ///
386 /// ```
387 /// let a = [0, 1, 2, 3, 4, 5];
388 /// let mut iter = a.iter().step_by(2);
389 ///
390 /// assert_eq!(iter.next(), Some(&0));
391 /// assert_eq!(iter.next(), Some(&2));
392 /// assert_eq!(iter.next(), Some(&4));
393 /// assert_eq!(iter.next(), None);
394 /// ```
395 #[inline]
396 #[stable(feature = "iterator_step_by", since = "1.28.0")]
397 fn step_by(self, step: usize) -> StepBy<Self>
398 where
399 Self: Sized,
400 {
401 StepBy::new(self, step)
402 }
403
404 /// Takes two iterators and creates a new iterator over both in sequence.
405 ///
406 /// `chain()` will return a new iterator which will first iterate over
407 /// values from the first iterator and then over values from the second
408 /// iterator.
409 ///
410 /// In other words, it links two iterators together, in a chain. 🔗
411 ///
412 /// [`once`] is commonly used to adapt a single value into a chain of
413 /// other kinds of iteration.
414 ///
415 /// # Examples
416 ///
417 /// Basic usage:
418 ///
419 /// ```
420 /// let a1 = [1, 2, 3];
421 /// let a2 = [4, 5, 6];
422 ///
423 /// let mut iter = a1.iter().chain(a2.iter());
424 ///
425 /// assert_eq!(iter.next(), Some(&1));
426 /// assert_eq!(iter.next(), Some(&2));
427 /// assert_eq!(iter.next(), Some(&3));
428 /// assert_eq!(iter.next(), Some(&4));
429 /// assert_eq!(iter.next(), Some(&5));
430 /// assert_eq!(iter.next(), Some(&6));
431 /// assert_eq!(iter.next(), None);
432 /// ```
433 ///
434 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
435 /// anything that can be converted into an [`Iterator`], not just an
436 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
437 /// [`IntoIterator`], and so can be passed to `chain()` directly:
438 ///
439 /// ```
440 /// let s1 = &[1, 2, 3];
441 /// let s2 = &[4, 5, 6];
442 ///
443 /// let mut iter = s1.iter().chain(s2);
444 ///
445 /// assert_eq!(iter.next(), Some(&1));
446 /// assert_eq!(iter.next(), Some(&2));
447 /// assert_eq!(iter.next(), Some(&3));
448 /// assert_eq!(iter.next(), Some(&4));
449 /// assert_eq!(iter.next(), Some(&5));
450 /// assert_eq!(iter.next(), Some(&6));
451 /// assert_eq!(iter.next(), None);
452 /// ```
453 ///
454 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
455 ///
456 /// ```
457 /// #[cfg(windows)]
458 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
459 /// use std::os::windows::ffi::OsStrExt;
460 /// s.encode_wide().chain(std::iter::once(0)).collect()
461 /// }
462 /// ```
463 ///
464 /// [`once`]: crate::iter::once
465 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
466 #[inline]
467 #[stable(feature = "rust1", since = "1.0.0")]
468 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
469 where
470 Self: Sized,
471 U: IntoIterator<Item = Self::Item>,
472 {
473 Chain::new(self, other.into_iter())
474 }
475
476 /// 'Zips up' two iterators into a single iterator of pairs.
477 ///
478 /// `zip()` returns a new iterator that will iterate over two other
479 /// iterators, returning a tuple where the first element comes from the
480 /// first iterator, and the second element comes from the second iterator.
481 ///
482 /// In other words, it zips two iterators together, into a single one.
483 ///
484 /// If either iterator returns [`None`], [`next`] from the zipped iterator
485 /// will return [`None`].
486 /// If the zipped iterator has no more elements to return then each further attempt to advance
487 /// it will first try to advance the first iterator at most one time and if it still yielded an item
488 /// try to advance the second iterator at most one time.
489 ///
490 /// To 'undo' the result of zipping up two iterators, see [`unzip`].
491 ///
492 /// [`unzip`]: Iterator::unzip
493 ///
494 /// # Examples
495 ///
496 /// Basic usage:
497 ///
498 /// ```
499 /// let a1 = [1, 2, 3];
500 /// let a2 = [4, 5, 6];
501 ///
502 /// let mut iter = a1.iter().zip(a2.iter());
503 ///
504 /// assert_eq!(iter.next(), Some((&1, &4)));
505 /// assert_eq!(iter.next(), Some((&2, &5)));
506 /// assert_eq!(iter.next(), Some((&3, &6)));
507 /// assert_eq!(iter.next(), None);
508 /// ```
509 ///
510 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
511 /// anything that can be converted into an [`Iterator`], not just an
512 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
513 /// [`IntoIterator`], and so can be passed to `zip()` directly:
514 ///
515 /// ```
516 /// let s1 = &[1, 2, 3];
517 /// let s2 = &[4, 5, 6];
518 ///
519 /// let mut iter = s1.iter().zip(s2);
520 ///
521 /// assert_eq!(iter.next(), Some((&1, &4)));
522 /// assert_eq!(iter.next(), Some((&2, &5)));
523 /// assert_eq!(iter.next(), Some((&3, &6)));
524 /// assert_eq!(iter.next(), None);
525 /// ```
526 ///
527 /// `zip()` is often used to zip an infinite iterator to a finite one.
528 /// This works because the finite iterator will eventually return [`None`],
529 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
530 ///
531 /// ```
532 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
533 ///
534 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
535 ///
536 /// assert_eq!((0, 'f'), enumerate[0]);
537 /// assert_eq!((0, 'f'), zipper[0]);
538 ///
539 /// assert_eq!((1, 'o'), enumerate[1]);
540 /// assert_eq!((1, 'o'), zipper[1]);
541 ///
542 /// assert_eq!((2, 'o'), enumerate[2]);
543 /// assert_eq!((2, 'o'), zipper[2]);
544 /// ```
545 ///
546 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
547 ///
548 /// ```
549 /// use std::iter::zip;
550 ///
551 /// let a = [1, 2, 3];
552 /// let b = [2, 3, 4];
553 ///
554 /// let mut zipped = zip(
555 /// a.into_iter().map(|x| x * 2).skip(1),
556 /// b.into_iter().map(|x| x * 2).skip(1),
557 /// );
558 ///
559 /// assert_eq!(zipped.next(), Some((4, 6)));
560 /// assert_eq!(zipped.next(), Some((6, 8)));
561 /// assert_eq!(zipped.next(), None);
562 /// ```
563 ///
564 /// compared to:
565 ///
566 /// ```
567 /// # let a = [1, 2, 3];
568 /// # let b = [2, 3, 4];
569 /// #
570 /// let mut zipped = a
571 /// .into_iter()
572 /// .map(|x| x * 2)
573 /// .skip(1)
574 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
575 /// #
576 /// # assert_eq!(zipped.next(), Some((4, 6)));
577 /// # assert_eq!(zipped.next(), Some((6, 8)));
578 /// # assert_eq!(zipped.next(), None);
579 /// ```
580 ///
581 /// [`enumerate`]: Iterator::enumerate
582 /// [`next`]: Iterator::next
583 /// [`zip`]: crate::iter::zip
584 #[inline]
585 #[stable(feature = "rust1", since = "1.0.0")]
586 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
587 where
588 Self: Sized,
589 U: IntoIterator,
590 {
591 Zip::new(self, other.into_iter())
592 }
593
594 /// Creates a new iterator which places a copy of `separator` between adjacent
595 /// items of the original iterator.
596 ///
597 /// In case `separator` does not implement [`Clone`] or needs to be
598 /// computed every time, use [`intersperse_with`].
599 ///
600 /// # Examples
601 ///
602 /// Basic usage:
603 ///
604 /// ```
605 /// #![feature(iter_intersperse)]
606 ///
607 /// let mut a = [0, 1, 2].iter().intersperse(&100);
608 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
609 /// assert_eq!(a.next(), Some(&100)); // The separator.
610 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
611 /// assert_eq!(a.next(), Some(&100)); // The separator.
612 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
613 /// assert_eq!(a.next(), None); // The iterator is finished.
614 /// ```
615 ///
616 /// `intersperse` can be very useful to join an iterator's items using a common element:
617 /// ```
618 /// #![feature(iter_intersperse)]
619 ///
620 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
621 /// assert_eq!(hello, "Hello World !");
622 /// ```
623 ///
624 /// [`Clone`]: crate::clone::Clone
625 /// [`intersperse_with`]: Iterator::intersperse_with
626 #[inline]
627 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
628 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
629 where
630 Self: Sized,
631 Self::Item: Clone,
632 {
633 Intersperse::new(self, separator)
634 }
635
636 /// Creates a new iterator which places an item generated by `separator`
637 /// between adjacent items of the original iterator.
638 ///
639 /// The closure will be called exactly once each time an item is placed
640 /// between two adjacent items from the underlying iterator; specifically,
641 /// the closure is not called if the underlying iterator yields less than
642 /// two items and after the last item is yielded.
643 ///
644 /// If the iterator's item implements [`Clone`], it may be easier to use
645 /// [`intersperse`].
646 ///
647 /// # Examples
648 ///
649 /// Basic usage:
650 ///
651 /// ```
652 /// #![feature(iter_intersperse)]
653 ///
654 /// #[derive(PartialEq, Debug)]
655 /// struct NotClone(usize);
656 ///
657 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
658 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
659 ///
660 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
661 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
662 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
663 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
664 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from `v`.
665 /// assert_eq!(it.next(), None); // The iterator is finished.
666 /// ```
667 ///
668 /// `intersperse_with` can be used in situations where the separator needs
669 /// to be computed:
670 /// ```
671 /// #![feature(iter_intersperse)]
672 ///
673 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
674 ///
675 /// // The closure mutably borrows its context to generate an item.
676 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
677 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
678 ///
679 /// let result = src.intersperse_with(separator).collect::<String>();
680 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
681 /// ```
682 /// [`Clone`]: crate::clone::Clone
683 /// [`intersperse`]: Iterator::intersperse
684 #[inline]
685 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
686 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
687 where
688 Self: Sized,
689 G: FnMut() -> Self::Item,
690 {
691 IntersperseWith::new(self, separator)
692 }
693
694 /// Takes a closure and creates an iterator which calls that closure on each
695 /// element.
696 ///
697 /// `map()` transforms one iterator into another, by means of its argument:
698 /// something that implements [`FnMut`]. It produces a new iterator which
699 /// calls this closure on each element of the original iterator.
700 ///
701 /// If you are good at thinking in types, you can think of `map()` like this:
702 /// If you have an iterator that gives you elements of some type `A`, and
703 /// you want an iterator of some other type `B`, you can use `map()`,
704 /// passing a closure that takes an `A` and returns a `B`.
705 ///
706 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
707 /// lazy, it is best used when you're already working with other iterators.
708 /// If you're doing some sort of looping for a side effect, it's considered
709 /// more idiomatic to use [`for`] than `map()`.
710 ///
711 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
712 ///
713 /// # Examples
714 ///
715 /// Basic usage:
716 ///
717 /// ```
718 /// let a = [1, 2, 3];
719 ///
720 /// let mut iter = a.iter().map(|x| 2 * x);
721 ///
722 /// assert_eq!(iter.next(), Some(2));
723 /// assert_eq!(iter.next(), Some(4));
724 /// assert_eq!(iter.next(), Some(6));
725 /// assert_eq!(iter.next(), None);
726 /// ```
727 ///
728 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
729 ///
730 /// ```
731 /// # #![allow(unused_must_use)]
732 /// // don't do this:
733 /// (0..5).map(|x| println!("{x}"));
734 ///
735 /// // it won't even execute, as it is lazy. Rust will warn you about this.
736 ///
737 /// // Instead, use for:
738 /// for x in 0..5 {
739 /// println!("{x}");
740 /// }
741 /// ```
742 #[rustc_diagnostic_item = "IteratorMap"]
743 #[inline]
744 #[stable(feature = "rust1", since = "1.0.0")]
745 fn map<B, F>(self, f: F) -> Map<Self, F>
746 where
747 Self: Sized,
748 F: FnMut(Self::Item) -> B,
749 {
750 Map::new(self, f)
751 }
752
753 /// Calls a closure on each element of an iterator.
754 ///
755 /// This is equivalent to using a [`for`] loop on the iterator, although
756 /// `break` and `continue` are not possible from a closure. It's generally
757 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
758 /// when processing items at the end of longer iterator chains. In some
759 /// cases `for_each` may also be faster than a loop, because it will use
760 /// internal iteration on adapters like `Chain`.
761 ///
762 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
763 ///
764 /// # Examples
765 ///
766 /// Basic usage:
767 ///
768 /// ```
769 /// use std::sync::mpsc::channel;
770 ///
771 /// let (tx, rx) = channel();
772 /// (0..5).map(|x| x * 2 + 1)
773 /// .for_each(move |x| tx.send(x).unwrap());
774 ///
775 /// let v: Vec<_> = rx.iter().collect();
776 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
777 /// ```
778 ///
779 /// For such a small example, a `for` loop may be cleaner, but `for_each`
780 /// might be preferable to keep a functional style with longer iterators:
781 ///
782 /// ```
783 /// (0..5).flat_map(|x| x * 100 .. x * 110)
784 /// .enumerate()
785 /// .filter(|&(i, x)| (i + x) % 3 == 0)
786 /// .for_each(|(i, x)| println!("{i}:{x}"));
787 /// ```
788 #[inline]
789 #[stable(feature = "iterator_for_each", since = "1.21.0")]
790 fn for_each<F>(self, f: F)
791 where
792 Self: Sized,
793 F: FnMut(Self::Item),
794 {
795 #[inline]
796 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
797 move |(), item| f(item)
798 }
799
800 self.fold((), call(f));
801 }
802
803 /// Creates an iterator which uses a closure to determine if an element
804 /// should be yielded.
805 ///
806 /// Given an element the closure must return `true` or `false`. The returned
807 /// iterator will yield only the elements for which the closure returns
808 /// `true`.
809 ///
810 /// # Examples
811 ///
812 /// Basic usage:
813 ///
814 /// ```
815 /// let a = [0i32, 1, 2];
816 ///
817 /// let mut iter = a.iter().filter(|x| x.is_positive());
818 ///
819 /// assert_eq!(iter.next(), Some(&1));
820 /// assert_eq!(iter.next(), Some(&2));
821 /// assert_eq!(iter.next(), None);
822 /// ```
823 ///
824 /// Because the closure passed to `filter()` takes a reference, and many
825 /// iterators iterate over references, this leads to a possibly confusing
826 /// situation, where the type of the closure is a double reference:
827 ///
828 /// ```
829 /// let a = [0, 1, 2];
830 ///
831 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
832 ///
833 /// assert_eq!(iter.next(), Some(&2));
834 /// assert_eq!(iter.next(), None);
835 /// ```
836 ///
837 /// It's common to instead use destructuring on the argument to strip away
838 /// one:
839 ///
840 /// ```
841 /// let a = [0, 1, 2];
842 ///
843 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
844 ///
845 /// assert_eq!(iter.next(), Some(&2));
846 /// assert_eq!(iter.next(), None);
847 /// ```
848 ///
849 /// or both:
850 ///
851 /// ```
852 /// let a = [0, 1, 2];
853 ///
854 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
855 ///
856 /// assert_eq!(iter.next(), Some(&2));
857 /// assert_eq!(iter.next(), None);
858 /// ```
859 ///
860 /// of these layers.
861 ///
862 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
863 #[inline]
864 #[stable(feature = "rust1", since = "1.0.0")]
865 #[rustc_diagnostic_item = "iter_filter"]
866 fn filter<P>(self, predicate: P) -> Filter<Self, P>
867 where
868 Self: Sized,
869 P: FnMut(&Self::Item) -> bool,
870 {
871 Filter::new(self, predicate)
872 }
873
874 /// Creates an iterator that both filters and maps.
875 ///
876 /// The returned iterator yields only the `value`s for which the supplied
877 /// closure returns `Some(value)`.
878 ///
879 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
880 /// concise. The example below shows how a `map().filter().map()` can be
881 /// shortened to a single call to `filter_map`.
882 ///
883 /// [`filter`]: Iterator::filter
884 /// [`map`]: Iterator::map
885 ///
886 /// # Examples
887 ///
888 /// Basic usage:
889 ///
890 /// ```
891 /// let a = ["1", "two", "NaN", "four", "5"];
892 ///
893 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
894 ///
895 /// assert_eq!(iter.next(), Some(1));
896 /// assert_eq!(iter.next(), Some(5));
897 /// assert_eq!(iter.next(), None);
898 /// ```
899 ///
900 /// Here's the same example, but with [`filter`] and [`map`]:
901 ///
902 /// ```
903 /// let a = ["1", "two", "NaN", "four", "5"];
904 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
905 /// assert_eq!(iter.next(), Some(1));
906 /// assert_eq!(iter.next(), Some(5));
907 /// assert_eq!(iter.next(), None);
908 /// ```
909 #[inline]
910 #[stable(feature = "rust1", since = "1.0.0")]
911 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
912 where
913 Self: Sized,
914 F: FnMut(Self::Item) -> Option<B>,
915 {
916 FilterMap::new(self, f)
917 }
918
919 /// Creates an iterator which gives the current iteration count as well as
920 /// the next value.
921 ///
922 /// The iterator returned yields pairs `(i, val)`, where `i` is the
923 /// current index of iteration and `val` is the value returned by the
924 /// iterator.
925 ///
926 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
927 /// different sized integer, the [`zip`] function provides similar
928 /// functionality.
929 ///
930 /// # Overflow Behavior
931 ///
932 /// The method does no guarding against overflows, so enumerating more than
933 /// [`usize::MAX`] elements either produces the wrong result or panics. If
934 /// debug assertions are enabled, a panic is guaranteed.
935 ///
936 /// # Panics
937 ///
938 /// The returned iterator might panic if the to-be-returned index would
939 /// overflow a [`usize`].
940 ///
941 /// [`zip`]: Iterator::zip
942 ///
943 /// # Examples
944 ///
945 /// ```
946 /// let a = ['a', 'b', 'c'];
947 ///
948 /// let mut iter = a.iter().enumerate();
949 ///
950 /// assert_eq!(iter.next(), Some((0, &'a')));
951 /// assert_eq!(iter.next(), Some((1, &'b')));
952 /// assert_eq!(iter.next(), Some((2, &'c')));
953 /// assert_eq!(iter.next(), None);
954 /// ```
955 #[inline]
956 #[stable(feature = "rust1", since = "1.0.0")]
957 #[rustc_diagnostic_item = "enumerate_method"]
958 fn enumerate(self) -> Enumerate<Self>
959 where
960 Self: Sized,
961 {
962 Enumerate::new(self)
963 }
964
965 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
966 /// to look at the next element of the iterator without consuming it. See
967 /// their documentation for more information.
968 ///
969 /// Note that the underlying iterator is still advanced when [`peek`] or
970 /// [`peek_mut`] are called for the first time: In order to retrieve the
971 /// next element, [`next`] is called on the underlying iterator, hence any
972 /// side effects (i.e. anything other than fetching the next value) of
973 /// the [`next`] method will occur.
974 ///
975 ///
976 /// # Examples
977 ///
978 /// Basic usage:
979 ///
980 /// ```
981 /// let xs = [1, 2, 3];
982 ///
983 /// let mut iter = xs.iter().peekable();
984 ///
985 /// // peek() lets us see into the future
986 /// assert_eq!(iter.peek(), Some(&&1));
987 /// assert_eq!(iter.next(), Some(&1));
988 ///
989 /// assert_eq!(iter.next(), Some(&2));
990 ///
991 /// // we can peek() multiple times, the iterator won't advance
992 /// assert_eq!(iter.peek(), Some(&&3));
993 /// assert_eq!(iter.peek(), Some(&&3));
994 ///
995 /// assert_eq!(iter.next(), Some(&3));
996 ///
997 /// // after the iterator is finished, so is peek()
998 /// assert_eq!(iter.peek(), None);
999 /// assert_eq!(iter.next(), None);
1000 /// ```
1001 ///
1002 /// Using [`peek_mut`] to mutate the next item without advancing the
1003 /// iterator:
1004 ///
1005 /// ```
1006 /// let xs = [1, 2, 3];
1007 ///
1008 /// let mut iter = xs.iter().peekable();
1009 ///
1010 /// // `peek_mut()` lets us see into the future
1011 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1012 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1013 /// assert_eq!(iter.next(), Some(&1));
1014 ///
1015 /// if let Some(mut p) = iter.peek_mut() {
1016 /// assert_eq!(*p, &2);
1017 /// // put a value into the iterator
1018 /// *p = &1000;
1019 /// }
1020 ///
1021 /// // The value reappears as the iterator continues
1022 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
1023 /// ```
1024 /// [`peek`]: Peekable::peek
1025 /// [`peek_mut`]: Peekable::peek_mut
1026 /// [`next`]: Iterator::next
1027 #[inline]
1028 #[stable(feature = "rust1", since = "1.0.0")]
1029 fn peekable(self) -> Peekable<Self>
1030 where
1031 Self: Sized,
1032 {
1033 Peekable::new(self)
1034 }
1035
1036 /// Creates an iterator that [`skip`]s elements based on a predicate.
1037 ///
1038 /// [`skip`]: Iterator::skip
1039 ///
1040 /// `skip_while()` takes a closure as an argument. It will call this
1041 /// closure on each element of the iterator, and ignore elements
1042 /// until it returns `false`.
1043 ///
1044 /// After `false` is returned, `skip_while()`'s job is over, and the
1045 /// rest of the elements are yielded.
1046 ///
1047 /// # Examples
1048 ///
1049 /// Basic usage:
1050 ///
1051 /// ```
1052 /// let a = [-1i32, 0, 1];
1053 ///
1054 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1055 ///
1056 /// assert_eq!(iter.next(), Some(&0));
1057 /// assert_eq!(iter.next(), Some(&1));
1058 /// assert_eq!(iter.next(), None);
1059 /// ```
1060 ///
1061 /// Because the closure passed to `skip_while()` takes a reference, and many
1062 /// iterators iterate over references, this leads to a possibly confusing
1063 /// situation, where the type of the closure argument is a double reference:
1064 ///
1065 /// ```
1066 /// let a = [-1, 0, 1];
1067 ///
1068 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1069 ///
1070 /// assert_eq!(iter.next(), Some(&0));
1071 /// assert_eq!(iter.next(), Some(&1));
1072 /// assert_eq!(iter.next(), None);
1073 /// ```
1074 ///
1075 /// Stopping after an initial `false`:
1076 ///
1077 /// ```
1078 /// let a = [-1, 0, 1, -2];
1079 ///
1080 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1081 ///
1082 /// assert_eq!(iter.next(), Some(&0));
1083 /// assert_eq!(iter.next(), Some(&1));
1084 ///
1085 /// // while this would have been false, since we already got a false,
1086 /// // skip_while() isn't used any more
1087 /// assert_eq!(iter.next(), Some(&-2));
1088 ///
1089 /// assert_eq!(iter.next(), None);
1090 /// ```
1091 #[inline]
1092 #[doc(alias = "drop_while")]
1093 #[stable(feature = "rust1", since = "1.0.0")]
1094 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1095 where
1096 Self: Sized,
1097 P: FnMut(&Self::Item) -> bool,
1098 {
1099 SkipWhile::new(self, predicate)
1100 }
1101
1102 /// Creates an iterator that yields elements based on a predicate.
1103 ///
1104 /// `take_while()` takes a closure as an argument. It will call this
1105 /// closure on each element of the iterator, and yield elements
1106 /// while it returns `true`.
1107 ///
1108 /// After `false` is returned, `take_while()`'s job is over, and the
1109 /// rest of the elements are ignored.
1110 ///
1111 /// # Examples
1112 ///
1113 /// Basic usage:
1114 ///
1115 /// ```
1116 /// let a = [-1i32, 0, 1];
1117 ///
1118 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1119 ///
1120 /// assert_eq!(iter.next(), Some(&-1));
1121 /// assert_eq!(iter.next(), None);
1122 /// ```
1123 ///
1124 /// Because the closure passed to `take_while()` takes a reference, and many
1125 /// iterators iterate over references, this leads to a possibly confusing
1126 /// situation, where the type of the closure is a double reference:
1127 ///
1128 /// ```
1129 /// let a = [-1, 0, 1];
1130 ///
1131 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1132 ///
1133 /// assert_eq!(iter.next(), Some(&-1));
1134 /// assert_eq!(iter.next(), None);
1135 /// ```
1136 ///
1137 /// Stopping after an initial `false`:
1138 ///
1139 /// ```
1140 /// let a = [-1, 0, 1, -2];
1141 ///
1142 /// let mut iter = a.iter().take_while(|x| **x < 0);
1143 ///
1144 /// assert_eq!(iter.next(), Some(&-1));
1145 ///
1146 /// // We have more elements that are less than zero, but since we already
1147 /// // got a false, take_while() isn't used any more
1148 /// assert_eq!(iter.next(), None);
1149 /// ```
1150 ///
1151 /// Because `take_while()` needs to look at the value in order to see if it
1152 /// should be included or not, consuming iterators will see that it is
1153 /// removed:
1154 ///
1155 /// ```
1156 /// let a = [1, 2, 3, 4];
1157 /// let mut iter = a.iter();
1158 ///
1159 /// let result: Vec<i32> = iter.by_ref()
1160 /// .take_while(|n| **n != 3)
1161 /// .cloned()
1162 /// .collect();
1163 ///
1164 /// assert_eq!(result, &[1, 2]);
1165 ///
1166 /// let result: Vec<i32> = iter.cloned().collect();
1167 ///
1168 /// assert_eq!(result, &[4]);
1169 /// ```
1170 ///
1171 /// The `3` is no longer there, because it was consumed in order to see if
1172 /// the iteration should stop, but wasn't placed back into the iterator.
1173 #[inline]
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1176 where
1177 Self: Sized,
1178 P: FnMut(&Self::Item) -> bool,
1179 {
1180 TakeWhile::new(self, predicate)
1181 }
1182
1183 /// Creates an iterator that both yields elements based on a predicate and maps.
1184 ///
1185 /// `map_while()` takes a closure as an argument. It will call this
1186 /// closure on each element of the iterator, and yield elements
1187 /// while it returns [`Some(_)`][`Some`].
1188 ///
1189 /// # Examples
1190 ///
1191 /// Basic usage:
1192 ///
1193 /// ```
1194 /// let a = [-1i32, 4, 0, 1];
1195 ///
1196 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1197 ///
1198 /// assert_eq!(iter.next(), Some(-16));
1199 /// assert_eq!(iter.next(), Some(4));
1200 /// assert_eq!(iter.next(), None);
1201 /// ```
1202 ///
1203 /// Here's the same example, but with [`take_while`] and [`map`]:
1204 ///
1205 /// [`take_while`]: Iterator::take_while
1206 /// [`map`]: Iterator::map
1207 ///
1208 /// ```
1209 /// let a = [-1i32, 4, 0, 1];
1210 ///
1211 /// let mut iter = a.iter()
1212 /// .map(|x| 16i32.checked_div(*x))
1213 /// .take_while(|x| x.is_some())
1214 /// .map(|x| x.unwrap());
1215 ///
1216 /// assert_eq!(iter.next(), Some(-16));
1217 /// assert_eq!(iter.next(), Some(4));
1218 /// assert_eq!(iter.next(), None);
1219 /// ```
1220 ///
1221 /// Stopping after an initial [`None`]:
1222 ///
1223 /// ```
1224 /// let a = [0, 1, 2, -3, 4, 5, -6];
1225 ///
1226 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1227 /// let vec = iter.collect::<Vec<_>>();
1228 ///
1229 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1230 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1231 /// assert_eq!(vec, vec![0, 1, 2]);
1232 /// ```
1233 ///
1234 /// Because `map_while()` needs to look at the value in order to see if it
1235 /// should be included or not, consuming iterators will see that it is
1236 /// removed:
1237 ///
1238 /// ```
1239 /// let a = [1, 2, -3, 4];
1240 /// let mut iter = a.iter();
1241 ///
1242 /// let result: Vec<u32> = iter.by_ref()
1243 /// .map_while(|n| u32::try_from(*n).ok())
1244 /// .collect();
1245 ///
1246 /// assert_eq!(result, &[1, 2]);
1247 ///
1248 /// let result: Vec<i32> = iter.cloned().collect();
1249 ///
1250 /// assert_eq!(result, &[4]);
1251 /// ```
1252 ///
1253 /// The `-3` is no longer there, because it was consumed in order to see if
1254 /// the iteration should stop, but wasn't placed back into the iterator.
1255 ///
1256 /// Note that unlike [`take_while`] this iterator is **not** fused.
1257 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1258 /// If you need fused iterator, use [`fuse`].
1259 ///
1260 /// [`fuse`]: Iterator::fuse
1261 #[inline]
1262 #[stable(feature = "iter_map_while", since = "1.57.0")]
1263 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1264 where
1265 Self: Sized,
1266 P: FnMut(Self::Item) -> Option<B>,
1267 {
1268 MapWhile::new(self, predicate)
1269 }
1270
1271 /// Creates an iterator that skips the first `n` elements.
1272 ///
1273 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1274 /// iterator is reached (whichever happens first). After that, all the remaining
1275 /// elements are yielded. In particular, if the original iterator is too short,
1276 /// then the returned iterator is empty.
1277 ///
1278 /// Rather than overriding this method directly, instead override the `nth` method.
1279 ///
1280 /// # Examples
1281 ///
1282 /// ```
1283 /// let a = [1, 2, 3];
1284 ///
1285 /// let mut iter = a.iter().skip(2);
1286 ///
1287 /// assert_eq!(iter.next(), Some(&3));
1288 /// assert_eq!(iter.next(), None);
1289 /// ```
1290 #[inline]
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 fn skip(self, n: usize) -> Skip<Self>
1293 where
1294 Self: Sized,
1295 {
1296 Skip::new(self, n)
1297 }
1298
1299 /// Creates an iterator that yields the first `n` elements, or fewer
1300 /// if the underlying iterator ends sooner.
1301 ///
1302 /// `take(n)` yields elements until `n` elements are yielded or the end of
1303 /// the iterator is reached (whichever happens first).
1304 /// The returned iterator is a prefix of length `n` if the original iterator
1305 /// contains at least `n` elements, otherwise it contains all of the
1306 /// (fewer than `n`) elements of the original iterator.
1307 ///
1308 /// # Examples
1309 ///
1310 /// Basic usage:
1311 ///
1312 /// ```
1313 /// let a = [1, 2, 3];
1314 ///
1315 /// let mut iter = a.iter().take(2);
1316 ///
1317 /// assert_eq!(iter.next(), Some(&1));
1318 /// assert_eq!(iter.next(), Some(&2));
1319 /// assert_eq!(iter.next(), None);
1320 /// ```
1321 ///
1322 /// `take()` is often used with an infinite iterator, to make it finite:
1323 ///
1324 /// ```
1325 /// let mut iter = (0..).take(3);
1326 ///
1327 /// assert_eq!(iter.next(), Some(0));
1328 /// assert_eq!(iter.next(), Some(1));
1329 /// assert_eq!(iter.next(), Some(2));
1330 /// assert_eq!(iter.next(), None);
1331 /// ```
1332 ///
1333 /// If less than `n` elements are available,
1334 /// `take` will limit itself to the size of the underlying iterator:
1335 ///
1336 /// ```
1337 /// let v = [1, 2];
1338 /// let mut iter = v.into_iter().take(5);
1339 /// assert_eq!(iter.next(), Some(1));
1340 /// assert_eq!(iter.next(), Some(2));
1341 /// assert_eq!(iter.next(), None);
1342 /// ```
1343 #[inline]
1344 #[stable(feature = "rust1", since = "1.0.0")]
1345 fn take(self, n: usize) -> Take<Self>
1346 where
1347 Self: Sized,
1348 {
1349 Take::new(self, n)
1350 }
1351
1352 /// An iterator adapter which, like [`fold`], holds internal state, but
1353 /// unlike [`fold`], produces a new iterator.
1354 ///
1355 /// [`fold`]: Iterator::fold
1356 ///
1357 /// `scan()` takes two arguments: an initial value which seeds the internal
1358 /// state, and a closure with two arguments, the first being a mutable
1359 /// reference to the internal state and the second an iterator element.
1360 /// The closure can assign to the internal state to share state between
1361 /// iterations.
1362 ///
1363 /// On iteration, the closure will be applied to each element of the
1364 /// iterator and the return value from the closure, an [`Option`], is
1365 /// returned by the `next` method. Thus the closure can return
1366 /// `Some(value)` to yield `value`, or `None` to end the iteration.
1367 ///
1368 /// # Examples
1369 ///
1370 /// ```
1371 /// let a = [1, 2, 3, 4];
1372 ///
1373 /// let mut iter = a.iter().scan(1, |state, &x| {
1374 /// // each iteration, we'll multiply the state by the element ...
1375 /// *state = *state * x;
1376 ///
1377 /// // ... and terminate if the state exceeds 6
1378 /// if *state > 6 {
1379 /// return None;
1380 /// }
1381 /// // ... else yield the negation of the state
1382 /// Some(-*state)
1383 /// });
1384 ///
1385 /// assert_eq!(iter.next(), Some(-1));
1386 /// assert_eq!(iter.next(), Some(-2));
1387 /// assert_eq!(iter.next(), Some(-6));
1388 /// assert_eq!(iter.next(), None);
1389 /// ```
1390 #[inline]
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1393 where
1394 Self: Sized,
1395 F: FnMut(&mut St, Self::Item) -> Option<B>,
1396 {
1397 Scan::new(self, initial_state, f)
1398 }
1399
1400 /// Creates an iterator that works like map, but flattens nested structure.
1401 ///
1402 /// The [`map`] adapter is very useful, but only when the closure
1403 /// argument produces values. If it produces an iterator instead, there's
1404 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1405 /// on its own.
1406 ///
1407 /// You can think of `flat_map(f)` as the semantic equivalent
1408 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1409 ///
1410 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1411 /// one item for each element, and `flat_map()`'s closure returns an
1412 /// iterator for each element.
1413 ///
1414 /// [`map`]: Iterator::map
1415 /// [`flatten`]: Iterator::flatten
1416 ///
1417 /// # Examples
1418 ///
1419 /// ```
1420 /// let words = ["alpha", "beta", "gamma"];
1421 ///
1422 /// // chars() returns an iterator
1423 /// let merged: String = words.iter()
1424 /// .flat_map(|s| s.chars())
1425 /// .collect();
1426 /// assert_eq!(merged, "alphabetagamma");
1427 /// ```
1428 #[inline]
1429 #[stable(feature = "rust1", since = "1.0.0")]
1430 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1431 where
1432 Self: Sized,
1433 U: IntoIterator,
1434 F: FnMut(Self::Item) -> U,
1435 {
1436 FlatMap::new(self, f)
1437 }
1438
1439 /// Creates an iterator that flattens nested structure.
1440 ///
1441 /// This is useful when you have an iterator of iterators or an iterator of
1442 /// things that can be turned into iterators and you want to remove one
1443 /// level of indirection.
1444 ///
1445 /// # Examples
1446 ///
1447 /// Basic usage:
1448 ///
1449 /// ```
1450 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1451 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1452 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1453 /// ```
1454 ///
1455 /// Mapping and then flattening:
1456 ///
1457 /// ```
1458 /// let words = ["alpha", "beta", "gamma"];
1459 ///
1460 /// // chars() returns an iterator
1461 /// let merged: String = words.iter()
1462 /// .map(|s| s.chars())
1463 /// .flatten()
1464 /// .collect();
1465 /// assert_eq!(merged, "alphabetagamma");
1466 /// ```
1467 ///
1468 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1469 /// in this case since it conveys intent more clearly:
1470 ///
1471 /// ```
1472 /// let words = ["alpha", "beta", "gamma"];
1473 ///
1474 /// // chars() returns an iterator
1475 /// let merged: String = words.iter()
1476 /// .flat_map(|s| s.chars())
1477 /// .collect();
1478 /// assert_eq!(merged, "alphabetagamma");
1479 /// ```
1480 ///
1481 /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
1482 ///
1483 /// ```
1484 /// let options = vec![Some(123), Some(321), None, Some(231)];
1485 /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
1486 /// assert_eq!(flattened_options, vec![123, 321, 231]);
1487 ///
1488 /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
1489 /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
1490 /// assert_eq!(flattened_results, vec![123, 321, 231]);
1491 /// ```
1492 ///
1493 /// Flattening only removes one level of nesting at a time:
1494 ///
1495 /// ```
1496 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1497 ///
1498 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1499 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1500 ///
1501 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1502 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1503 /// ```
1504 ///
1505 /// Here we see that `flatten()` does not perform a "deep" flatten.
1506 /// Instead, only one level of nesting is removed. That is, if you
1507 /// `flatten()` a three-dimensional array, the result will be
1508 /// two-dimensional and not one-dimensional. To get a one-dimensional
1509 /// structure, you have to `flatten()` again.
1510 ///
1511 /// [`flat_map()`]: Iterator::flat_map
1512 #[inline]
1513 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1514 fn flatten(self) -> Flatten<Self>
1515 where
1516 Self: Sized,
1517 Self::Item: IntoIterator,
1518 {
1519 Flatten::new(self)
1520 }
1521
1522 /// Calls the given function `f` for each contiguous window of size `N` over
1523 /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
1524 /// the windows during mapping overlap as well.
1525 ///
1526 /// In the following example, the closure is called three times with the
1527 /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
1528 ///
1529 /// ```
1530 /// #![feature(iter_map_windows)]
1531 ///
1532 /// let strings = "abcd".chars()
1533 /// .map_windows(|[x, y]| format!("{}+{}", x, y))
1534 /// .collect::<Vec<String>>();
1535 ///
1536 /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
1537 /// ```
1538 ///
1539 /// Note that the const parameter `N` is usually inferred by the
1540 /// destructured argument in the closure.
1541 ///
1542 /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
1543 /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
1544 /// empty iterator.
1545 ///
1546 /// The returned iterator implements [`FusedIterator`], because once `self`
1547 /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
1548 /// we cannot put it into a contiguous array buffer, and thus the returned iterator
1549 /// should be fused.
1550 ///
1551 /// [`slice::windows()`]: slice::windows
1552 /// [`FusedIterator`]: crate::iter::FusedIterator
1553 ///
1554 /// # Panics
1555 ///
1556 /// Panics if `N` is zero. This check will most probably get changed to a
1557 /// compile time error before this method gets stabilized.
1558 ///
1559 /// ```should_panic
1560 /// #![feature(iter_map_windows)]
1561 ///
1562 /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
1563 /// ```
1564 ///
1565 /// # Examples
1566 ///
1567 /// Building the sums of neighboring numbers.
1568 ///
1569 /// ```
1570 /// #![feature(iter_map_windows)]
1571 ///
1572 /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
1573 /// assert_eq!(it.next(), Some(4)); // 1 + 3
1574 /// assert_eq!(it.next(), Some(11)); // 3 + 8
1575 /// assert_eq!(it.next(), Some(9)); // 8 + 1
1576 /// assert_eq!(it.next(), None);
1577 /// ```
1578 ///
1579 /// Since the elements in the following example implement `Copy`, we can
1580 /// just copy the array and get an iterator over the windows.
1581 ///
1582 /// ```
1583 /// #![feature(iter_map_windows)]
1584 ///
1585 /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
1586 /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
1587 /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
1588 /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
1589 /// assert_eq!(it.next(), Some(['r', 'i', 's']));
1590 /// assert_eq!(it.next(), None);
1591 /// ```
1592 ///
1593 /// You can also use this function to check the sortedness of an iterator.
1594 /// For the simple case, rather use [`Iterator::is_sorted`].
1595 ///
1596 /// ```
1597 /// #![feature(iter_map_windows)]
1598 ///
1599 /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
1600 /// .map_windows(|[a, b]| a <= b);
1601 ///
1602 /// assert_eq!(it.next(), Some(true)); // 0.5 <= 1.0
1603 /// assert_eq!(it.next(), Some(true)); // 1.0 <= 3.5
1604 /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
1605 /// assert_eq!(it.next(), Some(true)); // 3.0 <= 8.5
1606 /// assert_eq!(it.next(), Some(true)); // 8.5 <= 8.5
1607 /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
1608 /// assert_eq!(it.next(), None);
1609 /// ```
1610 ///
1611 /// For non-fused iterators, they are fused after `map_windows`.
1612 ///
1613 /// ```
1614 /// #![feature(iter_map_windows)]
1615 ///
1616 /// #[derive(Default)]
1617 /// struct NonFusedIterator {
1618 /// state: i32,
1619 /// }
1620 ///
1621 /// impl Iterator for NonFusedIterator {
1622 /// type Item = i32;
1623 ///
1624 /// fn next(&mut self) -> Option<i32> {
1625 /// let val = self.state;
1626 /// self.state = self.state + 1;
1627 ///
1628 /// // yields `0..5` first, then only even numbers since `6..`.
1629 /// if val < 5 || val % 2 == 0 {
1630 /// Some(val)
1631 /// } else {
1632 /// None
1633 /// }
1634 /// }
1635 /// }
1636 ///
1637 ///
1638 /// let mut iter = NonFusedIterator::default();
1639 ///
1640 /// // yields 0..5 first.
1641 /// assert_eq!(iter.next(), Some(0));
1642 /// assert_eq!(iter.next(), Some(1));
1643 /// assert_eq!(iter.next(), Some(2));
1644 /// assert_eq!(iter.next(), Some(3));
1645 /// assert_eq!(iter.next(), Some(4));
1646 /// // then we can see our iterator going back and forth
1647 /// assert_eq!(iter.next(), None);
1648 /// assert_eq!(iter.next(), Some(6));
1649 /// assert_eq!(iter.next(), None);
1650 /// assert_eq!(iter.next(), Some(8));
1651 /// assert_eq!(iter.next(), None);
1652 ///
1653 /// // however, with `.map_windows()`, it is fused.
1654 /// let mut iter = NonFusedIterator::default()
1655 /// .map_windows(|arr: &[_; 2]| *arr);
1656 ///
1657 /// assert_eq!(iter.next(), Some([0, 1]));
1658 /// assert_eq!(iter.next(), Some([1, 2]));
1659 /// assert_eq!(iter.next(), Some([2, 3]));
1660 /// assert_eq!(iter.next(), Some([3, 4]));
1661 /// assert_eq!(iter.next(), None);
1662 ///
1663 /// // it will always return `None` after the first time.
1664 /// assert_eq!(iter.next(), None);
1665 /// assert_eq!(iter.next(), None);
1666 /// assert_eq!(iter.next(), None);
1667 /// ```
1668 #[inline]
1669 #[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
1670 fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
1671 where
1672 Self: Sized,
1673 F: FnMut(&[Self::Item; N]) -> R,
1674 {
1675 MapWindows::new(self, f)
1676 }
1677
1678 /// Creates an iterator which ends after the first [`None`].
1679 ///
1680 /// After an iterator returns [`None`], future calls may or may not yield
1681 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1682 /// [`None`] is given, it will always return [`None`] forever.
1683 ///
1684 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1685 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1686 /// if the [`FusedIterator`] trait is improperly implemented.
1687 ///
1688 /// [`Some(T)`]: Some
1689 /// [`FusedIterator`]: crate::iter::FusedIterator
1690 ///
1691 /// # Examples
1692 ///
1693 /// ```
1694 /// // an iterator which alternates between Some and None
1695 /// struct Alternate {
1696 /// state: i32,
1697 /// }
1698 ///
1699 /// impl Iterator for Alternate {
1700 /// type Item = i32;
1701 ///
1702 /// fn next(&mut self) -> Option<i32> {
1703 /// let val = self.state;
1704 /// self.state = self.state + 1;
1705 ///
1706 /// // if it's even, Some(i32), else None
1707 /// (val % 2 == 0).then_some(val)
1708 /// }
1709 /// }
1710 ///
1711 /// let mut iter = Alternate { state: 0 };
1712 ///
1713 /// // we can see our iterator going back and forth
1714 /// assert_eq!(iter.next(), Some(0));
1715 /// assert_eq!(iter.next(), None);
1716 /// assert_eq!(iter.next(), Some(2));
1717 /// assert_eq!(iter.next(), None);
1718 ///
1719 /// // however, once we fuse it...
1720 /// let mut iter = iter.fuse();
1721 ///
1722 /// assert_eq!(iter.next(), Some(4));
1723 /// assert_eq!(iter.next(), None);
1724 ///
1725 /// // it will always return `None` after the first time.
1726 /// assert_eq!(iter.next(), None);
1727 /// assert_eq!(iter.next(), None);
1728 /// assert_eq!(iter.next(), None);
1729 /// ```
1730 #[inline]
1731 #[stable(feature = "rust1", since = "1.0.0")]
1732 fn fuse(self) -> Fuse<Self>
1733 where
1734 Self: Sized,
1735 {
1736 Fuse::new(self)
1737 }
1738
1739 /// Does something with each element of an iterator, passing the value on.
1740 ///
1741 /// When using iterators, you'll often chain several of them together.
1742 /// While working on such code, you might want to check out what's
1743 /// happening at various parts in the pipeline. To do that, insert
1744 /// a call to `inspect()`.
1745 ///
1746 /// It's more common for `inspect()` to be used as a debugging tool than to
1747 /// exist in your final code, but applications may find it useful in certain
1748 /// situations when errors need to be logged before being discarded.
1749 ///
1750 /// # Examples
1751 ///
1752 /// Basic usage:
1753 ///
1754 /// ```
1755 /// let a = [1, 4, 2, 3];
1756 ///
1757 /// // this iterator sequence is complex.
1758 /// let sum = a.iter()
1759 /// .cloned()
1760 /// .filter(|x| x % 2 == 0)
1761 /// .fold(0, |sum, i| sum + i);
1762 ///
1763 /// println!("{sum}");
1764 ///
1765 /// // let's add some inspect() calls to investigate what's happening
1766 /// let sum = a.iter()
1767 /// .cloned()
1768 /// .inspect(|x| println!("about to filter: {x}"))
1769 /// .filter(|x| x % 2 == 0)
1770 /// .inspect(|x| println!("made it through filter: {x}"))
1771 /// .fold(0, |sum, i| sum + i);
1772 ///
1773 /// println!("{sum}");
1774 /// ```
1775 ///
1776 /// This will print:
1777 ///
1778 /// ```text
1779 /// 6
1780 /// about to filter: 1
1781 /// about to filter: 4
1782 /// made it through filter: 4
1783 /// about to filter: 2
1784 /// made it through filter: 2
1785 /// about to filter: 3
1786 /// 6
1787 /// ```
1788 ///
1789 /// Logging errors before discarding them:
1790 ///
1791 /// ```
1792 /// let lines = ["1", "2", "a"];
1793 ///
1794 /// let sum: i32 = lines
1795 /// .iter()
1796 /// .map(|line| line.parse::<i32>())
1797 /// .inspect(|num| {
1798 /// if let Err(ref e) = *num {
1799 /// println!("Parsing error: {e}");
1800 /// }
1801 /// })
1802 /// .filter_map(Result::ok)
1803 /// .sum();
1804 ///
1805 /// println!("Sum: {sum}");
1806 /// ```
1807 ///
1808 /// This will print:
1809 ///
1810 /// ```text
1811 /// Parsing error: invalid digit found in string
1812 /// Sum: 3
1813 /// ```
1814 #[inline]
1815 #[stable(feature = "rust1", since = "1.0.0")]
1816 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1817 where
1818 Self: Sized,
1819 F: FnMut(&Self::Item),
1820 {
1821 Inspect::new(self, f)
1822 }
1823
1824 /// Creates a "by reference" adapter for this instance of `Iterator`.
1825 ///
1826 /// Consuming method calls (direct or indirect calls to `next`)
1827 /// on the "by reference" adapter will consume the original iterator,
1828 /// but ownership-taking methods (those with a `self` parameter)
1829 /// only take ownership of the "by reference" iterator.
1830 ///
1831 /// This is useful for applying ownership-taking methods
1832 /// (such as `take` in the example below)
1833 /// without giving up ownership of the original iterator,
1834 /// so you can use the original iterator afterwards.
1835 ///
1836 /// Uses [impl<I: Iterator + ?Sized> Iterator for &mut I { type Item = I::Item; ...}](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#impl-Iterator-for-%26mut+I).
1837 ///
1838 /// # Examples
1839 ///
1840 /// ```
1841 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1842 ///
1843 /// // Take the first two words.
1844 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1845 /// assert_eq!(hello_world, vec!["hello", "world"]);
1846 ///
1847 /// // Collect the rest of the words.
1848 /// // We can only do this because we used `by_ref` earlier.
1849 /// let of_rust: Vec<_> = words.collect();
1850 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1851 /// ```
1852 #[stable(feature = "rust1", since = "1.0.0")]
1853 fn by_ref(&mut self) -> &mut Self
1854 where
1855 Self: Sized,
1856 {
1857 self
1858 }
1859
1860 /// Transforms an iterator into a collection.
1861 ///
1862 /// `collect()` can take anything iterable, and turn it into a relevant
1863 /// collection. This is one of the more powerful methods in the standard
1864 /// library, used in a variety of contexts.
1865 ///
1866 /// The most basic pattern in which `collect()` is used is to turn one
1867 /// collection into another. You take a collection, call [`iter`] on it,
1868 /// do a bunch of transformations, and then `collect()` at the end.
1869 ///
1870 /// `collect()` can also create instances of types that are not typical
1871 /// collections. For example, a [`String`] can be built from [`char`]s,
1872 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1873 /// into `Result<Collection<T>, E>`. See the examples below for more.
1874 ///
1875 /// Because `collect()` is so general, it can cause problems with type
1876 /// inference. As such, `collect()` is one of the few times you'll see
1877 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1878 /// helps the inference algorithm understand specifically which collection
1879 /// you're trying to collect into.
1880 ///
1881 /// # Examples
1882 ///
1883 /// Basic usage:
1884 ///
1885 /// ```
1886 /// let a = [1, 2, 3];
1887 ///
1888 /// let doubled: Vec<i32> = a.iter()
1889 /// .map(|&x| x * 2)
1890 /// .collect();
1891 ///
1892 /// assert_eq!(vec![2, 4, 6], doubled);
1893 /// ```
1894 ///
1895 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1896 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1897 ///
1898 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1899 ///
1900 /// ```
1901 /// use std::collections::VecDeque;
1902 ///
1903 /// let a = [1, 2, 3];
1904 ///
1905 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1906 ///
1907 /// assert_eq!(2, doubled[0]);
1908 /// assert_eq!(4, doubled[1]);
1909 /// assert_eq!(6, doubled[2]);
1910 /// ```
1911 ///
1912 /// Using the 'turbofish' instead of annotating `doubled`:
1913 ///
1914 /// ```
1915 /// let a = [1, 2, 3];
1916 ///
1917 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1918 ///
1919 /// assert_eq!(vec![2, 4, 6], doubled);
1920 /// ```
1921 ///
1922 /// Because `collect()` only cares about what you're collecting into, you can
1923 /// still use a partial type hint, `_`, with the turbofish:
1924 ///
1925 /// ```
1926 /// let a = [1, 2, 3];
1927 ///
1928 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1929 ///
1930 /// assert_eq!(vec![2, 4, 6], doubled);
1931 /// ```
1932 ///
1933 /// Using `collect()` to make a [`String`]:
1934 ///
1935 /// ```
1936 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1937 ///
1938 /// let hello: String = chars.iter()
1939 /// .map(|&x| x as u8)
1940 /// .map(|x| (x + 1) as char)
1941 /// .collect();
1942 ///
1943 /// assert_eq!("hello", hello);
1944 /// ```
1945 ///
1946 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1947 /// see if any of them failed:
1948 ///
1949 /// ```
1950 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1951 ///
1952 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1953 ///
1954 /// // gives us the first error
1955 /// assert_eq!(Err("nope"), result);
1956 ///
1957 /// let results = [Ok(1), Ok(3)];
1958 ///
1959 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1960 ///
1961 /// // gives us the list of answers
1962 /// assert_eq!(Ok(vec![1, 3]), result);
1963 /// ```
1964 ///
1965 /// [`iter`]: Iterator::next
1966 /// [`String`]: ../../std/string/struct.String.html
1967 /// [`char`]: type@char
1968 #[inline]
1969 #[stable(feature = "rust1", since = "1.0.0")]
1970 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1971 #[rustc_diagnostic_item = "iterator_collect_fn"]
1972 fn collect<B: FromIterator<Self::Item>>(self) -> B
1973 where
1974 Self: Sized,
1975 {
1976 // This is too aggressive to turn on for everything all the time, but PR#137908
1977 // accidentally noticed that some rustc iterators had malformed `size_hint`s,
1978 // so this will help catch such things in debug-assertions-std runners,
1979 // even if users won't actually ever see it.
1980 if cfg!(debug_assertions) {
1981 let hint = self.size_hint();
1982 assert!(hint.1.is_none_or(|high| high >= hint.0), "Malformed size_hint {hint:?}");
1983 }
1984
1985 FromIterator::from_iter(self)
1986 }
1987
1988 /// Fallibly transforms an iterator into a collection, short circuiting if
1989 /// a failure is encountered.
1990 ///
1991 /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
1992 /// conversions during collection. Its main use case is simplifying conversions from
1993 /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
1994 /// types (e.g. [`Result`]).
1995 ///
1996 /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
1997 /// only the inner type produced on `Try::Output` must implement it. Concretely,
1998 /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
1999 /// [`FromIterator`], even though [`ControlFlow`] doesn't.
2000 ///
2001 /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
2002 /// may continue to be used, in which case it will continue iterating starting after the element that
2003 /// triggered the failure. See the last example below for an example of how this works.
2004 ///
2005 /// # Examples
2006 /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
2007 /// ```
2008 /// #![feature(iterator_try_collect)]
2009 ///
2010 /// let u = vec![Some(1), Some(2), Some(3)];
2011 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2012 /// assert_eq!(v, Some(vec![1, 2, 3]));
2013 /// ```
2014 ///
2015 /// Failing to collect in the same way:
2016 /// ```
2017 /// #![feature(iterator_try_collect)]
2018 ///
2019 /// let u = vec![Some(1), Some(2), None, Some(3)];
2020 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2021 /// assert_eq!(v, None);
2022 /// ```
2023 ///
2024 /// A similar example, but with `Result`:
2025 /// ```
2026 /// #![feature(iterator_try_collect)]
2027 ///
2028 /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
2029 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2030 /// assert_eq!(v, Ok(vec![1, 2, 3]));
2031 ///
2032 /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
2033 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2034 /// assert_eq!(v, Err(()));
2035 /// ```
2036 ///
2037 /// Finally, even [`ControlFlow`] works, despite the fact that it
2038 /// doesn't implement [`FromIterator`]. Note also that the iterator can
2039 /// continue to be used, even if a failure is encountered:
2040 ///
2041 /// ```
2042 /// #![feature(iterator_try_collect)]
2043 ///
2044 /// use core::ops::ControlFlow::{Break, Continue};
2045 ///
2046 /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
2047 /// let mut it = u.into_iter();
2048 ///
2049 /// let v = it.try_collect::<Vec<_>>();
2050 /// assert_eq!(v, Break(3));
2051 ///
2052 /// let v = it.try_collect::<Vec<_>>();
2053 /// assert_eq!(v, Continue(vec![4, 5]));
2054 /// ```
2055 ///
2056 /// [`collect`]: Iterator::collect
2057 #[inline]
2058 #[unstable(feature = "iterator_try_collect", issue = "94047")]
2059 fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
2060 where
2061 Self: Sized,
2062 Self::Item: Try<Residual: Residual<B>>,
2063 B: FromIterator<<Self::Item as Try>::Output>,
2064 {
2065 try_process(ByRefSized(self), |i| i.collect())
2066 }
2067
2068 /// Collects all the items from an iterator into a collection.
2069 ///
2070 /// This method consumes the iterator and adds all its items to the
2071 /// passed collection. The collection is then returned, so the call chain
2072 /// can be continued.
2073 ///
2074 /// This is useful when you already have a collection and want to add
2075 /// the iterator items to it.
2076 ///
2077 /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
2078 /// but instead of being called on a collection, it's called on an iterator.
2079 ///
2080 /// # Examples
2081 ///
2082 /// Basic usage:
2083 ///
2084 /// ```
2085 /// #![feature(iter_collect_into)]
2086 ///
2087 /// let a = [1, 2, 3];
2088 /// let mut vec: Vec::<i32> = vec![0, 1];
2089 ///
2090 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2091 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2092 ///
2093 /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
2094 /// ```
2095 ///
2096 /// `Vec` can have a manual set capacity to avoid reallocating it:
2097 ///
2098 /// ```
2099 /// #![feature(iter_collect_into)]
2100 ///
2101 /// let a = [1, 2, 3];
2102 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2103 ///
2104 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2105 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2106 ///
2107 /// assert_eq!(6, vec.capacity());
2108 /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
2109 /// ```
2110 ///
2111 /// The returned mutable reference can be used to continue the call chain:
2112 ///
2113 /// ```
2114 /// #![feature(iter_collect_into)]
2115 ///
2116 /// let a = [1, 2, 3];
2117 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2118 ///
2119 /// let count = a.iter().collect_into(&mut vec).iter().count();
2120 ///
2121 /// assert_eq!(count, vec.len());
2122 /// assert_eq!(vec, vec![1, 2, 3]);
2123 ///
2124 /// let count = a.iter().collect_into(&mut vec).iter().count();
2125 ///
2126 /// assert_eq!(count, vec.len());
2127 /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
2128 /// ```
2129 #[inline]
2130 #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
2131 fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
2132 where
2133 Self: Sized,
2134 {
2135 collection.extend(self);
2136 collection
2137 }
2138
2139 /// Consumes an iterator, creating two collections from it.
2140 ///
2141 /// The predicate passed to `partition()` can return `true`, or `false`.
2142 /// `partition()` returns a pair, all of the elements for which it returned
2143 /// `true`, and all of the elements for which it returned `false`.
2144 ///
2145 /// See also [`is_partitioned()`] and [`partition_in_place()`].
2146 ///
2147 /// [`is_partitioned()`]: Iterator::is_partitioned
2148 /// [`partition_in_place()`]: Iterator::partition_in_place
2149 ///
2150 /// # Examples
2151 ///
2152 /// ```
2153 /// let a = [1, 2, 3];
2154 ///
2155 /// let (even, odd): (Vec<_>, Vec<_>) = a
2156 /// .into_iter()
2157 /// .partition(|n| n % 2 == 0);
2158 ///
2159 /// assert_eq!(even, vec![2]);
2160 /// assert_eq!(odd, vec![1, 3]);
2161 /// ```
2162 #[stable(feature = "rust1", since = "1.0.0")]
2163 fn partition<B, F>(self, f: F) -> (B, B)
2164 where
2165 Self: Sized,
2166 B: Default + Extend<Self::Item>,
2167 F: FnMut(&Self::Item) -> bool,
2168 {
2169 #[inline]
2170 fn extend<'a, T, B: Extend<T>>(
2171 mut f: impl FnMut(&T) -> bool + 'a,
2172 left: &'a mut B,
2173 right: &'a mut B,
2174 ) -> impl FnMut((), T) + 'a {
2175 move |(), x| {
2176 if f(&x) {
2177 left.extend_one(x);
2178 } else {
2179 right.extend_one(x);
2180 }
2181 }
2182 }
2183
2184 let mut left: B = Default::default();
2185 let mut right: B = Default::default();
2186
2187 self.fold((), extend(f, &mut left, &mut right));
2188
2189 (left, right)
2190 }
2191
2192 /// Reorders the elements of this iterator *in-place* according to the given predicate,
2193 /// such that all those that return `true` precede all those that return `false`.
2194 /// Returns the number of `true` elements found.
2195 ///
2196 /// The relative order of partitioned items is not maintained.
2197 ///
2198 /// # Current implementation
2199 ///
2200 /// The current algorithm tries to find the first element for which the predicate evaluates
2201 /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
2202 ///
2203 /// Time complexity: *O*(*n*)
2204 ///
2205 /// See also [`is_partitioned()`] and [`partition()`].
2206 ///
2207 /// [`is_partitioned()`]: Iterator::is_partitioned
2208 /// [`partition()`]: Iterator::partition
2209 ///
2210 /// # Examples
2211 ///
2212 /// ```
2213 /// #![feature(iter_partition_in_place)]
2214 ///
2215 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
2216 ///
2217 /// // Partition in-place between evens and odds
2218 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
2219 ///
2220 /// assert_eq!(i, 3);
2221 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
2222 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
2223 /// ```
2224 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
2225 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
2226 where
2227 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
2228 P: FnMut(&T) -> bool,
2229 {
2230 // FIXME: should we worry about the count overflowing? The only way to have more than
2231 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
2232
2233 // These closure "factory" functions exist to avoid genericity in `Self`.
2234
2235 #[inline]
2236 fn is_false<'a, T>(
2237 predicate: &'a mut impl FnMut(&T) -> bool,
2238 true_count: &'a mut usize,
2239 ) -> impl FnMut(&&mut T) -> bool + 'a {
2240 move |x| {
2241 let p = predicate(&**x);
2242 *true_count += p as usize;
2243 !p
2244 }
2245 }
2246
2247 #[inline]
2248 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
2249 move |x| predicate(&**x)
2250 }
2251
2252 // Repeatedly find the first `false` and swap it with the last `true`.
2253 let mut true_count = 0;
2254 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
2255 if let Some(tail) = self.rfind(is_true(predicate)) {
2256 crate::mem::swap(head, tail);
2257 true_count += 1;
2258 } else {
2259 break;
2260 }
2261 }
2262 true_count
2263 }
2264
2265 /// Checks if the elements of this iterator are partitioned according to the given predicate,
2266 /// such that all those that return `true` precede all those that return `false`.
2267 ///
2268 /// See also [`partition()`] and [`partition_in_place()`].
2269 ///
2270 /// [`partition()`]: Iterator::partition
2271 /// [`partition_in_place()`]: Iterator::partition_in_place
2272 ///
2273 /// # Examples
2274 ///
2275 /// ```
2276 /// #![feature(iter_is_partitioned)]
2277 ///
2278 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2279 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2280 /// ```
2281 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2282 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2283 where
2284 Self: Sized,
2285 P: FnMut(Self::Item) -> bool,
2286 {
2287 // Either all items test `true`, or the first clause stops at `false`
2288 // and we check that there are no more `true` items after that.
2289 self.all(&mut predicate) || !self.any(predicate)
2290 }
2291
2292 /// An iterator method that applies a function as long as it returns
2293 /// successfully, producing a single, final value.
2294 ///
2295 /// `try_fold()` takes two arguments: an initial value, and a closure with
2296 /// two arguments: an 'accumulator', and an element. The closure either
2297 /// returns successfully, with the value that the accumulator should have
2298 /// for the next iteration, or it returns failure, with an error value that
2299 /// is propagated back to the caller immediately (short-circuiting).
2300 ///
2301 /// The initial value is the value the accumulator will have on the first
2302 /// call. If applying the closure succeeded against every element of the
2303 /// iterator, `try_fold()` returns the final accumulator as success.
2304 ///
2305 /// Folding is useful whenever you have a collection of something, and want
2306 /// to produce a single value from it.
2307 ///
2308 /// # Note to Implementors
2309 ///
2310 /// Several of the other (forward) methods have default implementations in
2311 /// terms of this one, so try to implement this explicitly if it can
2312 /// do something better than the default `for` loop implementation.
2313 ///
2314 /// In particular, try to have this call `try_fold()` on the internal parts
2315 /// from which this iterator is composed. If multiple calls are needed,
2316 /// the `?` operator may be convenient for chaining the accumulator value
2317 /// along, but beware any invariants that need to be upheld before those
2318 /// early returns. This is a `&mut self` method, so iteration needs to be
2319 /// resumable after hitting an error here.
2320 ///
2321 /// # Examples
2322 ///
2323 /// Basic usage:
2324 ///
2325 /// ```
2326 /// let a = [1, 2, 3];
2327 ///
2328 /// // the checked sum of all of the elements of the array
2329 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
2330 ///
2331 /// assert_eq!(sum, Some(6));
2332 /// ```
2333 ///
2334 /// Short-circuiting:
2335 ///
2336 /// ```
2337 /// let a = [10, 20, 30, 100, 40, 50];
2338 /// let mut it = a.iter();
2339 ///
2340 /// // This sum overflows when adding the 100 element
2341 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
2342 /// assert_eq!(sum, None);
2343 ///
2344 /// // Because it short-circuited, the remaining elements are still
2345 /// // available through the iterator.
2346 /// assert_eq!(it.len(), 2);
2347 /// assert_eq!(it.next(), Some(&40));
2348 /// ```
2349 ///
2350 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2351 /// a similar idea:
2352 ///
2353 /// ```
2354 /// use std::ops::ControlFlow;
2355 ///
2356 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2357 /// if let Some(next) = prev.checked_add(x) {
2358 /// ControlFlow::Continue(next)
2359 /// } else {
2360 /// ControlFlow::Break(prev)
2361 /// }
2362 /// });
2363 /// assert_eq!(triangular, ControlFlow::Break(120));
2364 ///
2365 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2366 /// if let Some(next) = prev.checked_add(x) {
2367 /// ControlFlow::Continue(next)
2368 /// } else {
2369 /// ControlFlow::Break(prev)
2370 /// }
2371 /// });
2372 /// assert_eq!(triangular, ControlFlow::Continue(435));
2373 /// ```
2374 #[inline]
2375 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2376 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2377 where
2378 Self: Sized,
2379 F: FnMut(B, Self::Item) -> R,
2380 R: Try<Output = B>,
2381 {
2382 let mut accum = init;
2383 while let Some(x) = self.next() {
2384 accum = f(accum, x)?;
2385 }
2386 try { accum }
2387 }
2388
2389 /// An iterator method that applies a fallible function to each item in the
2390 /// iterator, stopping at the first error and returning that error.
2391 ///
2392 /// This can also be thought of as the fallible form of [`for_each()`]
2393 /// or as the stateless version of [`try_fold()`].
2394 ///
2395 /// [`for_each()`]: Iterator::for_each
2396 /// [`try_fold()`]: Iterator::try_fold
2397 ///
2398 /// # Examples
2399 ///
2400 /// ```
2401 /// use std::fs::rename;
2402 /// use std::io::{stdout, Write};
2403 /// use std::path::Path;
2404 ///
2405 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2406 ///
2407 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
2408 /// assert!(res.is_ok());
2409 ///
2410 /// let mut it = data.iter().cloned();
2411 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2412 /// assert!(res.is_err());
2413 /// // It short-circuited, so the remaining items are still in the iterator:
2414 /// assert_eq!(it.next(), Some("stale_bread.json"));
2415 /// ```
2416 ///
2417 /// The [`ControlFlow`] type can be used with this method for the situations
2418 /// in which you'd use `break` and `continue` in a normal loop:
2419 ///
2420 /// ```
2421 /// use std::ops::ControlFlow;
2422 ///
2423 /// let r = (2..100).try_for_each(|x| {
2424 /// if 323 % x == 0 {
2425 /// return ControlFlow::Break(x)
2426 /// }
2427 ///
2428 /// ControlFlow::Continue(())
2429 /// });
2430 /// assert_eq!(r, ControlFlow::Break(17));
2431 /// ```
2432 #[inline]
2433 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2434 fn try_for_each<F, R>(&mut self, f: F) -> R
2435 where
2436 Self: Sized,
2437 F: FnMut(Self::Item) -> R,
2438 R: Try<Output = ()>,
2439 {
2440 #[inline]
2441 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2442 move |(), x| f(x)
2443 }
2444
2445 self.try_fold((), call(f))
2446 }
2447
2448 /// Folds every element into an accumulator by applying an operation,
2449 /// returning the final result.
2450 ///
2451 /// `fold()` takes two arguments: an initial value, and a closure with two
2452 /// arguments: an 'accumulator', and an element. The closure returns the value that
2453 /// the accumulator should have for the next iteration.
2454 ///
2455 /// The initial value is the value the accumulator will have on the first
2456 /// call.
2457 ///
2458 /// After applying this closure to every element of the iterator, `fold()`
2459 /// returns the accumulator.
2460 ///
2461 /// This operation is sometimes called 'reduce' or 'inject'.
2462 ///
2463 /// Folding is useful whenever you have a collection of something, and want
2464 /// to produce a single value from it.
2465 ///
2466 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2467 /// might not terminate for infinite iterators, even on traits for which a
2468 /// result is determinable in finite time.
2469 ///
2470 /// Note: [`reduce()`] can be used to use the first element as the initial
2471 /// value, if the accumulator type and item type is the same.
2472 ///
2473 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2474 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2475 /// operators like `-` the order will affect the final result.
2476 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2477 ///
2478 /// # Note to Implementors
2479 ///
2480 /// Several of the other (forward) methods have default implementations in
2481 /// terms of this one, so try to implement this explicitly if it can
2482 /// do something better than the default `for` loop implementation.
2483 ///
2484 /// In particular, try to have this call `fold()` on the internal parts
2485 /// from which this iterator is composed.
2486 ///
2487 /// # Examples
2488 ///
2489 /// Basic usage:
2490 ///
2491 /// ```
2492 /// let a = [1, 2, 3];
2493 ///
2494 /// // the sum of all of the elements of the array
2495 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2496 ///
2497 /// assert_eq!(sum, 6);
2498 /// ```
2499 ///
2500 /// Let's walk through each step of the iteration here:
2501 ///
2502 /// | element | acc | x | result |
2503 /// |---------|-----|---|--------|
2504 /// | | 0 | | |
2505 /// | 1 | 0 | 1 | 1 |
2506 /// | 2 | 1 | 2 | 3 |
2507 /// | 3 | 3 | 3 | 6 |
2508 ///
2509 /// And so, our final result, `6`.
2510 ///
2511 /// This example demonstrates the left-associative nature of `fold()`:
2512 /// it builds a string, starting with an initial value
2513 /// and continuing with each element from the front until the back:
2514 ///
2515 /// ```
2516 /// let numbers = [1, 2, 3, 4, 5];
2517 ///
2518 /// let zero = "0".to_string();
2519 ///
2520 /// let result = numbers.iter().fold(zero, |acc, &x| {
2521 /// format!("({acc} + {x})")
2522 /// });
2523 ///
2524 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2525 /// ```
2526 /// It's common for people who haven't used iterators a lot to
2527 /// use a `for` loop with a list of things to build up a result. Those
2528 /// can be turned into `fold()`s:
2529 ///
2530 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2531 ///
2532 /// ```
2533 /// let numbers = [1, 2, 3, 4, 5];
2534 ///
2535 /// let mut result = 0;
2536 ///
2537 /// // for loop:
2538 /// for i in &numbers {
2539 /// result = result + i;
2540 /// }
2541 ///
2542 /// // fold:
2543 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2544 ///
2545 /// // they're the same
2546 /// assert_eq!(result, result2);
2547 /// ```
2548 ///
2549 /// [`reduce()`]: Iterator::reduce
2550 #[doc(alias = "inject", alias = "foldl")]
2551 #[inline]
2552 #[stable(feature = "rust1", since = "1.0.0")]
2553 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2554 where
2555 Self: Sized,
2556 F: FnMut(B, Self::Item) -> B,
2557 {
2558 let mut accum = init;
2559 while let Some(x) = self.next() {
2560 accum = f(accum, x);
2561 }
2562 accum
2563 }
2564
2565 /// Reduces the elements to a single one, by repeatedly applying a reducing
2566 /// operation.
2567 ///
2568 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2569 /// result of the reduction.
2570 ///
2571 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2572 /// For iterators with at least one element, this is the same as [`fold()`]
2573 /// with the first element of the iterator as the initial accumulator value, folding
2574 /// every subsequent element into it.
2575 ///
2576 /// [`fold()`]: Iterator::fold
2577 ///
2578 /// # Example
2579 ///
2580 /// ```
2581 /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap_or(0);
2582 /// assert_eq!(reduced, 45);
2583 ///
2584 /// // Which is equivalent to doing it with `fold`:
2585 /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
2586 /// assert_eq!(reduced, folded);
2587 /// ```
2588 #[inline]
2589 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2590 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2591 where
2592 Self: Sized,
2593 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2594 {
2595 let first = self.next()?;
2596 Some(self.fold(first, f))
2597 }
2598
2599 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2600 /// closure returns a failure, the failure is propagated back to the caller immediately.
2601 ///
2602 /// The return type of this method depends on the return type of the closure. If the closure
2603 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2604 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2605 /// `Option<Option<Self::Item>>`.
2606 ///
2607 /// When called on an empty iterator, this function will return either `Some(None)` or
2608 /// `Ok(None)` depending on the type of the provided closure.
2609 ///
2610 /// For iterators with at least one element, this is essentially the same as calling
2611 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2612 ///
2613 /// [`try_fold()`]: Iterator::try_fold
2614 ///
2615 /// # Examples
2616 ///
2617 /// Safely calculate the sum of a series of numbers:
2618 ///
2619 /// ```
2620 /// #![feature(iterator_try_reduce)]
2621 ///
2622 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2623 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2624 /// assert_eq!(sum, Some(Some(58)));
2625 /// ```
2626 ///
2627 /// Determine when a reduction short circuited:
2628 ///
2629 /// ```
2630 /// #![feature(iterator_try_reduce)]
2631 ///
2632 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2633 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2634 /// assert_eq!(sum, None);
2635 /// ```
2636 ///
2637 /// Determine when a reduction was not performed because there are no elements:
2638 ///
2639 /// ```
2640 /// #![feature(iterator_try_reduce)]
2641 ///
2642 /// let numbers: Vec<usize> = Vec::new();
2643 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2644 /// assert_eq!(sum, Some(None));
2645 /// ```
2646 ///
2647 /// Use a [`Result`] instead of an [`Option`]:
2648 ///
2649 /// ```
2650 /// #![feature(iterator_try_reduce)]
2651 ///
2652 /// let numbers = vec!["1", "2", "3", "4", "5"];
2653 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2654 /// numbers.into_iter().try_reduce(|x, y| {
2655 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2656 /// });
2657 /// assert_eq!(max, Ok(Some("5")));
2658 /// ```
2659 #[inline]
2660 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2661 fn try_reduce<R>(
2662 &mut self,
2663 f: impl FnMut(Self::Item, Self::Item) -> R,
2664 ) -> ChangeOutputType<R, Option<R::Output>>
2665 where
2666 Self: Sized,
2667 R: Try<Output = Self::Item, Residual: Residual<Option<Self::Item>>>,
2668 {
2669 let first = match self.next() {
2670 Some(i) => i,
2671 None => return Try::from_output(None),
2672 };
2673
2674 match self.try_fold(first, f).branch() {
2675 ControlFlow::Break(r) => FromResidual::from_residual(r),
2676 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2677 }
2678 }
2679
2680 /// Tests if every element of the iterator matches a predicate.
2681 ///
2682 /// `all()` takes a closure that returns `true` or `false`. It applies
2683 /// this closure to each element of the iterator, and if they all return
2684 /// `true`, then so does `all()`. If any of them return `false`, it
2685 /// returns `false`.
2686 ///
2687 /// `all()` is short-circuiting; in other words, it will stop processing
2688 /// as soon as it finds a `false`, given that no matter what else happens,
2689 /// the result will also be `false`.
2690 ///
2691 /// An empty iterator returns `true`.
2692 ///
2693 /// # Examples
2694 ///
2695 /// Basic usage:
2696 ///
2697 /// ```
2698 /// let a = [1, 2, 3];
2699 ///
2700 /// assert!(a.iter().all(|&x| x > 0));
2701 ///
2702 /// assert!(!a.iter().all(|&x| x > 2));
2703 /// ```
2704 ///
2705 /// Stopping at the first `false`:
2706 ///
2707 /// ```
2708 /// let a = [1, 2, 3];
2709 ///
2710 /// let mut iter = a.iter();
2711 ///
2712 /// assert!(!iter.all(|&x| x != 2));
2713 ///
2714 /// // we can still use `iter`, as there are more elements.
2715 /// assert_eq!(iter.next(), Some(&3));
2716 /// ```
2717 #[inline]
2718 #[stable(feature = "rust1", since = "1.0.0")]
2719 fn all<F>(&mut self, f: F) -> bool
2720 where
2721 Self: Sized,
2722 F: FnMut(Self::Item) -> bool,
2723 {
2724 #[inline]
2725 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2726 move |(), x| {
2727 if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
2728 }
2729 }
2730 self.try_fold((), check(f)) == ControlFlow::Continue(())
2731 }
2732
2733 /// Tests if any element of the iterator matches a predicate.
2734 ///
2735 /// `any()` takes a closure that returns `true` or `false`. It applies
2736 /// this closure to each element of the iterator, and if any of them return
2737 /// `true`, then so does `any()`. If they all return `false`, it
2738 /// returns `false`.
2739 ///
2740 /// `any()` is short-circuiting; in other words, it will stop processing
2741 /// as soon as it finds a `true`, given that no matter what else happens,
2742 /// the result will also be `true`.
2743 ///
2744 /// An empty iterator returns `false`.
2745 ///
2746 /// # Examples
2747 ///
2748 /// Basic usage:
2749 ///
2750 /// ```
2751 /// let a = [1, 2, 3];
2752 ///
2753 /// assert!(a.iter().any(|&x| x > 0));
2754 ///
2755 /// assert!(!a.iter().any(|&x| x > 5));
2756 /// ```
2757 ///
2758 /// Stopping at the first `true`:
2759 ///
2760 /// ```
2761 /// let a = [1, 2, 3];
2762 ///
2763 /// let mut iter = a.iter();
2764 ///
2765 /// assert!(iter.any(|&x| x != 2));
2766 ///
2767 /// // we can still use `iter`, as there are more elements.
2768 /// assert_eq!(iter.next(), Some(&2));
2769 /// ```
2770 #[inline]
2771 #[stable(feature = "rust1", since = "1.0.0")]
2772 fn any<F>(&mut self, f: F) -> bool
2773 where
2774 Self: Sized,
2775 F: FnMut(Self::Item) -> bool,
2776 {
2777 #[inline]
2778 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2779 move |(), x| {
2780 if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
2781 }
2782 }
2783
2784 self.try_fold((), check(f)) == ControlFlow::Break(())
2785 }
2786
2787 /// Searches for an element of an iterator that satisfies a predicate.
2788 ///
2789 /// `find()` takes a closure that returns `true` or `false`. It applies
2790 /// this closure to each element of the iterator, and if any of them return
2791 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2792 /// `false`, it returns [`None`].
2793 ///
2794 /// `find()` is short-circuiting; in other words, it will stop processing
2795 /// as soon as the closure returns `true`.
2796 ///
2797 /// Because `find()` takes a reference, and many iterators iterate over
2798 /// references, this leads to a possibly confusing situation where the
2799 /// argument is a double reference. You can see this effect in the
2800 /// examples below, with `&&x`.
2801 ///
2802 /// If you need the index of the element, see [`position()`].
2803 ///
2804 /// [`Some(element)`]: Some
2805 /// [`position()`]: Iterator::position
2806 ///
2807 /// # Examples
2808 ///
2809 /// Basic usage:
2810 ///
2811 /// ```
2812 /// let a = [1, 2, 3];
2813 ///
2814 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2815 ///
2816 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2817 /// ```
2818 ///
2819 /// Stopping at the first `true`:
2820 ///
2821 /// ```
2822 /// let a = [1, 2, 3];
2823 ///
2824 /// let mut iter = a.iter();
2825 ///
2826 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2827 ///
2828 /// // we can still use `iter`, as there are more elements.
2829 /// assert_eq!(iter.next(), Some(&3));
2830 /// ```
2831 ///
2832 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2833 #[inline]
2834 #[stable(feature = "rust1", since = "1.0.0")]
2835 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2836 where
2837 Self: Sized,
2838 P: FnMut(&Self::Item) -> bool,
2839 {
2840 #[inline]
2841 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2842 move |(), x| {
2843 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
2844 }
2845 }
2846
2847 self.try_fold((), check(predicate)).break_value()
2848 }
2849
2850 /// Applies function to the elements of iterator and returns
2851 /// the first non-none result.
2852 ///
2853 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2854 ///
2855 /// # Examples
2856 ///
2857 /// ```
2858 /// let a = ["lol", "NaN", "2", "5"];
2859 ///
2860 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2861 ///
2862 /// assert_eq!(first_number, Some(2));
2863 /// ```
2864 #[inline]
2865 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2866 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2867 where
2868 Self: Sized,
2869 F: FnMut(Self::Item) -> Option<B>,
2870 {
2871 #[inline]
2872 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2873 move |(), x| match f(x) {
2874 Some(x) => ControlFlow::Break(x),
2875 None => ControlFlow::Continue(()),
2876 }
2877 }
2878
2879 self.try_fold((), check(f)).break_value()
2880 }
2881
2882 /// Applies function to the elements of iterator and returns
2883 /// the first true result or the first error.
2884 ///
2885 /// The return type of this method depends on the return type of the closure.
2886 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
2887 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2888 ///
2889 /// # Examples
2890 ///
2891 /// ```
2892 /// #![feature(try_find)]
2893 ///
2894 /// let a = ["1", "2", "lol", "NaN", "5"];
2895 ///
2896 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2897 /// Ok(s.parse::<i32>()? == search)
2898 /// };
2899 ///
2900 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2901 /// assert_eq!(result, Ok(Some(&"2")));
2902 ///
2903 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2904 /// assert!(result.is_err());
2905 /// ```
2906 ///
2907 /// This also supports other types which implement [`Try`], not just [`Result`].
2908 ///
2909 /// ```
2910 /// #![feature(try_find)]
2911 ///
2912 /// use std::num::NonZero;
2913 ///
2914 /// let a = [3, 5, 7, 4, 9, 0, 11u32];
2915 /// let result = a.iter().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2916 /// assert_eq!(result, Some(Some(&4)));
2917 /// let result = a.iter().take(3).try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2918 /// assert_eq!(result, Some(None));
2919 /// let result = a.iter().rev().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2920 /// assert_eq!(result, None);
2921 /// ```
2922 #[inline]
2923 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2924 fn try_find<R>(
2925 &mut self,
2926 f: impl FnMut(&Self::Item) -> R,
2927 ) -> ChangeOutputType<R, Option<Self::Item>>
2928 where
2929 Self: Sized,
2930 R: Try<Output = bool, Residual: Residual<Option<Self::Item>>>,
2931 {
2932 #[inline]
2933 fn check<I, V, R>(
2934 mut f: impl FnMut(&I) -> V,
2935 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2936 where
2937 V: Try<Output = bool, Residual = R>,
2938 R: Residual<Option<I>>,
2939 {
2940 move |(), x| match f(&x).branch() {
2941 ControlFlow::Continue(false) => ControlFlow::Continue(()),
2942 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2943 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2944 }
2945 }
2946
2947 match self.try_fold((), check(f)) {
2948 ControlFlow::Break(x) => x,
2949 ControlFlow::Continue(()) => Try::from_output(None),
2950 }
2951 }
2952
2953 /// Searches for an element in an iterator, returning its index.
2954 ///
2955 /// `position()` takes a closure that returns `true` or `false`. It applies
2956 /// this closure to each element of the iterator, and if one of them
2957 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2958 /// them return `false`, it returns [`None`].
2959 ///
2960 /// `position()` is short-circuiting; in other words, it will stop
2961 /// processing as soon as it finds a `true`.
2962 ///
2963 /// # Overflow Behavior
2964 ///
2965 /// The method does no guarding against overflows, so if there are more
2966 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2967 /// result or panics. If debug assertions are enabled, a panic is
2968 /// guaranteed.
2969 ///
2970 /// # Panics
2971 ///
2972 /// This function might panic if the iterator has more than `usize::MAX`
2973 /// non-matching elements.
2974 ///
2975 /// [`Some(index)`]: Some
2976 ///
2977 /// # Examples
2978 ///
2979 /// Basic usage:
2980 ///
2981 /// ```
2982 /// let a = [1, 2, 3];
2983 ///
2984 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2985 ///
2986 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2987 /// ```
2988 ///
2989 /// Stopping at the first `true`:
2990 ///
2991 /// ```
2992 /// let a = [1, 2, 3, 4];
2993 ///
2994 /// let mut iter = a.iter();
2995 ///
2996 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2997 ///
2998 /// // we can still use `iter`, as there are more elements.
2999 /// assert_eq!(iter.next(), Some(&3));
3000 ///
3001 /// // The returned index depends on iterator state
3002 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
3003 ///
3004 /// ```
3005 #[inline]
3006 #[stable(feature = "rust1", since = "1.0.0")]
3007 fn position<P>(&mut self, predicate: P) -> Option<usize>
3008 where
3009 Self: Sized,
3010 P: FnMut(Self::Item) -> bool,
3011 {
3012 #[inline]
3013 fn check<'a, T>(
3014 mut predicate: impl FnMut(T) -> bool + 'a,
3015 acc: &'a mut usize,
3016 ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
3017 #[rustc_inherit_overflow_checks]
3018 move |_, x| {
3019 if predicate(x) {
3020 ControlFlow::Break(*acc)
3021 } else {
3022 *acc += 1;
3023 ControlFlow::Continue(())
3024 }
3025 }
3026 }
3027
3028 let mut acc = 0;
3029 self.try_fold((), check(predicate, &mut acc)).break_value()
3030 }
3031
3032 /// Searches for an element in an iterator from the right, returning its
3033 /// index.
3034 ///
3035 /// `rposition()` takes a closure that returns `true` or `false`. It applies
3036 /// this closure to each element of the iterator, starting from the end,
3037 /// and if one of them returns `true`, then `rposition()` returns
3038 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
3039 ///
3040 /// `rposition()` is short-circuiting; in other words, it will stop
3041 /// processing as soon as it finds a `true`.
3042 ///
3043 /// [`Some(index)`]: Some
3044 ///
3045 /// # Examples
3046 ///
3047 /// Basic usage:
3048 ///
3049 /// ```
3050 /// let a = [1, 2, 3];
3051 ///
3052 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
3053 ///
3054 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
3055 /// ```
3056 ///
3057 /// Stopping at the first `true`:
3058 ///
3059 /// ```
3060 /// let a = [-1, 2, 3, 4];
3061 ///
3062 /// let mut iter = a.iter();
3063 ///
3064 /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
3065 ///
3066 /// // we can still use `iter`, as there are more elements.
3067 /// assert_eq!(iter.next(), Some(&-1));
3068 /// assert_eq!(iter.next_back(), Some(&3));
3069 /// ```
3070 #[inline]
3071 #[stable(feature = "rust1", since = "1.0.0")]
3072 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
3073 where
3074 P: FnMut(Self::Item) -> bool,
3075 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
3076 {
3077 // No need for an overflow check here, because `ExactSizeIterator`
3078 // implies that the number of elements fits into a `usize`.
3079 #[inline]
3080 fn check<T>(
3081 mut predicate: impl FnMut(T) -> bool,
3082 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
3083 move |i, x| {
3084 let i = i - 1;
3085 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
3086 }
3087 }
3088
3089 let n = self.len();
3090 self.try_rfold(n, check(predicate)).break_value()
3091 }
3092
3093 /// Returns the maximum element of an iterator.
3094 ///
3095 /// If several elements are equally maximum, the last element is
3096 /// returned. If the iterator is empty, [`None`] is returned.
3097 ///
3098 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3099 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3100 /// ```
3101 /// assert_eq!(
3102 /// [2.4, f32::NAN, 1.3]
3103 /// .into_iter()
3104 /// .reduce(f32::max)
3105 /// .unwrap_or(0.),
3106 /// 2.4
3107 /// );
3108 /// ```
3109 ///
3110 /// # Examples
3111 ///
3112 /// ```
3113 /// let a = [1, 2, 3];
3114 /// let b: Vec<u32> = Vec::new();
3115 ///
3116 /// assert_eq!(a.iter().max(), Some(&3));
3117 /// assert_eq!(b.iter().max(), None);
3118 /// ```
3119 #[inline]
3120 #[stable(feature = "rust1", since = "1.0.0")]
3121 fn max(self) -> Option<Self::Item>
3122 where
3123 Self: Sized,
3124 Self::Item: Ord,
3125 {
3126 self.max_by(Ord::cmp)
3127 }
3128
3129 /// Returns the minimum element of an iterator.
3130 ///
3131 /// If several elements are equally minimum, the first element is returned.
3132 /// If the iterator is empty, [`None`] is returned.
3133 ///
3134 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3135 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3136 /// ```
3137 /// assert_eq!(
3138 /// [2.4, f32::NAN, 1.3]
3139 /// .into_iter()
3140 /// .reduce(f32::min)
3141 /// .unwrap_or(0.),
3142 /// 1.3
3143 /// );
3144 /// ```
3145 ///
3146 /// # Examples
3147 ///
3148 /// ```
3149 /// let a = [1, 2, 3];
3150 /// let b: Vec<u32> = Vec::new();
3151 ///
3152 /// assert_eq!(a.iter().min(), Some(&1));
3153 /// assert_eq!(b.iter().min(), None);
3154 /// ```
3155 #[inline]
3156 #[stable(feature = "rust1", since = "1.0.0")]
3157 fn min(self) -> Option<Self::Item>
3158 where
3159 Self: Sized,
3160 Self::Item: Ord,
3161 {
3162 self.min_by(Ord::cmp)
3163 }
3164
3165 /// Returns the element that gives the maximum value from the
3166 /// specified function.
3167 ///
3168 /// If several elements are equally maximum, the last element is
3169 /// returned. If the iterator is empty, [`None`] is returned.
3170 ///
3171 /// # Examples
3172 ///
3173 /// ```
3174 /// let a = [-3_i32, 0, 1, 5, -10];
3175 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
3176 /// ```
3177 #[inline]
3178 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3179 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3180 where
3181 Self: Sized,
3182 F: FnMut(&Self::Item) -> B,
3183 {
3184 #[inline]
3185 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3186 move |x| (f(&x), x)
3187 }
3188
3189 #[inline]
3190 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3191 x_p.cmp(y_p)
3192 }
3193
3194 let (_, x) = self.map(key(f)).max_by(compare)?;
3195 Some(x)
3196 }
3197
3198 /// Returns the element that gives the maximum value with respect to the
3199 /// specified comparison function.
3200 ///
3201 /// If several elements are equally maximum, the last element is
3202 /// returned. If the iterator is empty, [`None`] is returned.
3203 ///
3204 /// # Examples
3205 ///
3206 /// ```
3207 /// let a = [-3_i32, 0, 1, 5, -10];
3208 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
3209 /// ```
3210 #[inline]
3211 #[stable(feature = "iter_max_by", since = "1.15.0")]
3212 fn max_by<F>(self, compare: F) -> Option<Self::Item>
3213 where
3214 Self: Sized,
3215 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3216 {
3217 #[inline]
3218 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3219 move |x, y| cmp::max_by(x, y, &mut compare)
3220 }
3221
3222 self.reduce(fold(compare))
3223 }
3224
3225 /// Returns the element that gives the minimum value from the
3226 /// specified function.
3227 ///
3228 /// If several elements are equally minimum, the first element is
3229 /// returned. If the iterator is empty, [`None`] is returned.
3230 ///
3231 /// # Examples
3232 ///
3233 /// ```
3234 /// let a = [-3_i32, 0, 1, 5, -10];
3235 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
3236 /// ```
3237 #[inline]
3238 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3239 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3240 where
3241 Self: Sized,
3242 F: FnMut(&Self::Item) -> B,
3243 {
3244 #[inline]
3245 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3246 move |x| (f(&x), x)
3247 }
3248
3249 #[inline]
3250 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3251 x_p.cmp(y_p)
3252 }
3253
3254 let (_, x) = self.map(key(f)).min_by(compare)?;
3255 Some(x)
3256 }
3257
3258 /// Returns the element that gives the minimum value with respect to the
3259 /// specified comparison function.
3260 ///
3261 /// If several elements are equally minimum, the first element is
3262 /// returned. If the iterator is empty, [`None`] is returned.
3263 ///
3264 /// # Examples
3265 ///
3266 /// ```
3267 /// let a = [-3_i32, 0, 1, 5, -10];
3268 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
3269 /// ```
3270 #[inline]
3271 #[stable(feature = "iter_min_by", since = "1.15.0")]
3272 fn min_by<F>(self, compare: F) -> Option<Self::Item>
3273 where
3274 Self: Sized,
3275 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3276 {
3277 #[inline]
3278 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3279 move |x, y| cmp::min_by(x, y, &mut compare)
3280 }
3281
3282 self.reduce(fold(compare))
3283 }
3284
3285 /// Reverses an iterator's direction.
3286 ///
3287 /// Usually, iterators iterate from left to right. After using `rev()`,
3288 /// an iterator will instead iterate from right to left.
3289 ///
3290 /// This is only possible if the iterator has an end, so `rev()` only
3291 /// works on [`DoubleEndedIterator`]s.
3292 ///
3293 /// # Examples
3294 ///
3295 /// ```
3296 /// let a = [1, 2, 3];
3297 ///
3298 /// let mut iter = a.iter().rev();
3299 ///
3300 /// assert_eq!(iter.next(), Some(&3));
3301 /// assert_eq!(iter.next(), Some(&2));
3302 /// assert_eq!(iter.next(), Some(&1));
3303 ///
3304 /// assert_eq!(iter.next(), None);
3305 /// ```
3306 #[inline]
3307 #[doc(alias = "reverse")]
3308 #[stable(feature = "rust1", since = "1.0.0")]
3309 fn rev(self) -> Rev<Self>
3310 where
3311 Self: Sized + DoubleEndedIterator,
3312 {
3313 Rev::new(self)
3314 }
3315
3316 /// Converts an iterator of pairs into a pair of containers.
3317 ///
3318 /// `unzip()` consumes an entire iterator of pairs, producing two
3319 /// collections: one from the left elements of the pairs, and one
3320 /// from the right elements.
3321 ///
3322 /// This function is, in some sense, the opposite of [`zip`].
3323 ///
3324 /// [`zip`]: Iterator::zip
3325 ///
3326 /// # Examples
3327 ///
3328 /// ```
3329 /// let a = [(1, 2), (3, 4), (5, 6)];
3330 ///
3331 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
3332 ///
3333 /// assert_eq!(left, [1, 3, 5]);
3334 /// assert_eq!(right, [2, 4, 6]);
3335 ///
3336 /// // you can also unzip multiple nested tuples at once
3337 /// let a = [(1, (2, 3)), (4, (5, 6))];
3338 ///
3339 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
3340 /// assert_eq!(x, [1, 4]);
3341 /// assert_eq!(y, [2, 5]);
3342 /// assert_eq!(z, [3, 6]);
3343 /// ```
3344 #[stable(feature = "rust1", since = "1.0.0")]
3345 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3346 where
3347 FromA: Default + Extend<A>,
3348 FromB: Default + Extend<B>,
3349 Self: Sized + Iterator<Item = (A, B)>,
3350 {
3351 let mut unzipped: (FromA, FromB) = Default::default();
3352 unzipped.extend(self);
3353 unzipped
3354 }
3355
3356 /// Creates an iterator which copies all of its elements.
3357 ///
3358 /// This is useful when you have an iterator over `&T`, but you need an
3359 /// iterator over `T`.
3360 ///
3361 /// # Examples
3362 ///
3363 /// ```
3364 /// let a = [1, 2, 3];
3365 ///
3366 /// let v_copied: Vec<_> = a.iter().copied().collect();
3367 ///
3368 /// // copied is the same as .map(|&x| x)
3369 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3370 ///
3371 /// assert_eq!(v_copied, vec![1, 2, 3]);
3372 /// assert_eq!(v_map, vec![1, 2, 3]);
3373 /// ```
3374 #[stable(feature = "iter_copied", since = "1.36.0")]
3375 #[rustc_diagnostic_item = "iter_copied"]
3376 fn copied<'a, T: 'a>(self) -> Copied<Self>
3377 where
3378 Self: Sized + Iterator<Item = &'a T>,
3379 T: Copy,
3380 {
3381 Copied::new(self)
3382 }
3383
3384 /// Creates an iterator which [`clone`]s all of its elements.
3385 ///
3386 /// This is useful when you have an iterator over `&T`, but you need an
3387 /// iterator over `T`.
3388 ///
3389 /// There is no guarantee whatsoever about the `clone` method actually
3390 /// being called *or* optimized away. So code should not depend on
3391 /// either.
3392 ///
3393 /// [`clone`]: Clone::clone
3394 ///
3395 /// # Examples
3396 ///
3397 /// Basic usage:
3398 ///
3399 /// ```
3400 /// let a = [1, 2, 3];
3401 ///
3402 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3403 ///
3404 /// // cloned is the same as .map(|&x| x), for integers
3405 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3406 ///
3407 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3408 /// assert_eq!(v_map, vec![1, 2, 3]);
3409 /// ```
3410 ///
3411 /// To get the best performance, try to clone late:
3412 ///
3413 /// ```
3414 /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
3415 /// // don't do this:
3416 /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
3417 /// assert_eq!(&[vec![23]], &slower[..]);
3418 /// // instead call `cloned` late
3419 /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
3420 /// assert_eq!(&[vec![23]], &faster[..]);
3421 /// ```
3422 #[stable(feature = "rust1", since = "1.0.0")]
3423 #[rustc_diagnostic_item = "iter_cloned"]
3424 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3425 where
3426 Self: Sized + Iterator<Item = &'a T>,
3427 T: Clone,
3428 {
3429 Cloned::new(self)
3430 }
3431
3432 /// Repeats an iterator endlessly.
3433 ///
3434 /// Instead of stopping at [`None`], the iterator will instead start again,
3435 /// from the beginning. After iterating again, it will start at the
3436 /// beginning again. And again. And again. Forever. Note that in case the
3437 /// original iterator is empty, the resulting iterator will also be empty.
3438 ///
3439 /// # Examples
3440 ///
3441 /// ```
3442 /// let a = [1, 2, 3];
3443 ///
3444 /// let mut it = a.iter().cycle();
3445 ///
3446 /// assert_eq!(it.next(), Some(&1));
3447 /// assert_eq!(it.next(), Some(&2));
3448 /// assert_eq!(it.next(), Some(&3));
3449 /// assert_eq!(it.next(), Some(&1));
3450 /// assert_eq!(it.next(), Some(&2));
3451 /// assert_eq!(it.next(), Some(&3));
3452 /// assert_eq!(it.next(), Some(&1));
3453 /// ```
3454 #[stable(feature = "rust1", since = "1.0.0")]
3455 #[inline]
3456 fn cycle(self) -> Cycle<Self>
3457 where
3458 Self: Sized + Clone,
3459 {
3460 Cycle::new(self)
3461 }
3462
3463 /// Returns an iterator over `N` elements of the iterator at a time.
3464 ///
3465 /// The chunks do not overlap. If `N` does not divide the length of the
3466 /// iterator, then the last up to `N-1` elements will be omitted and can be
3467 /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
3468 /// function of the iterator.
3469 ///
3470 /// # Panics
3471 ///
3472 /// Panics if `N` is zero.
3473 ///
3474 /// # Examples
3475 ///
3476 /// Basic usage:
3477 ///
3478 /// ```
3479 /// #![feature(iter_array_chunks)]
3480 ///
3481 /// let mut iter = "lorem".chars().array_chunks();
3482 /// assert_eq!(iter.next(), Some(['l', 'o']));
3483 /// assert_eq!(iter.next(), Some(['r', 'e']));
3484 /// assert_eq!(iter.next(), None);
3485 /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
3486 /// ```
3487 ///
3488 /// ```
3489 /// #![feature(iter_array_chunks)]
3490 ///
3491 /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
3492 /// // ^-----^ ^------^
3493 /// for [x, y, z] in data.iter().array_chunks() {
3494 /// assert_eq!(x + y + z, 4);
3495 /// }
3496 /// ```
3497 #[track_caller]
3498 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
3499 fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
3500 where
3501 Self: Sized,
3502 {
3503 ArrayChunks::new(self)
3504 }
3505
3506 /// Sums the elements of an iterator.
3507 ///
3508 /// Takes each element, adds them together, and returns the result.
3509 ///
3510 /// An empty iterator returns the *additive identity* ("zero") of the type,
3511 /// which is `0` for integers and `-0.0` for floats.
3512 ///
3513 /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
3514 /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
3515 ///
3516 /// # Panics
3517 ///
3518 /// When calling `sum()` and a primitive integer type is being returned, this
3519 /// method will panic if the computation overflows and debug assertions are
3520 /// enabled.
3521 ///
3522 /// # Examples
3523 ///
3524 /// ```
3525 /// let a = [1, 2, 3];
3526 /// let sum: i32 = a.iter().sum();
3527 ///
3528 /// assert_eq!(sum, 6);
3529 ///
3530 /// let b: Vec<f32> = vec![];
3531 /// let sum: f32 = b.iter().sum();
3532 /// assert_eq!(sum, -0.0_f32);
3533 /// ```
3534 #[stable(feature = "iter_arith", since = "1.11.0")]
3535 fn sum<S>(self) -> S
3536 where
3537 Self: Sized,
3538 S: Sum<Self::Item>,
3539 {
3540 Sum::sum(self)
3541 }
3542
3543 /// Iterates over the entire iterator, multiplying all the elements
3544 ///
3545 /// An empty iterator returns the one value of the type.
3546 ///
3547 /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
3548 /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
3549 ///
3550 /// # Panics
3551 ///
3552 /// When calling `product()` and a primitive integer type is being returned,
3553 /// method will panic if the computation overflows and debug assertions are
3554 /// enabled.
3555 ///
3556 /// # Examples
3557 ///
3558 /// ```
3559 /// fn factorial(n: u32) -> u32 {
3560 /// (1..=n).product()
3561 /// }
3562 /// assert_eq!(factorial(0), 1);
3563 /// assert_eq!(factorial(1), 1);
3564 /// assert_eq!(factorial(5), 120);
3565 /// ```
3566 #[stable(feature = "iter_arith", since = "1.11.0")]
3567 fn product<P>(self) -> P
3568 where
3569 Self: Sized,
3570 P: Product<Self::Item>,
3571 {
3572 Product::product(self)
3573 }
3574
3575 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3576 /// of another.
3577 ///
3578 /// # Examples
3579 ///
3580 /// ```
3581 /// use std::cmp::Ordering;
3582 ///
3583 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3584 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3585 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3586 /// ```
3587 #[stable(feature = "iter_order", since = "1.5.0")]
3588 fn cmp<I>(self, other: I) -> Ordering
3589 where
3590 I: IntoIterator<Item = Self::Item>,
3591 Self::Item: Ord,
3592 Self: Sized,
3593 {
3594 self.cmp_by(other, |x, y| x.cmp(&y))
3595 }
3596
3597 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3598 /// of another with respect to the specified comparison function.
3599 ///
3600 /// # Examples
3601 ///
3602 /// ```
3603 /// #![feature(iter_order_by)]
3604 ///
3605 /// use std::cmp::Ordering;
3606 ///
3607 /// let xs = [1, 2, 3, 4];
3608 /// let ys = [1, 4, 9, 16];
3609 ///
3610 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3611 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3612 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3613 /// ```
3614 #[unstable(feature = "iter_order_by", issue = "64295")]
3615 fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
3616 where
3617 Self: Sized,
3618 I: IntoIterator,
3619 F: FnMut(Self::Item, I::Item) -> Ordering,
3620 {
3621 #[inline]
3622 fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
3623 where
3624 F: FnMut(X, Y) -> Ordering,
3625 {
3626 move |x, y| match cmp(x, y) {
3627 Ordering::Equal => ControlFlow::Continue(()),
3628 non_eq => ControlFlow::Break(non_eq),
3629 }
3630 }
3631
3632 match iter_compare(self, other.into_iter(), compare(cmp)) {
3633 ControlFlow::Continue(ord) => ord,
3634 ControlFlow::Break(ord) => ord,
3635 }
3636 }
3637
3638 /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
3639 /// this [`Iterator`] with those of another. The comparison works like short-circuit
3640 /// evaluation, returning a result without comparing the remaining elements.
3641 /// As soon as an order can be determined, the evaluation stops and a result is returned.
3642 ///
3643 /// # Examples
3644 ///
3645 /// ```
3646 /// use std::cmp::Ordering;
3647 ///
3648 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3649 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3650 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3651 /// ```
3652 ///
3653 /// For floating-point numbers, NaN does not have a total order and will result
3654 /// in `None` when compared:
3655 ///
3656 /// ```
3657 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3658 /// ```
3659 ///
3660 /// The results are determined by the order of evaluation.
3661 ///
3662 /// ```
3663 /// use std::cmp::Ordering;
3664 ///
3665 /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
3666 /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
3667 /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
3668 /// ```
3669 ///
3670 #[stable(feature = "iter_order", since = "1.5.0")]
3671 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3672 where
3673 I: IntoIterator,
3674 Self::Item: PartialOrd<I::Item>,
3675 Self: Sized,
3676 {
3677 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3678 }
3679
3680 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3681 /// of another with respect to the specified comparison function.
3682 ///
3683 /// # Examples
3684 ///
3685 /// ```
3686 /// #![feature(iter_order_by)]
3687 ///
3688 /// use std::cmp::Ordering;
3689 ///
3690 /// let xs = [1.0, 2.0, 3.0, 4.0];
3691 /// let ys = [1.0, 4.0, 9.0, 16.0];
3692 ///
3693 /// assert_eq!(
3694 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3695 /// Some(Ordering::Less)
3696 /// );
3697 /// assert_eq!(
3698 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3699 /// Some(Ordering::Equal)
3700 /// );
3701 /// assert_eq!(
3702 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3703 /// Some(Ordering::Greater)
3704 /// );
3705 /// ```
3706 #[unstable(feature = "iter_order_by", issue = "64295")]
3707 fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
3708 where
3709 Self: Sized,
3710 I: IntoIterator,
3711 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3712 {
3713 #[inline]
3714 fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
3715 where
3716 F: FnMut(X, Y) -> Option<Ordering>,
3717 {
3718 move |x, y| match partial_cmp(x, y) {
3719 Some(Ordering::Equal) => ControlFlow::Continue(()),
3720 non_eq => ControlFlow::Break(non_eq),
3721 }
3722 }
3723
3724 match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
3725 ControlFlow::Continue(ord) => Some(ord),
3726 ControlFlow::Break(ord) => ord,
3727 }
3728 }
3729
3730 /// Determines if the elements of this [`Iterator`] are equal to those of
3731 /// another.
3732 ///
3733 /// # Examples
3734 ///
3735 /// ```
3736 /// assert_eq!([1].iter().eq([1].iter()), true);
3737 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3738 /// ```
3739 #[stable(feature = "iter_order", since = "1.5.0")]
3740 fn eq<I>(self, other: I) -> bool
3741 where
3742 I: IntoIterator,
3743 Self::Item: PartialEq<I::Item>,
3744 Self: Sized,
3745 {
3746 self.eq_by(other, |x, y| x == y)
3747 }
3748
3749 /// Determines if the elements of this [`Iterator`] are equal to those of
3750 /// another with respect to the specified equality function.
3751 ///
3752 /// # Examples
3753 ///
3754 /// ```
3755 /// #![feature(iter_order_by)]
3756 ///
3757 /// let xs = [1, 2, 3, 4];
3758 /// let ys = [1, 4, 9, 16];
3759 ///
3760 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3761 /// ```
3762 #[unstable(feature = "iter_order_by", issue = "64295")]
3763 fn eq_by<I, F>(self, other: I, eq: F) -> bool
3764 where
3765 Self: Sized,
3766 I: IntoIterator,
3767 F: FnMut(Self::Item, I::Item) -> bool,
3768 {
3769 #[inline]
3770 fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
3771 where
3772 F: FnMut(X, Y) -> bool,
3773 {
3774 move |x, y| {
3775 if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
3776 }
3777 }
3778
3779 match iter_compare(self, other.into_iter(), compare(eq)) {
3780 ControlFlow::Continue(ord) => ord == Ordering::Equal,
3781 ControlFlow::Break(()) => false,
3782 }
3783 }
3784
3785 /// Determines if the elements of this [`Iterator`] are not equal to those of
3786 /// another.
3787 ///
3788 /// # Examples
3789 ///
3790 /// ```
3791 /// assert_eq!([1].iter().ne([1].iter()), false);
3792 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3793 /// ```
3794 #[stable(feature = "iter_order", since = "1.5.0")]
3795 fn ne<I>(self, other: I) -> bool
3796 where
3797 I: IntoIterator,
3798 Self::Item: PartialEq<I::Item>,
3799 Self: Sized,
3800 {
3801 !self.eq(other)
3802 }
3803
3804 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3805 /// less than those of another.
3806 ///
3807 /// # Examples
3808 ///
3809 /// ```
3810 /// assert_eq!([1].iter().lt([1].iter()), false);
3811 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3812 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3813 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3814 /// ```
3815 #[stable(feature = "iter_order", since = "1.5.0")]
3816 fn lt<I>(self, other: I) -> bool
3817 where
3818 I: IntoIterator,
3819 Self::Item: PartialOrd<I::Item>,
3820 Self: Sized,
3821 {
3822 self.partial_cmp(other) == Some(Ordering::Less)
3823 }
3824
3825 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3826 /// less or equal to those of another.
3827 ///
3828 /// # Examples
3829 ///
3830 /// ```
3831 /// assert_eq!([1].iter().le([1].iter()), true);
3832 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3833 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3834 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3835 /// ```
3836 #[stable(feature = "iter_order", since = "1.5.0")]
3837 fn le<I>(self, other: I) -> bool
3838 where
3839 I: IntoIterator,
3840 Self::Item: PartialOrd<I::Item>,
3841 Self: Sized,
3842 {
3843 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3844 }
3845
3846 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3847 /// greater than those of another.
3848 ///
3849 /// # Examples
3850 ///
3851 /// ```
3852 /// assert_eq!([1].iter().gt([1].iter()), false);
3853 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3854 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3855 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3856 /// ```
3857 #[stable(feature = "iter_order", since = "1.5.0")]
3858 fn gt<I>(self, other: I) -> bool
3859 where
3860 I: IntoIterator,
3861 Self::Item: PartialOrd<I::Item>,
3862 Self: Sized,
3863 {
3864 self.partial_cmp(other) == Some(Ordering::Greater)
3865 }
3866
3867 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3868 /// greater than or equal to those of another.
3869 ///
3870 /// # Examples
3871 ///
3872 /// ```
3873 /// assert_eq!([1].iter().ge([1].iter()), true);
3874 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3875 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3876 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3877 /// ```
3878 #[stable(feature = "iter_order", since = "1.5.0")]
3879 fn ge<I>(self, other: I) -> bool
3880 where
3881 I: IntoIterator,
3882 Self::Item: PartialOrd<I::Item>,
3883 Self: Sized,
3884 {
3885 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3886 }
3887
3888 /// Checks if the elements of this iterator are sorted.
3889 ///
3890 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3891 /// iterator yields exactly zero or one element, `true` is returned.
3892 ///
3893 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3894 /// implies that this function returns `false` if any two consecutive items are not
3895 /// comparable.
3896 ///
3897 /// # Examples
3898 ///
3899 /// ```
3900 /// assert!([1, 2, 2, 9].iter().is_sorted());
3901 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3902 /// assert!([0].iter().is_sorted());
3903 /// assert!(std::iter::empty::<i32>().is_sorted());
3904 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3905 /// ```
3906 #[inline]
3907 #[stable(feature = "is_sorted", since = "1.82.0")]
3908 fn is_sorted(self) -> bool
3909 where
3910 Self: Sized,
3911 Self::Item: PartialOrd,
3912 {
3913 self.is_sorted_by(|a, b| a <= b)
3914 }
3915
3916 /// Checks if the elements of this iterator are sorted using the given comparator function.
3917 ///
3918 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3919 /// function to determine whether two elements are to be considered in sorted order.
3920 ///
3921 /// # Examples
3922 ///
3923 /// ```
3924 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
3925 /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
3926 ///
3927 /// assert!([0].iter().is_sorted_by(|a, b| true));
3928 /// assert!([0].iter().is_sorted_by(|a, b| false));
3929 ///
3930 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
3931 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
3932 /// ```
3933 #[stable(feature = "is_sorted", since = "1.82.0")]
3934 fn is_sorted_by<F>(mut self, compare: F) -> bool
3935 where
3936 Self: Sized,
3937 F: FnMut(&Self::Item, &Self::Item) -> bool,
3938 {
3939 #[inline]
3940 fn check<'a, T>(
3941 last: &'a mut T,
3942 mut compare: impl FnMut(&T, &T) -> bool + 'a,
3943 ) -> impl FnMut(T) -> bool + 'a {
3944 move |curr| {
3945 if !compare(&last, &curr) {
3946 return false;
3947 }
3948 *last = curr;
3949 true
3950 }
3951 }
3952
3953 let mut last = match self.next() {
3954 Some(e) => e,
3955 None => return true,
3956 };
3957
3958 self.all(check(&mut last, compare))
3959 }
3960
3961 /// Checks if the elements of this iterator are sorted using the given key extraction
3962 /// function.
3963 ///
3964 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3965 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3966 /// its documentation for more information.
3967 ///
3968 /// [`is_sorted`]: Iterator::is_sorted
3969 ///
3970 /// # Examples
3971 ///
3972 /// ```
3973 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3974 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3975 /// ```
3976 #[inline]
3977 #[stable(feature = "is_sorted", since = "1.82.0")]
3978 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3979 where
3980 Self: Sized,
3981 F: FnMut(Self::Item) -> K,
3982 K: PartialOrd,
3983 {
3984 self.map(f).is_sorted()
3985 }
3986
3987 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3988 // The unusual name is to avoid name collisions in method resolution
3989 // see #76479.
3990 #[inline]
3991 #[doc(hidden)]
3992 #[unstable(feature = "trusted_random_access", issue = "none")]
3993 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3994 where
3995 Self: TrustedRandomAccessNoCoerce,
3996 {
3997 unreachable!("Always specialized");
3998 }
3999}
4000
4001/// Compares two iterators element-wise using the given function.
4002///
4003/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
4004/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
4005/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
4006/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
4007/// the iterators.
4008///
4009/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
4010/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
4011#[inline]
4012fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
4013where
4014 A: Iterator,
4015 B: Iterator,
4016 F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
4017{
4018 #[inline]
4019 fn compare<'a, B, X, T>(
4020 b: &'a mut B,
4021 mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
4022 ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
4023 where
4024 B: Iterator,
4025 {
4026 move |x: X| match b.next() {
4027 None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
4028 Some(y: ::Item) => f(x, y).map_break(ControlFlow::Break),
4029 }
4030 }
4031
4032 match a.try_for_each(compare(&mut b, f)) {
4033 ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
4034 None => Ordering::Equal,
4035 Some(_) => Ordering::Less,
4036 }),
4037 ControlFlow::Break(x: ControlFlow) => x,
4038 }
4039}
4040
4041/// Implements `Iterator` for mutable references to iterators, such as those produced by [`Iterator::by_ref`].
4042///
4043/// This implementation passes all method calls on to the original iterator.
4044#[stable(feature = "rust1", since = "1.0.0")]
4045impl<I: Iterator + ?Sized> Iterator for &mut I {
4046 type Item = I::Item;
4047 #[inline]
4048 fn next(&mut self) -> Option<I::Item> {
4049 (**self).next()
4050 }
4051 fn size_hint(&self) -> (usize, Option<usize>) {
4052 (**self).size_hint()
4053 }
4054 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
4055 (**self).advance_by(n)
4056 }
4057 fn nth(&mut self, n: usize) -> Option<Self::Item> {
4058 (**self).nth(n)
4059 }
4060 fn fold<B, F>(self, init: B, f: F) -> B
4061 where
4062 F: FnMut(B, Self::Item) -> B,
4063 {
4064 self.spec_fold(init, f)
4065 }
4066 fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4067 where
4068 F: FnMut(B, Self::Item) -> R,
4069 R: Try<Output = B>,
4070 {
4071 self.spec_try_fold(init, f)
4072 }
4073}
4074
4075/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
4076trait IteratorRefSpec: Iterator {
4077 fn spec_fold<B, F>(self, init: B, f: F) -> B
4078 where
4079 F: FnMut(B, Self::Item) -> B;
4080
4081 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4082 where
4083 F: FnMut(B, Self::Item) -> R,
4084 R: Try<Output = B>;
4085}
4086
4087impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
4088 default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
4089 where
4090 F: FnMut(B, Self::Item) -> B,
4091 {
4092 let mut accum: B = init;
4093 while let Some(x: ::Item) = self.next() {
4094 accum = f(accum, x);
4095 }
4096 accum
4097 }
4098
4099 default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
4100 where
4101 F: FnMut(B, Self::Item) -> R,
4102 R: Try<Output = B>,
4103 {
4104 let mut accum: B = init;
4105 while let Some(x: ::Item) = self.next() {
4106 accum = f(accum, x)?;
4107 }
4108 try { accum }
4109 }
4110}
4111
4112impl<I: Iterator> IteratorRefSpec for &mut I {
4113 impl_fold_via_try_fold! { spec_fold -> spec_try_fold }
4114
4115 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4116 where
4117 F: FnMut(B, Self::Item) -> R,
4118 R: Try<Output = B>,
4119 {
4120 (**self).try_fold(init, f)
4121 }
4122}
4123