2 use crate::cmp::{self, Ordering};
3 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
5 use super::super::try_process;
6 use super::super::ByRefSized;
7 use super::super::TrustedRandomAccessNoCoerce;
8 use super::super::{ArrayChunks, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
9 use super::super::{FlatMap, Flatten};
10 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
12 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
15 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
17 /// A trait for dealing with iterators.
19 /// This is the main iterator trait. For more about the concept of iterators
20 /// generally, please see the [module-level documentation]. In particular, you
21 /// may want to know how to [implement `Iterator`][impl].
23 /// [module-level documentation]: crate::iter
24 /// [impl]: crate::iter#implementing-iterator
25 #[stable(feature = "rust1", since = "1.0.0")]
26 #[rustc_on_unimplemented(
28 _Self = "std::ops::RangeTo<Idx>",
29 label = "if you meant to iterate until a value, add a starting value",
30 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
31 bounded `Range`: `0..end`"
34 _Self = "std::ops::RangeToInclusive<Idx>",
35 label = "if you meant to iterate until a value (including it), add a starting value",
36 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
37 to have a bounded `RangeInclusive`: `0..=end`"
41 label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
43 on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
45 _Self = "std::vec::Vec<T, A>",
46 label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
50 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
53 _Self = "std::string::String",
54 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
58 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
59 syntax `start..end` or the inclusive range syntax `start..=end`"
61 label = "`{Self}` is not an iterator",
62 message = "`{Self}` is not an iterator"
65 #[rustc_diagnostic_item = "Iterator"]
66 #[must_use = "iterators are lazy and do nothing unless consumed"]
68 /// The type of the elements being iterated over.
69 #[rustc_diagnostic_item = "IteratorItem"]
70 #[stable(feature = "rust1", since = "1.0.0")]
73 /// Advances the iterator and returns the next value.
75 /// Returns [`None`] when iteration is finished. Individual iterator
76 /// implementations may choose to resume iteration, and so calling `next()`
77 /// again may or may not eventually start returning [`Some(Item)`] again at some
80 /// [`Some(Item)`]: Some
87 /// let a = [1, 2, 3];
89 /// let mut iter = a.iter();
91 /// // A call to next() returns the next value...
92 /// assert_eq!(Some(&1), iter.next());
93 /// assert_eq!(Some(&2), iter.next());
94 /// assert_eq!(Some(&3), iter.next());
96 /// // ... and then None once it's over.
97 /// assert_eq!(None, iter.next());
99 /// // More calls may or may not return `None`. Here, they always will.
100 /// assert_eq!(None, iter.next());
101 /// assert_eq!(None, iter.next());
104 #[stable(feature = "rust1", since = "1.0.0")]
105 fn next(&mut self) -> Option<Self::Item>;
107 /// Advances the iterator and returns an array containing the next `N` values.
109 /// If there are not enough elements to fill the array then `Err` is returned
110 /// containing an iterator over the remaining elements.
117 /// #![feature(iter_next_chunk)]
119 /// let mut iter = "lorem".chars();
121 /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2
122 /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3
123 /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
126 /// Split a string and get the first three items.
129 /// #![feature(iter_next_chunk)]
131 /// let quote = "not all those who wander are lost";
132 /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
133 /// assert_eq!(first, "not");
134 /// assert_eq!(second, "all");
135 /// assert_eq!(third, "those");
138 #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
139 fn next_chunk<const N: usize>(
141 ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
145 array::iter_next_chunk(self)
148 /// Returns the bounds on the remaining length of the iterator.
150 /// Specifically, `size_hint()` returns a tuple where the first element
151 /// is the lower bound, and the second element is the upper bound.
153 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
154 /// A [`None`] here means that either there is no known upper bound, or the
155 /// upper bound is larger than [`usize`].
157 /// # Implementation notes
159 /// It is not enforced that an iterator implementation yields the declared
160 /// number of elements. A buggy iterator may yield less than the lower bound
161 /// or more than the upper bound of elements.
163 /// `size_hint()` is primarily intended to be used for optimizations such as
164 /// reserving space for the elements of the iterator, but must not be
165 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
166 /// implementation of `size_hint()` should not lead to memory safety
169 /// That said, the implementation should provide a correct estimation,
170 /// because otherwise it would be a violation of the trait's protocol.
172 /// The default implementation returns <code>(0, [None])</code> which is correct for any
180 /// let a = [1, 2, 3];
181 /// let mut iter = a.iter();
183 /// assert_eq!((3, Some(3)), iter.size_hint());
184 /// let _ = iter.next();
185 /// assert_eq!((2, Some(2)), iter.size_hint());
188 /// A more complex example:
191 /// // The even numbers in the range of zero to nine.
192 /// let iter = (0..10).filter(|x| x % 2 == 0);
194 /// // We might iterate from zero to ten times. Knowing that it's five
195 /// // exactly wouldn't be possible without executing filter().
196 /// assert_eq!((0, Some(10)), iter.size_hint());
198 /// // Let's add five more numbers with chain()
199 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
201 /// // now both bounds are increased by five
202 /// assert_eq!((5, Some(15)), iter.size_hint());
205 /// Returning `None` for an upper bound:
208 /// // an infinite iterator has no upper bound
209 /// // and the maximum possible lower bound
212 /// assert_eq!((usize::MAX, None), iter.size_hint());
215 #[stable(feature = "rust1", since = "1.0.0")]
216 fn size_hint(&self) -> (usize, Option<usize>) {
220 /// Consumes the iterator, counting the number of iterations and returning it.
222 /// This method will call [`next`] repeatedly until [`None`] is encountered,
223 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
224 /// called at least once even if the iterator does not have any elements.
226 /// [`next`]: Iterator::next
228 /// # Overflow Behavior
230 /// The method does no guarding against overflows, so counting elements of
231 /// an iterator with more than [`usize::MAX`] elements either produces the
232 /// wrong result or panics. If debug assertions are enabled, a panic is
237 /// This function might panic if the iterator has more than [`usize::MAX`]
245 /// let a = [1, 2, 3];
246 /// assert_eq!(a.iter().count(), 3);
248 /// let a = [1, 2, 3, 4, 5];
249 /// assert_eq!(a.iter().count(), 5);
252 #[stable(feature = "rust1", since = "1.0.0")]
253 fn count(self) -> usize
259 #[rustc_inherit_overflow_checks]
260 |count, _| count + 1,
264 /// Consumes the iterator, returning the last element.
266 /// This method will evaluate the iterator until it returns [`None`]. While
267 /// doing so, it keeps track of the current element. After [`None`] is
268 /// returned, `last()` will then return the last element it saw.
275 /// let a = [1, 2, 3];
276 /// assert_eq!(a.iter().last(), Some(&3));
278 /// let a = [1, 2, 3, 4, 5];
279 /// assert_eq!(a.iter().last(), Some(&5));
282 #[stable(feature = "rust1", since = "1.0.0")]
283 fn last(self) -> Option<Self::Item>
288 fn some<T>(_: Option<T>, x: T) -> Option<T> {
292 self.fold(None, some)
295 /// Advances the iterator by `n` elements.
297 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
298 /// times until [`None`] is encountered.
300 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
301 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
302 /// of elements the iterator is advanced by before running out of elements (i.e. the
303 /// length of the iterator). Note that `k` is always less than `n`.
305 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
306 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
307 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
309 /// [`Flatten`]: crate::iter::Flatten
310 /// [`next`]: Iterator::next
317 /// #![feature(iter_advance_by)]
319 /// let a = [1, 2, 3, 4];
320 /// let mut iter = a.iter();
322 /// assert_eq!(iter.advance_by(2), Ok(()));
323 /// assert_eq!(iter.next(), Some(&3));
324 /// assert_eq!(iter.advance_by(0), Ok(()));
325 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
328 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
329 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
331 self.next().ok_or(i)?;
336 /// Returns the `n`th element of the iterator.
338 /// Like most indexing operations, the count starts from zero, so `nth(0)`
339 /// returns the first value, `nth(1)` the second, and so on.
341 /// Note that all preceding elements, as well as the returned element, will be
342 /// consumed from the iterator. That means that the preceding elements will be
343 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
344 /// will return different elements.
346 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
354 /// let a = [1, 2, 3];
355 /// assert_eq!(a.iter().nth(1), Some(&2));
358 /// Calling `nth()` multiple times doesn't rewind the iterator:
361 /// let a = [1, 2, 3];
363 /// let mut iter = a.iter();
365 /// assert_eq!(iter.nth(1), Some(&2));
366 /// assert_eq!(iter.nth(1), None);
369 /// Returning `None` if there are less than `n + 1` elements:
372 /// let a = [1, 2, 3];
373 /// assert_eq!(a.iter().nth(10), None);
376 #[stable(feature = "rust1", since = "1.0.0")]
377 fn nth(&mut self, n: usize) -> Option<Self::Item> {
378 self.advance_by(n).ok()?;
382 /// Creates an iterator starting at the same point, but stepping by
383 /// the given amount at each iteration.
385 /// Note 1: The first element of the iterator will always be returned,
386 /// regardless of the step given.
388 /// Note 2: The time at which ignored elements are pulled is not fixed.
389 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
390 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
391 /// `advance_n_and_return_first(&mut self, step)`,
392 /// `advance_n_and_return_first(&mut self, step)`, …
393 /// Which way is used may change for some iterators for performance reasons.
394 /// The second way will advance the iterator earlier and may consume more items.
396 /// `advance_n_and_return_first` is the equivalent of:
398 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
402 /// let next = iter.next();
412 /// The method will panic if the given step is `0`.
419 /// let a = [0, 1, 2, 3, 4, 5];
420 /// let mut iter = a.iter().step_by(2);
422 /// assert_eq!(iter.next(), Some(&0));
423 /// assert_eq!(iter.next(), Some(&2));
424 /// assert_eq!(iter.next(), Some(&4));
425 /// assert_eq!(iter.next(), None);
428 #[stable(feature = "iterator_step_by", since = "1.28.0")]
429 fn step_by(self, step: usize) -> StepBy<Self>
433 StepBy::new(self, step)
436 /// Takes two iterators and creates a new iterator over both in sequence.
438 /// `chain()` will return a new iterator which will first iterate over
439 /// values from the first iterator and then over values from the second
442 /// In other words, it links two iterators together, in a chain. 🔗
444 /// [`once`] is commonly used to adapt a single value into a chain of
445 /// other kinds of iteration.
452 /// let a1 = [1, 2, 3];
453 /// let a2 = [4, 5, 6];
455 /// let mut iter = a1.iter().chain(a2.iter());
457 /// assert_eq!(iter.next(), Some(&1));
458 /// assert_eq!(iter.next(), Some(&2));
459 /// assert_eq!(iter.next(), Some(&3));
460 /// assert_eq!(iter.next(), Some(&4));
461 /// assert_eq!(iter.next(), Some(&5));
462 /// assert_eq!(iter.next(), Some(&6));
463 /// assert_eq!(iter.next(), None);
466 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
467 /// anything that can be converted into an [`Iterator`], not just an
468 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
469 /// [`IntoIterator`], and so can be passed to `chain()` directly:
472 /// let s1 = &[1, 2, 3];
473 /// let s2 = &[4, 5, 6];
475 /// let mut iter = s1.iter().chain(s2);
477 /// assert_eq!(iter.next(), Some(&1));
478 /// assert_eq!(iter.next(), Some(&2));
479 /// assert_eq!(iter.next(), Some(&3));
480 /// assert_eq!(iter.next(), Some(&4));
481 /// assert_eq!(iter.next(), Some(&5));
482 /// assert_eq!(iter.next(), Some(&6));
483 /// assert_eq!(iter.next(), None);
486 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
490 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
491 /// use std::os::windows::ffi::OsStrExt;
492 /// s.encode_wide().chain(std::iter::once(0)).collect()
496 /// [`once`]: crate::iter::once
497 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
499 #[stable(feature = "rust1", since = "1.0.0")]
500 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
503 U: IntoIterator<Item = Self::Item>,
505 Chain::new(self, other.into_iter())
508 /// 'Zips up' two iterators into a single iterator of pairs.
510 /// `zip()` returns a new iterator that will iterate over two other
511 /// iterators, returning a tuple where the first element comes from the
512 /// first iterator, and the second element comes from the second iterator.
514 /// In other words, it zips two iterators together, into a single one.
516 /// If either iterator returns [`None`], [`next`] from the zipped iterator
517 /// will return [`None`].
518 /// If the zipped iterator has no more elements to return then each further attempt to advance
519 /// it will first try to advance the first iterator at most one time and if it still yielded an item
520 /// try to advance the second iterator at most one time.
522 /// To 'undo' the result of zipping up two iterators, see [`unzip`].
524 /// [`unzip`]: Iterator::unzip
531 /// let a1 = [1, 2, 3];
532 /// let a2 = [4, 5, 6];
534 /// let mut iter = a1.iter().zip(a2.iter());
536 /// assert_eq!(iter.next(), Some((&1, &4)));
537 /// assert_eq!(iter.next(), Some((&2, &5)));
538 /// assert_eq!(iter.next(), Some((&3, &6)));
539 /// assert_eq!(iter.next(), None);
542 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
543 /// anything that can be converted into an [`Iterator`], not just an
544 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
545 /// [`IntoIterator`], and so can be passed to `zip()` directly:
548 /// let s1 = &[1, 2, 3];
549 /// let s2 = &[4, 5, 6];
551 /// let mut iter = s1.iter().zip(s2);
553 /// assert_eq!(iter.next(), Some((&1, &4)));
554 /// assert_eq!(iter.next(), Some((&2, &5)));
555 /// assert_eq!(iter.next(), Some((&3, &6)));
556 /// assert_eq!(iter.next(), None);
559 /// `zip()` is often used to zip an infinite iterator to a finite one.
560 /// This works because the finite iterator will eventually return [`None`],
561 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
564 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
566 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
568 /// assert_eq!((0, 'f'), enumerate[0]);
569 /// assert_eq!((0, 'f'), zipper[0]);
571 /// assert_eq!((1, 'o'), enumerate[1]);
572 /// assert_eq!((1, 'o'), zipper[1]);
574 /// assert_eq!((2, 'o'), enumerate[2]);
575 /// assert_eq!((2, 'o'), zipper[2]);
578 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
581 /// use std::iter::zip;
583 /// let a = [1, 2, 3];
584 /// let b = [2, 3, 4];
586 /// let mut zipped = zip(
587 /// a.into_iter().map(|x| x * 2).skip(1),
588 /// b.into_iter().map(|x| x * 2).skip(1),
591 /// assert_eq!(zipped.next(), Some((4, 6)));
592 /// assert_eq!(zipped.next(), Some((6, 8)));
593 /// assert_eq!(zipped.next(), None);
599 /// # let a = [1, 2, 3];
600 /// # let b = [2, 3, 4];
602 /// let mut zipped = a
606 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
608 /// # assert_eq!(zipped.next(), Some((4, 6)));
609 /// # assert_eq!(zipped.next(), Some((6, 8)));
610 /// # assert_eq!(zipped.next(), None);
613 /// [`enumerate`]: Iterator::enumerate
614 /// [`next`]: Iterator::next
615 /// [`zip`]: crate::iter::zip
617 #[stable(feature = "rust1", since = "1.0.0")]
618 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
623 Zip::new(self, other.into_iter())
626 /// Creates a new iterator which places a copy of `separator` between adjacent
627 /// items of the original iterator.
629 /// In case `separator` does not implement [`Clone`] or needs to be
630 /// computed every time, use [`intersperse_with`].
637 /// #![feature(iter_intersperse)]
639 /// let mut a = [0, 1, 2].iter().intersperse(&100);
640 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
641 /// assert_eq!(a.next(), Some(&100)); // The separator.
642 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
643 /// assert_eq!(a.next(), Some(&100)); // The separator.
644 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
645 /// assert_eq!(a.next(), None); // The iterator is finished.
648 /// `intersperse` can be very useful to join an iterator's items using a common element:
650 /// #![feature(iter_intersperse)]
652 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
653 /// assert_eq!(hello, "Hello World !");
656 /// [`Clone`]: crate::clone::Clone
657 /// [`intersperse_with`]: Iterator::intersperse_with
659 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
660 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
665 Intersperse::new(self, separator)
668 /// Creates a new iterator which places an item generated by `separator`
669 /// between adjacent items of the original iterator.
671 /// The closure will be called exactly once each time an item is placed
672 /// between two adjacent items from the underlying iterator; specifically,
673 /// the closure is not called if the underlying iterator yields less than
674 /// two items and after the last item is yielded.
676 /// If the iterator's item implements [`Clone`], it may be easier to use
684 /// #![feature(iter_intersperse)]
686 /// #[derive(PartialEq, Debug)]
687 /// struct NotClone(usize);
689 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
690 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
692 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
693 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
694 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
695 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
696 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from `v`.
697 /// assert_eq!(it.next(), None); // The iterator is finished.
700 /// `intersperse_with` can be used in situations where the separator needs
703 /// #![feature(iter_intersperse)]
705 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
707 /// // The closure mutably borrows its context to generate an item.
708 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
709 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
711 /// let result = src.intersperse_with(separator).collect::<String>();
712 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
714 /// [`Clone`]: crate::clone::Clone
715 /// [`intersperse`]: Iterator::intersperse
717 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
718 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
721 G: FnMut() -> Self::Item,
723 IntersperseWith::new(self, separator)
726 /// Takes a closure and creates an iterator which calls that closure on each
729 /// `map()` transforms one iterator into another, by means of its argument:
730 /// something that implements [`FnMut`]. It produces a new iterator which
731 /// calls this closure on each element of the original iterator.
733 /// If you are good at thinking in types, you can think of `map()` like this:
734 /// If you have an iterator that gives you elements of some type `A`, and
735 /// you want an iterator of some other type `B`, you can use `map()`,
736 /// passing a closure that takes an `A` and returns a `B`.
738 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
739 /// lazy, it is best used when you're already working with other iterators.
740 /// If you're doing some sort of looping for a side effect, it's considered
741 /// more idiomatic to use [`for`] than `map()`.
743 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
744 /// [`FnMut`]: crate::ops::FnMut
751 /// let a = [1, 2, 3];
753 /// let mut iter = a.iter().map(|x| 2 * x);
755 /// assert_eq!(iter.next(), Some(2));
756 /// assert_eq!(iter.next(), Some(4));
757 /// assert_eq!(iter.next(), Some(6));
758 /// assert_eq!(iter.next(), None);
761 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
764 /// # #![allow(unused_must_use)]
765 /// // don't do this:
766 /// (0..5).map(|x| println!("{x}"));
768 /// // it won't even execute, as it is lazy. Rust will warn you about this.
770 /// // Instead, use for:
776 #[stable(feature = "rust1", since = "1.0.0")]
777 fn map<B, F>(self, f: F) -> Map<Self, F>
780 F: FnMut(Self::Item) -> B,
785 /// Calls a closure on each element of an iterator.
787 /// This is equivalent to using a [`for`] loop on the iterator, although
788 /// `break` and `continue` are not possible from a closure. It's generally
789 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
790 /// when processing items at the end of longer iterator chains. In some
791 /// cases `for_each` may also be faster than a loop, because it will use
792 /// internal iteration on adapters like `Chain`.
794 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
801 /// use std::sync::mpsc::channel;
803 /// let (tx, rx) = channel();
804 /// (0..5).map(|x| x * 2 + 1)
805 /// .for_each(move |x| tx.send(x).unwrap());
807 /// let v: Vec<_> = rx.iter().collect();
808 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
811 /// For such a small example, a `for` loop may be cleaner, but `for_each`
812 /// might be preferable to keep a functional style with longer iterators:
815 /// (0..5).flat_map(|x| x * 100 .. x * 110)
817 /// .filter(|&(i, x)| (i + x) % 3 == 0)
818 /// .for_each(|(i, x)| println!("{i}:{x}"));
821 #[stable(feature = "iterator_for_each", since = "1.21.0")]
822 fn for_each<F>(self, f: F)
825 F: FnMut(Self::Item),
828 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
829 move |(), item| f(item)
832 self.fold((), call(f));
835 /// Creates an iterator which uses a closure to determine if an element
836 /// should be yielded.
838 /// Given an element the closure must return `true` or `false`. The returned
839 /// iterator will yield only the elements for which the closure returns
847 /// let a = [0i32, 1, 2];
849 /// let mut iter = a.iter().filter(|x| x.is_positive());
851 /// assert_eq!(iter.next(), Some(&1));
852 /// assert_eq!(iter.next(), Some(&2));
853 /// assert_eq!(iter.next(), None);
856 /// Because the closure passed to `filter()` takes a reference, and many
857 /// iterators iterate over references, this leads to a possibly confusing
858 /// situation, where the type of the closure is a double reference:
861 /// let a = [0, 1, 2];
863 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
865 /// assert_eq!(iter.next(), Some(&2));
866 /// assert_eq!(iter.next(), None);
869 /// It's common to instead use destructuring on the argument to strip away
873 /// let a = [0, 1, 2];
875 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
877 /// assert_eq!(iter.next(), Some(&2));
878 /// assert_eq!(iter.next(), None);
884 /// let a = [0, 1, 2];
886 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
888 /// assert_eq!(iter.next(), Some(&2));
889 /// assert_eq!(iter.next(), None);
894 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
896 #[stable(feature = "rust1", since = "1.0.0")]
897 fn filter<P>(self, predicate: P) -> Filter<Self, P>
900 P: FnMut(&Self::Item) -> bool,
902 Filter::new(self, predicate)
905 /// Creates an iterator that both filters and maps.
907 /// The returned iterator yields only the `value`s for which the supplied
908 /// closure returns `Some(value)`.
910 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
911 /// concise. The example below shows how a `map().filter().map()` can be
912 /// shortened to a single call to `filter_map`.
914 /// [`filter`]: Iterator::filter
915 /// [`map`]: Iterator::map
922 /// let a = ["1", "two", "NaN", "four", "5"];
924 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
926 /// assert_eq!(iter.next(), Some(1));
927 /// assert_eq!(iter.next(), Some(5));
928 /// assert_eq!(iter.next(), None);
931 /// Here's the same example, but with [`filter`] and [`map`]:
934 /// let a = ["1", "two", "NaN", "four", "5"];
935 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
936 /// assert_eq!(iter.next(), Some(1));
937 /// assert_eq!(iter.next(), Some(5));
938 /// assert_eq!(iter.next(), None);
941 #[stable(feature = "rust1", since = "1.0.0")]
942 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
945 F: FnMut(Self::Item) -> Option<B>,
947 FilterMap::new(self, f)
950 /// Creates an iterator which gives the current iteration count as well as
953 /// The iterator returned yields pairs `(i, val)`, where `i` is the
954 /// current index of iteration and `val` is the value returned by the
957 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
958 /// different sized integer, the [`zip`] function provides similar
961 /// # Overflow Behavior
963 /// The method does no guarding against overflows, so enumerating more than
964 /// [`usize::MAX`] elements either produces the wrong result or panics. If
965 /// debug assertions are enabled, a panic is guaranteed.
969 /// The returned iterator might panic if the to-be-returned index would
970 /// overflow a [`usize`].
972 /// [`zip`]: Iterator::zip
977 /// let a = ['a', 'b', 'c'];
979 /// let mut iter = a.iter().enumerate();
981 /// assert_eq!(iter.next(), Some((0, &'a')));
982 /// assert_eq!(iter.next(), Some((1, &'b')));
983 /// assert_eq!(iter.next(), Some((2, &'c')));
984 /// assert_eq!(iter.next(), None);
987 #[stable(feature = "rust1", since = "1.0.0")]
988 fn enumerate(self) -> Enumerate<Self>
995 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
996 /// to look at the next element of the iterator without consuming it. See
997 /// their documentation for more information.
999 /// Note that the underlying iterator is still advanced when [`peek`] or
1000 /// [`peek_mut`] are called for the first time: In order to retrieve the
1001 /// next element, [`next`] is called on the underlying iterator, hence any
1002 /// side effects (i.e. anything other than fetching the next value) of
1003 /// the [`next`] method will occur.
1011 /// let xs = [1, 2, 3];
1013 /// let mut iter = xs.iter().peekable();
1015 /// // peek() lets us see into the future
1016 /// assert_eq!(iter.peek(), Some(&&1));
1017 /// assert_eq!(iter.next(), Some(&1));
1019 /// assert_eq!(iter.next(), Some(&2));
1021 /// // we can peek() multiple times, the iterator won't advance
1022 /// assert_eq!(iter.peek(), Some(&&3));
1023 /// assert_eq!(iter.peek(), Some(&&3));
1025 /// assert_eq!(iter.next(), Some(&3));
1027 /// // after the iterator is finished, so is peek()
1028 /// assert_eq!(iter.peek(), None);
1029 /// assert_eq!(iter.next(), None);
1032 /// Using [`peek_mut`] to mutate the next item without advancing the
1036 /// let xs = [1, 2, 3];
1038 /// let mut iter = xs.iter().peekable();
1040 /// // `peek_mut()` lets us see into the future
1041 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1042 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1043 /// assert_eq!(iter.next(), Some(&1));
1045 /// if let Some(mut p) = iter.peek_mut() {
1046 /// assert_eq!(*p, &2);
1047 /// // put a value into the iterator
1051 /// // The value reappears as the iterator continues
1052 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
1054 /// [`peek`]: Peekable::peek
1055 /// [`peek_mut`]: Peekable::peek_mut
1056 /// [`next`]: Iterator::next
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 fn peekable(self) -> Peekable<Self>
1066 /// Creates an iterator that [`skip`]s elements based on a predicate.
1068 /// [`skip`]: Iterator::skip
1070 /// `skip_while()` takes a closure as an argument. It will call this
1071 /// closure on each element of the iterator, and ignore elements
1072 /// until it returns `false`.
1074 /// After `false` is returned, `skip_while()`'s job is over, and the
1075 /// rest of the elements are yielded.
1082 /// let a = [-1i32, 0, 1];
1084 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1086 /// assert_eq!(iter.next(), Some(&0));
1087 /// assert_eq!(iter.next(), Some(&1));
1088 /// assert_eq!(iter.next(), None);
1091 /// Because the closure passed to `skip_while()` takes a reference, and many
1092 /// iterators iterate over references, this leads to a possibly confusing
1093 /// situation, where the type of the closure argument is a double reference:
1096 /// let a = [-1, 0, 1];
1098 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1100 /// assert_eq!(iter.next(), Some(&0));
1101 /// assert_eq!(iter.next(), Some(&1));
1102 /// assert_eq!(iter.next(), None);
1105 /// Stopping after an initial `false`:
1108 /// let a = [-1, 0, 1, -2];
1110 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1112 /// assert_eq!(iter.next(), Some(&0));
1113 /// assert_eq!(iter.next(), Some(&1));
1115 /// // while this would have been false, since we already got a false,
1116 /// // skip_while() isn't used any more
1117 /// assert_eq!(iter.next(), Some(&-2));
1119 /// assert_eq!(iter.next(), None);
1122 #[doc(alias = "drop_while")]
1123 #[stable(feature = "rust1", since = "1.0.0")]
1124 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1127 P: FnMut(&Self::Item) -> bool,
1129 SkipWhile::new(self, predicate)
1132 /// Creates an iterator that yields elements based on a predicate.
1134 /// `take_while()` takes a closure as an argument. It will call this
1135 /// closure on each element of the iterator, and yield elements
1136 /// while it returns `true`.
1138 /// After `false` is returned, `take_while()`'s job is over, and the
1139 /// rest of the elements are ignored.
1146 /// let a = [-1i32, 0, 1];
1148 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1150 /// assert_eq!(iter.next(), Some(&-1));
1151 /// assert_eq!(iter.next(), None);
1154 /// Because the closure passed to `take_while()` takes a reference, and many
1155 /// iterators iterate over references, this leads to a possibly confusing
1156 /// situation, where the type of the closure is a double reference:
1159 /// let a = [-1, 0, 1];
1161 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1163 /// assert_eq!(iter.next(), Some(&-1));
1164 /// assert_eq!(iter.next(), None);
1167 /// Stopping after an initial `false`:
1170 /// let a = [-1, 0, 1, -2];
1172 /// let mut iter = a.iter().take_while(|x| **x < 0);
1174 /// assert_eq!(iter.next(), Some(&-1));
1176 /// // We have more elements that are less than zero, but since we already
1177 /// // got a false, take_while() isn't used any more
1178 /// assert_eq!(iter.next(), None);
1181 /// Because `take_while()` needs to look at the value in order to see if it
1182 /// should be included or not, consuming iterators will see that it is
1186 /// let a = [1, 2, 3, 4];
1187 /// let mut iter = a.iter();
1189 /// let result: Vec<i32> = iter.by_ref()
1190 /// .take_while(|n| **n != 3)
1194 /// assert_eq!(result, &[1, 2]);
1196 /// let result: Vec<i32> = iter.cloned().collect();
1198 /// assert_eq!(result, &[4]);
1201 /// The `3` is no longer there, because it was consumed in order to see if
1202 /// the iteration should stop, but wasn't placed back into the iterator.
1204 #[stable(feature = "rust1", since = "1.0.0")]
1205 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1208 P: FnMut(&Self::Item) -> bool,
1210 TakeWhile::new(self, predicate)
1213 /// Creates an iterator that both yields elements based on a predicate and maps.
1215 /// `map_while()` takes a closure as an argument. It will call this
1216 /// closure on each element of the iterator, and yield elements
1217 /// while it returns [`Some(_)`][`Some`].
1224 /// let a = [-1i32, 4, 0, 1];
1226 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1228 /// assert_eq!(iter.next(), Some(-16));
1229 /// assert_eq!(iter.next(), Some(4));
1230 /// assert_eq!(iter.next(), None);
1233 /// Here's the same example, but with [`take_while`] and [`map`]:
1235 /// [`take_while`]: Iterator::take_while
1236 /// [`map`]: Iterator::map
1239 /// let a = [-1i32, 4, 0, 1];
1241 /// let mut iter = a.iter()
1242 /// .map(|x| 16i32.checked_div(*x))
1243 /// .take_while(|x| x.is_some())
1244 /// .map(|x| x.unwrap());
1246 /// assert_eq!(iter.next(), Some(-16));
1247 /// assert_eq!(iter.next(), Some(4));
1248 /// assert_eq!(iter.next(), None);
1251 /// Stopping after an initial [`None`]:
1254 /// let a = [0, 1, 2, -3, 4, 5, -6];
1256 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1257 /// let vec = iter.collect::<Vec<_>>();
1259 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1260 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1261 /// assert_eq!(vec, vec![0, 1, 2]);
1264 /// Because `map_while()` needs to look at the value in order to see if it
1265 /// should be included or not, consuming iterators will see that it is
1269 /// let a = [1, 2, -3, 4];
1270 /// let mut iter = a.iter();
1272 /// let result: Vec<u32> = iter.by_ref()
1273 /// .map_while(|n| u32::try_from(*n).ok())
1276 /// assert_eq!(result, &[1, 2]);
1278 /// let result: Vec<i32> = iter.cloned().collect();
1280 /// assert_eq!(result, &[4]);
1283 /// The `-3` is no longer there, because it was consumed in order to see if
1284 /// the iteration should stop, but wasn't placed back into the iterator.
1286 /// Note that unlike [`take_while`] this iterator is **not** fused.
1287 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1288 /// If you need fused iterator, use [`fuse`].
1290 /// [`fuse`]: Iterator::fuse
1292 #[stable(feature = "iter_map_while", since = "1.57.0")]
1293 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1296 P: FnMut(Self::Item) -> Option<B>,
1298 MapWhile::new(self, predicate)
1301 /// Creates an iterator that skips the first `n` elements.
1303 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1304 /// iterator is reached (whichever happens first). After that, all the remaining
1305 /// elements are yielded. In particular, if the original iterator is too short,
1306 /// then the returned iterator is empty.
1308 /// Rather than overriding this method directly, instead override the `nth` method.
1315 /// let a = [1, 2, 3];
1317 /// let mut iter = a.iter().skip(2);
1319 /// assert_eq!(iter.next(), Some(&3));
1320 /// assert_eq!(iter.next(), None);
1323 #[stable(feature = "rust1", since = "1.0.0")]
1324 fn skip(self, n: usize) -> Skip<Self>
1331 /// Creates an iterator that yields the first `n` elements, or fewer
1332 /// if the underlying iterator ends sooner.
1334 /// `take(n)` yields elements until `n` elements are yielded or the end of
1335 /// the iterator is reached (whichever happens first).
1336 /// The returned iterator is a prefix of length `n` if the original iterator
1337 /// contains at least `n` elements, otherwise it contains all of the
1338 /// (fewer than `n`) elements of the original iterator.
1345 /// let a = [1, 2, 3];
1347 /// let mut iter = a.iter().take(2);
1349 /// assert_eq!(iter.next(), Some(&1));
1350 /// assert_eq!(iter.next(), Some(&2));
1351 /// assert_eq!(iter.next(), None);
1354 /// `take()` is often used with an infinite iterator, to make it finite:
1357 /// let mut iter = (0..).take(3);
1359 /// assert_eq!(iter.next(), Some(0));
1360 /// assert_eq!(iter.next(), Some(1));
1361 /// assert_eq!(iter.next(), Some(2));
1362 /// assert_eq!(iter.next(), None);
1365 /// If less than `n` elements are available,
1366 /// `take` will limit itself to the size of the underlying iterator:
1370 /// let mut iter = v.into_iter().take(5);
1371 /// assert_eq!(iter.next(), Some(1));
1372 /// assert_eq!(iter.next(), Some(2));
1373 /// assert_eq!(iter.next(), None);
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 fn take(self, n: usize) -> Take<Self>
1384 /// An iterator adapter similar to [`fold`] that holds internal state and
1385 /// produces a new iterator.
1387 /// [`fold`]: Iterator::fold
1389 /// `scan()` takes two arguments: an initial value which seeds the internal
1390 /// state, and a closure with two arguments, the first being a mutable
1391 /// reference to the internal state and the second an iterator element.
1392 /// The closure can assign to the internal state to share state between
1395 /// On iteration, the closure will be applied to each element of the
1396 /// iterator and the return value from the closure, an [`Option`], is
1397 /// yielded by the iterator.
1404 /// let a = [1, 2, 3];
1406 /// let mut iter = a.iter().scan(1, |state, &x| {
1407 /// // each iteration, we'll multiply the state by the element
1408 /// *state = *state * x;
1410 /// // then, we'll yield the negation of the state
1414 /// assert_eq!(iter.next(), Some(-1));
1415 /// assert_eq!(iter.next(), Some(-2));
1416 /// assert_eq!(iter.next(), Some(-6));
1417 /// assert_eq!(iter.next(), None);
1420 #[stable(feature = "rust1", since = "1.0.0")]
1421 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1424 F: FnMut(&mut St, Self::Item) -> Option<B>,
1426 Scan::new(self, initial_state, f)
1429 /// Creates an iterator that works like map, but flattens nested structure.
1431 /// The [`map`] adapter is very useful, but only when the closure
1432 /// argument produces values. If it produces an iterator instead, there's
1433 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1436 /// You can think of `flat_map(f)` as the semantic equivalent
1437 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1439 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1440 /// one item for each element, and `flat_map()`'s closure returns an
1441 /// iterator for each element.
1443 /// [`map`]: Iterator::map
1444 /// [`flatten`]: Iterator::flatten
1451 /// let words = ["alpha", "beta", "gamma"];
1453 /// // chars() returns an iterator
1454 /// let merged: String = words.iter()
1455 /// .flat_map(|s| s.chars())
1457 /// assert_eq!(merged, "alphabetagamma");
1460 #[stable(feature = "rust1", since = "1.0.0")]
1461 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1465 F: FnMut(Self::Item) -> U,
1467 FlatMap::new(self, f)
1470 /// Creates an iterator that flattens nested structure.
1472 /// This is useful when you have an iterator of iterators or an iterator of
1473 /// things that can be turned into iterators and you want to remove one
1474 /// level of indirection.
1481 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1482 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1483 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1486 /// Mapping and then flattening:
1489 /// let words = ["alpha", "beta", "gamma"];
1491 /// // chars() returns an iterator
1492 /// let merged: String = words.iter()
1493 /// .map(|s| s.chars())
1496 /// assert_eq!(merged, "alphabetagamma");
1499 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1500 /// in this case since it conveys intent more clearly:
1503 /// let words = ["alpha", "beta", "gamma"];
1505 /// // chars() returns an iterator
1506 /// let merged: String = words.iter()
1507 /// .flat_map(|s| s.chars())
1509 /// assert_eq!(merged, "alphabetagamma");
1512 /// Flattening only removes one level of nesting at a time:
1515 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1517 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1518 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1520 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1521 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1524 /// Here we see that `flatten()` does not perform a "deep" flatten.
1525 /// Instead, only one level of nesting is removed. That is, if you
1526 /// `flatten()` a three-dimensional array, the result will be
1527 /// two-dimensional and not one-dimensional. To get a one-dimensional
1528 /// structure, you have to `flatten()` again.
1530 /// [`flat_map()`]: Iterator::flat_map
1532 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1533 fn flatten(self) -> Flatten<Self>
1536 Self::Item: IntoIterator,
1541 /// Creates an iterator which ends after the first [`None`].
1543 /// After an iterator returns [`None`], future calls may or may not yield
1544 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1545 /// [`None`] is given, it will always return [`None`] forever.
1547 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1548 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1549 /// if the [`FusedIterator`] trait is improperly implemented.
1551 /// [`Some(T)`]: Some
1552 /// [`FusedIterator`]: crate::iter::FusedIterator
1559 /// // an iterator which alternates between Some and None
1560 /// struct Alternate {
1564 /// impl Iterator for Alternate {
1565 /// type Item = i32;
1567 /// fn next(&mut self) -> Option<i32> {
1568 /// let val = self.state;
1569 /// self.state = self.state + 1;
1571 /// // if it's even, Some(i32), else None
1572 /// if val % 2 == 0 {
1580 /// let mut iter = Alternate { state: 0 };
1582 /// // we can see our iterator going back and forth
1583 /// assert_eq!(iter.next(), Some(0));
1584 /// assert_eq!(iter.next(), None);
1585 /// assert_eq!(iter.next(), Some(2));
1586 /// assert_eq!(iter.next(), None);
1588 /// // however, once we fuse it...
1589 /// let mut iter = iter.fuse();
1591 /// assert_eq!(iter.next(), Some(4));
1592 /// assert_eq!(iter.next(), None);
1594 /// // it will always return `None` after the first time.
1595 /// assert_eq!(iter.next(), None);
1596 /// assert_eq!(iter.next(), None);
1597 /// assert_eq!(iter.next(), None);
1600 #[stable(feature = "rust1", since = "1.0.0")]
1601 fn fuse(self) -> Fuse<Self>
1608 /// Does something with each element of an iterator, passing the value on.
1610 /// When using iterators, you'll often chain several of them together.
1611 /// While working on such code, you might want to check out what's
1612 /// happening at various parts in the pipeline. To do that, insert
1613 /// a call to `inspect()`.
1615 /// It's more common for `inspect()` to be used as a debugging tool than to
1616 /// exist in your final code, but applications may find it useful in certain
1617 /// situations when errors need to be logged before being discarded.
1624 /// let a = [1, 4, 2, 3];
1626 /// // this iterator sequence is complex.
1627 /// let sum = a.iter()
1629 /// .filter(|x| x % 2 == 0)
1630 /// .fold(0, |sum, i| sum + i);
1632 /// println!("{sum}");
1634 /// // let's add some inspect() calls to investigate what's happening
1635 /// let sum = a.iter()
1637 /// .inspect(|x| println!("about to filter: {x}"))
1638 /// .filter(|x| x % 2 == 0)
1639 /// .inspect(|x| println!("made it through filter: {x}"))
1640 /// .fold(0, |sum, i| sum + i);
1642 /// println!("{sum}");
1645 /// This will print:
1649 /// about to filter: 1
1650 /// about to filter: 4
1651 /// made it through filter: 4
1652 /// about to filter: 2
1653 /// made it through filter: 2
1654 /// about to filter: 3
1658 /// Logging errors before discarding them:
1661 /// let lines = ["1", "2", "a"];
1663 /// let sum: i32 = lines
1665 /// .map(|line| line.parse::<i32>())
1666 /// .inspect(|num| {
1667 /// if let Err(ref e) = *num {
1668 /// println!("Parsing error: {e}");
1671 /// .filter_map(Result::ok)
1674 /// println!("Sum: {sum}");
1677 /// This will print:
1680 /// Parsing error: invalid digit found in string
1684 #[stable(feature = "rust1", since = "1.0.0")]
1685 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1688 F: FnMut(&Self::Item),
1690 Inspect::new(self, f)
1693 /// Borrows an iterator, rather than consuming it.
1695 /// This is useful to allow applying iterator adapters while still
1696 /// retaining ownership of the original iterator.
1703 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1705 /// // Take the first two words.
1706 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1707 /// assert_eq!(hello_world, vec!["hello", "world"]);
1709 /// // Collect the rest of the words.
1710 /// // We can only do this because we used `by_ref` earlier.
1711 /// let of_rust: Vec<_> = words.collect();
1712 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1714 #[stable(feature = "rust1", since = "1.0.0")]
1715 fn by_ref(&mut self) -> &mut Self
1722 /// Transforms an iterator into a collection.
1724 /// `collect()` can take anything iterable, and turn it into a relevant
1725 /// collection. This is one of the more powerful methods in the standard
1726 /// library, used in a variety of contexts.
1728 /// The most basic pattern in which `collect()` is used is to turn one
1729 /// collection into another. You take a collection, call [`iter`] on it,
1730 /// do a bunch of transformations, and then `collect()` at the end.
1732 /// `collect()` can also create instances of types that are not typical
1733 /// collections. For example, a [`String`] can be built from [`char`]s,
1734 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1735 /// into `Result<Collection<T>, E>`. See the examples below for more.
1737 /// Because `collect()` is so general, it can cause problems with type
1738 /// inference. As such, `collect()` is one of the few times you'll see
1739 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1740 /// helps the inference algorithm understand specifically which collection
1741 /// you're trying to collect into.
1748 /// let a = [1, 2, 3];
1750 /// let doubled: Vec<i32> = a.iter()
1751 /// .map(|&x| x * 2)
1754 /// assert_eq!(vec![2, 4, 6], doubled);
1757 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1758 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1760 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1763 /// use std::collections::VecDeque;
1765 /// let a = [1, 2, 3];
1767 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1769 /// assert_eq!(2, doubled[0]);
1770 /// assert_eq!(4, doubled[1]);
1771 /// assert_eq!(6, doubled[2]);
1774 /// Using the 'turbofish' instead of annotating `doubled`:
1777 /// let a = [1, 2, 3];
1779 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1781 /// assert_eq!(vec![2, 4, 6], doubled);
1784 /// Because `collect()` only cares about what you're collecting into, you can
1785 /// still use a partial type hint, `_`, with the turbofish:
1788 /// let a = [1, 2, 3];
1790 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1792 /// assert_eq!(vec![2, 4, 6], doubled);
1795 /// Using `collect()` to make a [`String`]:
1798 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1800 /// let hello: String = chars.iter()
1801 /// .map(|&x| x as u8)
1802 /// .map(|x| (x + 1) as char)
1805 /// assert_eq!("hello", hello);
1808 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1809 /// see if any of them failed:
1812 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1814 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1816 /// // gives us the first error
1817 /// assert_eq!(Err("nope"), result);
1819 /// let results = [Ok(1), Ok(3)];
1821 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1823 /// // gives us the list of answers
1824 /// assert_eq!(Ok(vec![1, 3]), result);
1827 /// [`iter`]: Iterator::next
1828 /// [`String`]: ../../std/string/struct.String.html
1829 /// [`char`]: type@char
1831 #[stable(feature = "rust1", since = "1.0.0")]
1832 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1833 #[cfg_attr(not(test), rustc_diagnostic_item = "iterator_collect_fn")]
1834 fn collect<B: FromIterator<Self::Item>>(self) -> B
1838 FromIterator::from_iter(self)
1841 /// Fallibly transforms an iterator into a collection, short circuiting if
1842 /// a failure is encountered.
1844 /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
1845 /// conversions during collection. Its main use case is simplifying conversions from
1846 /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
1847 /// types (e.g. [`Result`]).
1849 /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
1850 /// only the inner type produced on `Try::Output` must implement it. Concretely,
1851 /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
1852 /// [`FromIterator`], even though [`ControlFlow`] doesn't.
1854 /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
1855 /// may continue to be used, in which case it will continue iterating starting after the element that
1856 /// triggered the failure. See the last example below for an example of how this works.
1859 /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
1861 /// #![feature(iterator_try_collect)]
1863 /// let u = vec![Some(1), Some(2), Some(3)];
1864 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1865 /// assert_eq!(v, Some(vec![1, 2, 3]));
1868 /// Failing to collect in the same way:
1870 /// #![feature(iterator_try_collect)]
1872 /// let u = vec![Some(1), Some(2), None, Some(3)];
1873 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1874 /// assert_eq!(v, None);
1877 /// A similar example, but with `Result`:
1879 /// #![feature(iterator_try_collect)]
1881 /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
1882 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1883 /// assert_eq!(v, Ok(vec![1, 2, 3]));
1885 /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
1886 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1887 /// assert_eq!(v, Err(()));
1890 /// Finally, even [`ControlFlow`] works, despite the fact that it
1891 /// doesn't implement [`FromIterator`]. Note also that the iterator can
1892 /// continue to be used, even if a failure is encountered:
1895 /// #![feature(iterator_try_collect)]
1897 /// use core::ops::ControlFlow::{Break, Continue};
1899 /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
1900 /// let mut it = u.into_iter();
1902 /// let v = it.try_collect::<Vec<_>>();
1903 /// assert_eq!(v, Break(3));
1905 /// let v = it.try_collect::<Vec<_>>();
1906 /// assert_eq!(v, Continue(vec![4, 5]));
1909 /// [`collect`]: Iterator::collect
1911 #[unstable(feature = "iterator_try_collect", issue = "94047")]
1912 fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
1915 <Self as Iterator>::Item: Try,
1916 <<Self as Iterator>::Item as Try>::Residual: Residual<B>,
1917 B: FromIterator<<Self::Item as Try>::Output>,
1919 try_process(ByRefSized(self), |i| i.collect())
1922 /// Collects all the items from an iterator into a collection.
1924 /// This method consumes the iterator and adds all its items to the
1925 /// passed collection. The collection is then returned, so the call chain
1926 /// can be continued.
1928 /// This is useful when you already have a collection and wants to add
1929 /// the iterator items to it.
1931 /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
1932 /// but instead of being called on a collection, it's called on an iterator.
1939 /// #![feature(iter_collect_into)]
1941 /// let a = [1, 2, 3];
1942 /// let mut vec: Vec::<i32> = vec![0, 1];
1944 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
1945 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
1947 /// assert_eq!(vec![0, 1, 2, 4, 6, 10, 20, 30], vec);
1950 /// `Vec` can have a manual set capacity to avoid reallocating it:
1953 /// #![feature(iter_collect_into)]
1955 /// let a = [1, 2, 3];
1956 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
1958 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
1959 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
1961 /// assert_eq!(6, vec.capacity());
1962 /// println!("{:?}", vec);
1965 /// The returned mutable reference can be used to continue the call chain:
1968 /// #![feature(iter_collect_into)]
1970 /// let a = [1, 2, 3];
1971 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
1973 /// let count = a.iter().collect_into(&mut vec).iter().count();
1975 /// assert_eq!(count, vec.len());
1976 /// println!("Vec len is {}", count);
1978 /// let count = a.iter().collect_into(&mut vec).iter().count();
1980 /// assert_eq!(count, vec.len());
1981 /// println!("Vec len now is {}", count);
1984 #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
1985 fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
1989 collection.extend(self);
1993 /// Consumes an iterator, creating two collections from it.
1995 /// The predicate passed to `partition()` can return `true`, or `false`.
1996 /// `partition()` returns a pair, all of the elements for which it returned
1997 /// `true`, and all of the elements for which it returned `false`.
1999 /// See also [`is_partitioned()`] and [`partition_in_place()`].
2001 /// [`is_partitioned()`]: Iterator::is_partitioned
2002 /// [`partition_in_place()`]: Iterator::partition_in_place
2009 /// let a = [1, 2, 3];
2011 /// let (even, odd): (Vec<_>, Vec<_>) = a
2013 /// .partition(|n| n % 2 == 0);
2015 /// assert_eq!(even, vec![2]);
2016 /// assert_eq!(odd, vec![1, 3]);
2018 #[stable(feature = "rust1", since = "1.0.0")]
2019 fn partition<B, F>(self, f: F) -> (B, B)
2022 B: Default + Extend<Self::Item>,
2023 F: FnMut(&Self::Item) -> bool,
2026 fn extend<'a, T, B: Extend<T>>(
2027 mut f: impl FnMut(&T) -> bool + 'a,
2030 ) -> impl FnMut((), T) + 'a {
2035 right.extend_one(x);
2040 let mut left: B = Default::default();
2041 let mut right: B = Default::default();
2043 self.fold((), extend(f, &mut left, &mut right));
2048 /// Reorders the elements of this iterator *in-place* according to the given predicate,
2049 /// such that all those that return `true` precede all those that return `false`.
2050 /// Returns the number of `true` elements found.
2052 /// The relative order of partitioned items is not maintained.
2054 /// # Current implementation
2056 /// Current algorithms tries finding the first element for which the predicate evaluates
2057 /// to false, and the last element for which it evaluates to true and repeatedly swaps them.
2059 /// Time complexity: *O*(*n*)
2061 /// See also [`is_partitioned()`] and [`partition()`].
2063 /// [`is_partitioned()`]: Iterator::is_partitioned
2064 /// [`partition()`]: Iterator::partition
2069 /// #![feature(iter_partition_in_place)]
2071 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
2073 /// // Partition in-place between evens and odds
2074 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
2076 /// assert_eq!(i, 3);
2077 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
2078 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
2080 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
2081 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
2083 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
2084 P: FnMut(&T) -> bool,
2086 // FIXME: should we worry about the count overflowing? The only way to have more than
2087 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
2089 // These closure "factory" functions exist to avoid genericity in `Self`.
2093 predicate: &'a mut impl FnMut(&T) -> bool,
2094 true_count: &'a mut usize,
2095 ) -> impl FnMut(&&mut T) -> bool + 'a {
2097 let p = predicate(&**x);
2098 *true_count += p as usize;
2104 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
2105 move |x| predicate(&**x)
2108 // Repeatedly find the first `false` and swap it with the last `true`.
2109 let mut true_count = 0;
2110 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
2111 if let Some(tail) = self.rfind(is_true(predicate)) {
2112 crate::mem::swap(head, tail);
2121 /// Checks if the elements of this iterator are partitioned according to the given predicate,
2122 /// such that all those that return `true` precede all those that return `false`.
2124 /// See also [`partition()`] and [`partition_in_place()`].
2126 /// [`partition()`]: Iterator::partition
2127 /// [`partition_in_place()`]: Iterator::partition_in_place
2132 /// #![feature(iter_is_partitioned)]
2134 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2135 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2137 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2138 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2141 P: FnMut(Self::Item) -> bool,
2143 // Either all items test `true`, or the first clause stops at `false`
2144 // and we check that there are no more `true` items after that.
2145 self.all(&mut predicate) || !self.any(predicate)
2148 /// An iterator method that applies a function as long as it returns
2149 /// successfully, producing a single, final value.
2151 /// `try_fold()` takes two arguments: an initial value, and a closure with
2152 /// two arguments: an 'accumulator', and an element. The closure either
2153 /// returns successfully, with the value that the accumulator should have
2154 /// for the next iteration, or it returns failure, with an error value that
2155 /// is propagated back to the caller immediately (short-circuiting).
2157 /// The initial value is the value the accumulator will have on the first
2158 /// call. If applying the closure succeeded against every element of the
2159 /// iterator, `try_fold()` returns the final accumulator as success.
2161 /// Folding is useful whenever you have a collection of something, and want
2162 /// to produce a single value from it.
2164 /// # Note to Implementors
2166 /// Several of the other (forward) methods have default implementations in
2167 /// terms of this one, so try to implement this explicitly if it can
2168 /// do something better than the default `for` loop implementation.
2170 /// In particular, try to have this call `try_fold()` on the internal parts
2171 /// from which this iterator is composed. If multiple calls are needed,
2172 /// the `?` operator may be convenient for chaining the accumulator value
2173 /// along, but beware any invariants that need to be upheld before those
2174 /// early returns. This is a `&mut self` method, so iteration needs to be
2175 /// resumable after hitting an error here.
2182 /// let a = [1, 2, 3];
2184 /// // the checked sum of all of the elements of the array
2185 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
2187 /// assert_eq!(sum, Some(6));
2190 /// Short-circuiting:
2193 /// let a = [10, 20, 30, 100, 40, 50];
2194 /// let mut it = a.iter();
2196 /// // This sum overflows when adding the 100 element
2197 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
2198 /// assert_eq!(sum, None);
2200 /// // Because it short-circuited, the remaining elements are still
2201 /// // available through the iterator.
2202 /// assert_eq!(it.len(), 2);
2203 /// assert_eq!(it.next(), Some(&40));
2206 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2210 /// use std::ops::ControlFlow;
2212 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2213 /// if let Some(next) = prev.checked_add(x) {
2214 /// ControlFlow::Continue(next)
2216 /// ControlFlow::Break(prev)
2219 /// assert_eq!(triangular, ControlFlow::Break(120));
2221 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2222 /// if let Some(next) = prev.checked_add(x) {
2223 /// ControlFlow::Continue(next)
2225 /// ControlFlow::Break(prev)
2228 /// assert_eq!(triangular, ControlFlow::Continue(435));
2231 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2232 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2235 F: FnMut(B, Self::Item) -> R,
2238 let mut accum = init;
2239 while let Some(x) = self.next() {
2240 accum = f(accum, x)?;
2245 /// An iterator method that applies a fallible function to each item in the
2246 /// iterator, stopping at the first error and returning that error.
2248 /// This can also be thought of as the fallible form of [`for_each()`]
2249 /// or as the stateless version of [`try_fold()`].
2251 /// [`for_each()`]: Iterator::for_each
2252 /// [`try_fold()`]: Iterator::try_fold
2257 /// use std::fs::rename;
2258 /// use std::io::{stdout, Write};
2259 /// use std::path::Path;
2261 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2263 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
2264 /// assert!(res.is_ok());
2266 /// let mut it = data.iter().cloned();
2267 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2268 /// assert!(res.is_err());
2269 /// // It short-circuited, so the remaining items are still in the iterator:
2270 /// assert_eq!(it.next(), Some("stale_bread.json"));
2273 /// The [`ControlFlow`] type can be used with this method for the situations
2274 /// in which you'd use `break` and `continue` in a normal loop:
2277 /// use std::ops::ControlFlow;
2279 /// let r = (2..100).try_for_each(|x| {
2280 /// if 323 % x == 0 {
2281 /// return ControlFlow::Break(x)
2284 /// ControlFlow::Continue(())
2286 /// assert_eq!(r, ControlFlow::Break(17));
2289 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2290 fn try_for_each<F, R>(&mut self, f: F) -> R
2293 F: FnMut(Self::Item) -> R,
2294 R: Try<Output = ()>,
2297 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2301 self.try_fold((), call(f))
2304 /// Folds every element into an accumulator by applying an operation,
2305 /// returning the final result.
2307 /// `fold()` takes two arguments: an initial value, and a closure with two
2308 /// arguments: an 'accumulator', and an element. The closure returns the value that
2309 /// the accumulator should have for the next iteration.
2311 /// The initial value is the value the accumulator will have on the first
2314 /// After applying this closure to every element of the iterator, `fold()`
2315 /// returns the accumulator.
2317 /// This operation is sometimes called 'reduce' or 'inject'.
2319 /// Folding is useful whenever you have a collection of something, and want
2320 /// to produce a single value from it.
2322 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2323 /// might not terminate for infinite iterators, even on traits for which a
2324 /// result is determinable in finite time.
2326 /// Note: [`reduce()`] can be used to use the first element as the initial
2327 /// value, if the accumulator type and item type is the same.
2329 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2330 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2331 /// operators like `-` the order will affect the final result.
2332 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2334 /// # Note to Implementors
2336 /// Several of the other (forward) methods have default implementations in
2337 /// terms of this one, so try to implement this explicitly if it can
2338 /// do something better than the default `for` loop implementation.
2340 /// In particular, try to have this call `fold()` on the internal parts
2341 /// from which this iterator is composed.
2348 /// let a = [1, 2, 3];
2350 /// // the sum of all of the elements of the array
2351 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2353 /// assert_eq!(sum, 6);
2356 /// Let's walk through each step of the iteration here:
2358 /// | element | acc | x | result |
2359 /// |---------|-----|---|--------|
2361 /// | 1 | 0 | 1 | 1 |
2362 /// | 2 | 1 | 2 | 3 |
2363 /// | 3 | 3 | 3 | 6 |
2365 /// And so, our final result, `6`.
2367 /// This example demonstrates the left-associative nature of `fold()`:
2368 /// it builds a string, starting with an initial value
2369 /// and continuing with each element from the front until the back:
2372 /// let numbers = [1, 2, 3, 4, 5];
2374 /// let zero = "0".to_string();
2376 /// let result = numbers.iter().fold(zero, |acc, &x| {
2377 /// format!("({acc} + {x})")
2380 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2382 /// It's common for people who haven't used iterators a lot to
2383 /// use a `for` loop with a list of things to build up a result. Those
2384 /// can be turned into `fold()`s:
2386 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2389 /// let numbers = [1, 2, 3, 4, 5];
2391 /// let mut result = 0;
2394 /// for i in &numbers {
2395 /// result = result + i;
2399 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2401 /// // they're the same
2402 /// assert_eq!(result, result2);
2405 /// [`reduce()`]: Iterator::reduce
2406 #[doc(alias = "inject", alias = "foldl")]
2408 #[stable(feature = "rust1", since = "1.0.0")]
2409 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2412 F: FnMut(B, Self::Item) -> B,
2414 let mut accum = init;
2415 while let Some(x) = self.next() {
2416 accum = f(accum, x);
2421 /// Reduces the elements to a single one, by repeatedly applying a reducing
2424 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2425 /// result of the reduction.
2427 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2428 /// For iterators with at least one element, this is the same as [`fold()`]
2429 /// with the first element of the iterator as the initial accumulator value, folding
2430 /// every subsequent element into it.
2432 /// [`fold()`]: Iterator::fold
2437 /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap();
2438 /// assert_eq!(reduced, 45);
2440 /// // Which is equivalent to doing it with `fold`:
2441 /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
2442 /// assert_eq!(reduced, folded);
2445 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2446 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2449 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2451 let first = self.next()?;
2452 Some(self.fold(first, f))
2455 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2456 /// closure returns a failure, the failure is propagated back to the caller immediately.
2458 /// The return type of this method depends on the return type of the closure. If the closure
2459 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2460 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2461 /// `Option<Option<Self::Item>>`.
2463 /// When called on an empty iterator, this function will return either `Some(None)` or
2464 /// `Ok(None)` depending on the type of the provided closure.
2466 /// For iterators with at least one element, this is essentially the same as calling
2467 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2469 /// [`try_fold()`]: Iterator::try_fold
2473 /// Safely calculate the sum of a series of numbers:
2476 /// #![feature(iterator_try_reduce)]
2478 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2479 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2480 /// assert_eq!(sum, Some(Some(58)));
2483 /// Determine when a reduction short circuited:
2486 /// #![feature(iterator_try_reduce)]
2488 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2489 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2490 /// assert_eq!(sum, None);
2493 /// Determine when a reduction was not performed because there are no elements:
2496 /// #![feature(iterator_try_reduce)]
2498 /// let numbers: Vec<usize> = Vec::new();
2499 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2500 /// assert_eq!(sum, Some(None));
2503 /// Use a [`Result`] instead of an [`Option`]:
2506 /// #![feature(iterator_try_reduce)]
2508 /// let numbers = vec!["1", "2", "3", "4", "5"];
2509 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2510 /// numbers.into_iter().try_reduce(|x, y| {
2511 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2513 /// assert_eq!(max, Ok(Some("5")));
2516 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2517 fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
2520 F: FnMut(Self::Item, Self::Item) -> R,
2521 R: Try<Output = Self::Item>,
2522 R::Residual: Residual<Option<Self::Item>>,
2524 let first = match self.next() {
2526 None => return Try::from_output(None),
2529 match self.try_fold(first, f).branch() {
2530 ControlFlow::Break(r) => FromResidual::from_residual(r),
2531 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2535 /// Tests if every element of the iterator matches a predicate.
2537 /// `all()` takes a closure that returns `true` or `false`. It applies
2538 /// this closure to each element of the iterator, and if they all return
2539 /// `true`, then so does `all()`. If any of them return `false`, it
2540 /// returns `false`.
2542 /// `all()` is short-circuiting; in other words, it will stop processing
2543 /// as soon as it finds a `false`, given that no matter what else happens,
2544 /// the result will also be `false`.
2546 /// An empty iterator returns `true`.
2553 /// let a = [1, 2, 3];
2555 /// assert!(a.iter().all(|&x| x > 0));
2557 /// assert!(!a.iter().all(|&x| x > 2));
2560 /// Stopping at the first `false`:
2563 /// let a = [1, 2, 3];
2565 /// let mut iter = a.iter();
2567 /// assert!(!iter.all(|&x| x != 2));
2569 /// // we can still use `iter`, as there are more elements.
2570 /// assert_eq!(iter.next(), Some(&3));
2573 #[stable(feature = "rust1", since = "1.0.0")]
2574 fn all<F>(&mut self, f: F) -> bool
2577 F: FnMut(Self::Item) -> bool,
2580 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2582 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2585 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2588 /// Tests if any element of the iterator matches a predicate.
2590 /// `any()` takes a closure that returns `true` or `false`. It applies
2591 /// this closure to each element of the iterator, and if any of them return
2592 /// `true`, then so does `any()`. If they all return `false`, it
2593 /// returns `false`.
2595 /// `any()` is short-circuiting; in other words, it will stop processing
2596 /// as soon as it finds a `true`, given that no matter what else happens,
2597 /// the result will also be `true`.
2599 /// An empty iterator returns `false`.
2606 /// let a = [1, 2, 3];
2608 /// assert!(a.iter().any(|&x| x > 0));
2610 /// assert!(!a.iter().any(|&x| x > 5));
2613 /// Stopping at the first `true`:
2616 /// let a = [1, 2, 3];
2618 /// let mut iter = a.iter();
2620 /// assert!(iter.any(|&x| x != 2));
2622 /// // we can still use `iter`, as there are more elements.
2623 /// assert_eq!(iter.next(), Some(&2));
2626 #[stable(feature = "rust1", since = "1.0.0")]
2627 fn any<F>(&mut self, f: F) -> bool
2630 F: FnMut(Self::Item) -> bool,
2633 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2635 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2639 self.try_fold((), check(f)) == ControlFlow::BREAK
2642 /// Searches for an element of an iterator that satisfies a predicate.
2644 /// `find()` takes a closure that returns `true` or `false`. It applies
2645 /// this closure to each element of the iterator, and if any of them return
2646 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2647 /// `false`, it returns [`None`].
2649 /// `find()` is short-circuiting; in other words, it will stop processing
2650 /// as soon as the closure returns `true`.
2652 /// Because `find()` takes a reference, and many iterators iterate over
2653 /// references, this leads to a possibly confusing situation where the
2654 /// argument is a double reference. You can see this effect in the
2655 /// examples below, with `&&x`.
2657 /// [`Some(element)`]: Some
2664 /// let a = [1, 2, 3];
2666 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2668 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2671 /// Stopping at the first `true`:
2674 /// let a = [1, 2, 3];
2676 /// let mut iter = a.iter();
2678 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2680 /// // we can still use `iter`, as there are more elements.
2681 /// assert_eq!(iter.next(), Some(&3));
2684 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2686 #[stable(feature = "rust1", since = "1.0.0")]
2687 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2690 P: FnMut(&Self::Item) -> bool,
2693 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2695 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2699 self.try_fold((), check(predicate)).break_value()
2702 /// Applies function to the elements of iterator and returns
2703 /// the first non-none result.
2705 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2710 /// let a = ["lol", "NaN", "2", "5"];
2712 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2714 /// assert_eq!(first_number, Some(2));
2717 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2718 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2721 F: FnMut(Self::Item) -> Option<B>,
2724 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2725 move |(), x| match f(x) {
2726 Some(x) => ControlFlow::Break(x),
2727 None => ControlFlow::CONTINUE,
2731 self.try_fold((), check(f)).break_value()
2734 /// Applies function to the elements of iterator and returns
2735 /// the first true result or the first error.
2737 /// The return type of this method depends on the return type of the closure.
2738 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
2739 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2744 /// #![feature(try_find)]
2746 /// let a = ["1", "2", "lol", "NaN", "5"];
2748 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2749 /// Ok(s.parse::<i32>()? == search)
2752 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2753 /// assert_eq!(result, Ok(Some(&"2")));
2755 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2756 /// assert!(result.is_err());
2759 /// This also supports other types which implement `Try`, not just `Result`.
2761 /// #![feature(try_find)]
2763 /// use std::num::NonZeroU32;
2764 /// let a = [3, 5, 7, 4, 9, 0, 11];
2765 /// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2766 /// assert_eq!(result, Some(Some(&4)));
2767 /// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2768 /// assert_eq!(result, Some(None));
2769 /// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2770 /// assert_eq!(result, None);
2773 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2774 fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>>
2777 F: FnMut(&Self::Item) -> R,
2778 R: Try<Output = bool>,
2779 R::Residual: Residual<Option<Self::Item>>,
2783 mut f: impl FnMut(&I) -> V,
2784 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2786 V: Try<Output = bool, Residual = R>,
2787 R: Residual<Option<I>>,
2789 move |(), x| match f(&x).branch() {
2790 ControlFlow::Continue(false) => ControlFlow::CONTINUE,
2791 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2792 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2796 match self.try_fold((), check(f)) {
2797 ControlFlow::Break(x) => x,
2798 ControlFlow::Continue(()) => Try::from_output(None),
2802 /// Searches for an element in an iterator, returning its index.
2804 /// `position()` takes a closure that returns `true` or `false`. It applies
2805 /// this closure to each element of the iterator, and if one of them
2806 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2807 /// them return `false`, it returns [`None`].
2809 /// `position()` is short-circuiting; in other words, it will stop
2810 /// processing as soon as it finds a `true`.
2812 /// # Overflow Behavior
2814 /// The method does no guarding against overflows, so if there are more
2815 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2816 /// result or panics. If debug assertions are enabled, a panic is
2821 /// This function might panic if the iterator has more than `usize::MAX`
2822 /// non-matching elements.
2824 /// [`Some(index)`]: Some
2831 /// let a = [1, 2, 3];
2833 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2835 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2838 /// Stopping at the first `true`:
2841 /// let a = [1, 2, 3, 4];
2843 /// let mut iter = a.iter();
2845 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2847 /// // we can still use `iter`, as there are more elements.
2848 /// assert_eq!(iter.next(), Some(&3));
2850 /// // The returned index depends on iterator state
2851 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2855 #[stable(feature = "rust1", since = "1.0.0")]
2856 fn position<P>(&mut self, predicate: P) -> Option<usize>
2859 P: FnMut(Self::Item) -> bool,
2863 mut predicate: impl FnMut(T) -> bool,
2864 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2865 #[rustc_inherit_overflow_checks]
2867 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2871 self.try_fold(0, check(predicate)).break_value()
2874 /// Searches for an element in an iterator from the right, returning its
2877 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2878 /// this closure to each element of the iterator, starting from the end,
2879 /// and if one of them returns `true`, then `rposition()` returns
2880 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2882 /// `rposition()` is short-circuiting; in other words, it will stop
2883 /// processing as soon as it finds a `true`.
2885 /// [`Some(index)`]: Some
2892 /// let a = [1, 2, 3];
2894 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2896 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2899 /// Stopping at the first `true`:
2902 /// let a = [-1, 2, 3, 4];
2904 /// let mut iter = a.iter();
2906 /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
2908 /// // we can still use `iter`, as there are more elements.
2909 /// assert_eq!(iter.next(), Some(&-1));
2912 #[stable(feature = "rust1", since = "1.0.0")]
2913 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2915 P: FnMut(Self::Item) -> bool,
2916 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2918 // No need for an overflow check here, because `ExactSizeIterator`
2919 // implies that the number of elements fits into a `usize`.
2922 mut predicate: impl FnMut(T) -> bool,
2923 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2926 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2931 self.try_rfold(n, check(predicate)).break_value()
2934 /// Returns the maximum element of an iterator.
2936 /// If several elements are equally maximum, the last element is
2937 /// returned. If the iterator is empty, [`None`] is returned.
2939 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2940 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2943 /// [2.4, f32::NAN, 1.3]
2945 /// .reduce(f32::max)
2956 /// let a = [1, 2, 3];
2957 /// let b: Vec<u32> = Vec::new();
2959 /// assert_eq!(a.iter().max(), Some(&3));
2960 /// assert_eq!(b.iter().max(), None);
2963 #[stable(feature = "rust1", since = "1.0.0")]
2964 fn max(self) -> Option<Self::Item>
2969 self.max_by(Ord::cmp)
2972 /// Returns the minimum element of an iterator.
2974 /// If several elements are equally minimum, the first element is returned.
2975 /// If the iterator is empty, [`None`] is returned.
2977 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2978 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2981 /// [2.4, f32::NAN, 1.3]
2983 /// .reduce(f32::min)
2994 /// let a = [1, 2, 3];
2995 /// let b: Vec<u32> = Vec::new();
2997 /// assert_eq!(a.iter().min(), Some(&1));
2998 /// assert_eq!(b.iter().min(), None);
3001 #[stable(feature = "rust1", since = "1.0.0")]
3002 fn min(self) -> Option<Self::Item>
3007 self.min_by(Ord::cmp)
3010 /// Returns the element that gives the maximum value from the
3011 /// specified function.
3013 /// If several elements are equally maximum, the last element is
3014 /// returned. If the iterator is empty, [`None`] is returned.
3019 /// let a = [-3_i32, 0, 1, 5, -10];
3020 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
3023 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3024 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3027 F: FnMut(&Self::Item) -> B,
3030 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3035 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3039 let (_, x) = self.map(key(f)).max_by(compare)?;
3043 /// Returns the element that gives the maximum value with respect to the
3044 /// specified comparison function.
3046 /// If several elements are equally maximum, the last element is
3047 /// returned. If the iterator is empty, [`None`] is returned.
3052 /// let a = [-3_i32, 0, 1, 5, -10];
3053 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
3056 #[stable(feature = "iter_max_by", since = "1.15.0")]
3057 fn max_by<F>(self, compare: F) -> Option<Self::Item>
3060 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3063 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3064 move |x, y| cmp::max_by(x, y, &mut compare)
3067 self.reduce(fold(compare))
3070 /// Returns the element that gives the minimum value from the
3071 /// specified function.
3073 /// If several elements are equally minimum, the first element is
3074 /// returned. If the iterator is empty, [`None`] is returned.
3079 /// let a = [-3_i32, 0, 1, 5, -10];
3080 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
3083 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3084 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3087 F: FnMut(&Self::Item) -> B,
3090 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3095 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3099 let (_, x) = self.map(key(f)).min_by(compare)?;
3103 /// Returns the element that gives the minimum value with respect to the
3104 /// specified comparison function.
3106 /// If several elements are equally minimum, the first element is
3107 /// returned. If the iterator is empty, [`None`] is returned.
3112 /// let a = [-3_i32, 0, 1, 5, -10];
3113 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
3116 #[stable(feature = "iter_min_by", since = "1.15.0")]
3117 fn min_by<F>(self, compare: F) -> Option<Self::Item>
3120 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3123 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3124 move |x, y| cmp::min_by(x, y, &mut compare)
3127 self.reduce(fold(compare))
3130 /// Reverses an iterator's direction.
3132 /// Usually, iterators iterate from left to right. After using `rev()`,
3133 /// an iterator will instead iterate from right to left.
3135 /// This is only possible if the iterator has an end, so `rev()` only
3136 /// works on [`DoubleEndedIterator`]s.
3141 /// let a = [1, 2, 3];
3143 /// let mut iter = a.iter().rev();
3145 /// assert_eq!(iter.next(), Some(&3));
3146 /// assert_eq!(iter.next(), Some(&2));
3147 /// assert_eq!(iter.next(), Some(&1));
3149 /// assert_eq!(iter.next(), None);
3152 #[doc(alias = "reverse")]
3153 #[stable(feature = "rust1", since = "1.0.0")]
3154 fn rev(self) -> Rev<Self>
3156 Self: Sized + DoubleEndedIterator,
3161 /// Converts an iterator of pairs into a pair of containers.
3163 /// `unzip()` consumes an entire iterator of pairs, producing two
3164 /// collections: one from the left elements of the pairs, and one
3165 /// from the right elements.
3167 /// This function is, in some sense, the opposite of [`zip`].
3169 /// [`zip`]: Iterator::zip
3176 /// let a = [(1, 2), (3, 4), (5, 6)];
3178 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
3180 /// assert_eq!(left, [1, 3, 5]);
3181 /// assert_eq!(right, [2, 4, 6]);
3183 /// // you can also unzip multiple nested tuples at once
3184 /// let a = [(1, (2, 3)), (4, (5, 6))];
3186 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
3187 /// assert_eq!(x, [1, 4]);
3188 /// assert_eq!(y, [2, 5]);
3189 /// assert_eq!(z, [3, 6]);
3191 #[stable(feature = "rust1", since = "1.0.0")]
3192 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3194 FromA: Default + Extend<A>,
3195 FromB: Default + Extend<B>,
3196 Self: Sized + Iterator<Item = (A, B)>,
3198 let mut unzipped: (FromA, FromB) = Default::default();
3199 unzipped.extend(self);
3203 /// Creates an iterator which copies all of its elements.
3205 /// This is useful when you have an iterator over `&T`, but you need an
3206 /// iterator over `T`.
3213 /// let a = [1, 2, 3];
3215 /// let v_copied: Vec<_> = a.iter().copied().collect();
3217 /// // copied is the same as .map(|&x| x)
3218 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3220 /// assert_eq!(v_copied, vec![1, 2, 3]);
3221 /// assert_eq!(v_map, vec![1, 2, 3]);
3223 #[stable(feature = "iter_copied", since = "1.36.0")]
3224 fn copied<'a, T: 'a>(self) -> Copied<Self>
3226 Self: Sized + Iterator<Item = &'a T>,
3232 /// Creates an iterator which [`clone`]s all of its elements.
3234 /// This is useful when you have an iterator over `&T`, but you need an
3235 /// iterator over `T`.
3237 /// There is no guarantee whatsoever about the `clone` method actually
3238 /// being called *or* optimized away. So code should not depend on
3241 /// [`clone`]: Clone::clone
3248 /// let a = [1, 2, 3];
3250 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3252 /// // cloned is the same as .map(|&x| x), for integers
3253 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3255 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3256 /// assert_eq!(v_map, vec![1, 2, 3]);
3259 /// To get the best performance, try to clone late:
3262 /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
3263 /// // don't do this:
3264 /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
3265 /// assert_eq!(&[vec![23]], &slower[..]);
3266 /// // instead call `cloned` late
3267 /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
3268 /// assert_eq!(&[vec![23]], &faster[..]);
3270 #[stable(feature = "rust1", since = "1.0.0")]
3271 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3273 Self: Sized + Iterator<Item = &'a T>,
3279 /// Repeats an iterator endlessly.
3281 /// Instead of stopping at [`None`], the iterator will instead start again,
3282 /// from the beginning. After iterating again, it will start at the
3283 /// beginning again. And again. And again. Forever. Note that in case the
3284 /// original iterator is empty, the resulting iterator will also be empty.
3291 /// let a = [1, 2, 3];
3293 /// let mut it = a.iter().cycle();
3295 /// assert_eq!(it.next(), Some(&1));
3296 /// assert_eq!(it.next(), Some(&2));
3297 /// assert_eq!(it.next(), Some(&3));
3298 /// assert_eq!(it.next(), Some(&1));
3299 /// assert_eq!(it.next(), Some(&2));
3300 /// assert_eq!(it.next(), Some(&3));
3301 /// assert_eq!(it.next(), Some(&1));
3303 #[stable(feature = "rust1", since = "1.0.0")]
3305 fn cycle(self) -> Cycle<Self>
3307 Self: Sized + Clone,
3312 /// Returns an iterator over `N` elements of the iterator at a time.
3314 /// The chunks do not overlap. If `N` does not divide the length of the
3315 /// iterator, then the last up to `N-1` elements will be omitted and can be
3316 /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
3317 /// function of the iterator.
3321 /// Panics if `N` is 0.
3328 /// #![feature(iter_array_chunks)]
3330 /// let mut iter = "lorem".chars().array_chunks();
3331 /// assert_eq!(iter.next(), Some(['l', 'o']));
3332 /// assert_eq!(iter.next(), Some(['r', 'e']));
3333 /// assert_eq!(iter.next(), None);
3334 /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
3338 /// #![feature(iter_array_chunks)]
3340 /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
3341 /// // ^-----^ ^------^
3342 /// for [x, y, z] in data.iter().array_chunks() {
3343 /// assert_eq!(x + y + z, 4);
3347 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
3348 fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
3352 ArrayChunks::new(self)
3355 /// Sums the elements of an iterator.
3357 /// Takes each element, adds them together, and returns the result.
3359 /// An empty iterator returns the zero value of the type.
3363 /// When calling `sum()` and a primitive integer type is being returned, this
3364 /// method will panic if the computation overflows and debug assertions are
3372 /// let a = [1, 2, 3];
3373 /// let sum: i32 = a.iter().sum();
3375 /// assert_eq!(sum, 6);
3377 #[stable(feature = "iter_arith", since = "1.11.0")]
3378 fn sum<S>(self) -> S
3386 /// Iterates over the entire iterator, multiplying all the elements
3388 /// An empty iterator returns the one value of the type.
3392 /// When calling `product()` and a primitive integer type is being returned,
3393 /// method will panic if the computation overflows and debug assertions are
3399 /// fn factorial(n: u32) -> u32 {
3400 /// (1..=n).product()
3402 /// assert_eq!(factorial(0), 1);
3403 /// assert_eq!(factorial(1), 1);
3404 /// assert_eq!(factorial(5), 120);
3406 #[stable(feature = "iter_arith", since = "1.11.0")]
3407 fn product<P>(self) -> P
3410 P: Product<Self::Item>,
3412 Product::product(self)
3415 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3421 /// use std::cmp::Ordering;
3423 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3424 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3425 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3427 #[stable(feature = "iter_order", since = "1.5.0")]
3428 fn cmp<I>(self, other: I) -> Ordering
3430 I: IntoIterator<Item = Self::Item>,
3434 self.cmp_by(other, |x, y| x.cmp(&y))
3437 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3438 /// of another with respect to the specified comparison function.
3445 /// #![feature(iter_order_by)]
3447 /// use std::cmp::Ordering;
3449 /// let xs = [1, 2, 3, 4];
3450 /// let ys = [1, 4, 9, 16];
3452 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3453 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3454 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3456 #[unstable(feature = "iter_order_by", issue = "64295")]
3457 fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
3461 F: FnMut(Self::Item, I::Item) -> Ordering,
3464 fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
3466 F: FnMut(X, Y) -> Ordering,
3468 move |x, y| match cmp(x, y) {
3469 Ordering::Equal => ControlFlow::CONTINUE,
3470 non_eq => ControlFlow::Break(non_eq),
3474 match iter_compare(self, other.into_iter(), compare(cmp)) {
3475 ControlFlow::Continue(ord) => ord,
3476 ControlFlow::Break(ord) => ord,
3480 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3486 /// use std::cmp::Ordering;
3488 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3489 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3490 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3492 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3494 #[stable(feature = "iter_order", since = "1.5.0")]
3495 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3498 Self::Item: PartialOrd<I::Item>,
3501 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3504 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3505 /// of another with respect to the specified comparison function.
3512 /// #![feature(iter_order_by)]
3514 /// use std::cmp::Ordering;
3516 /// let xs = [1.0, 2.0, 3.0, 4.0];
3517 /// let ys = [1.0, 4.0, 9.0, 16.0];
3520 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3521 /// Some(Ordering::Less)
3524 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3525 /// Some(Ordering::Equal)
3528 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3529 /// Some(Ordering::Greater)
3532 #[unstable(feature = "iter_order_by", issue = "64295")]
3533 fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
3537 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3540 fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
3542 F: FnMut(X, Y) -> Option<Ordering>,
3544 move |x, y| match partial_cmp(x, y) {
3545 Some(Ordering::Equal) => ControlFlow::CONTINUE,
3546 non_eq => ControlFlow::Break(non_eq),
3550 match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
3551 ControlFlow::Continue(ord) => Some(ord),
3552 ControlFlow::Break(ord) => ord,
3556 /// Determines if the elements of this [`Iterator`] are equal to those of
3562 /// assert_eq!([1].iter().eq([1].iter()), true);
3563 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3565 #[stable(feature = "iter_order", since = "1.5.0")]
3566 fn eq<I>(self, other: I) -> bool
3569 Self::Item: PartialEq<I::Item>,
3572 self.eq_by(other, |x, y| x == y)
3575 /// Determines if the elements of this [`Iterator`] are equal to those of
3576 /// another with respect to the specified equality function.
3583 /// #![feature(iter_order_by)]
3585 /// let xs = [1, 2, 3, 4];
3586 /// let ys = [1, 4, 9, 16];
3588 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3590 #[unstable(feature = "iter_order_by", issue = "64295")]
3591 fn eq_by<I, F>(self, other: I, eq: F) -> bool
3595 F: FnMut(Self::Item, I::Item) -> bool,
3598 fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
3600 F: FnMut(X, Y) -> bool,
3603 if eq(x, y) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
3607 match iter_compare(self, other.into_iter(), compare(eq)) {
3608 ControlFlow::Continue(ord) => ord == Ordering::Equal,
3609 ControlFlow::Break(()) => false,
3613 /// Determines if the elements of this [`Iterator`] are unequal to those of
3619 /// assert_eq!([1].iter().ne([1].iter()), false);
3620 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3622 #[stable(feature = "iter_order", since = "1.5.0")]
3623 fn ne<I>(self, other: I) -> bool
3626 Self::Item: PartialEq<I::Item>,
3632 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3633 /// less than those of another.
3638 /// assert_eq!([1].iter().lt([1].iter()), false);
3639 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3640 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3641 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3643 #[stable(feature = "iter_order", since = "1.5.0")]
3644 fn lt<I>(self, other: I) -> bool
3647 Self::Item: PartialOrd<I::Item>,
3650 self.partial_cmp(other) == Some(Ordering::Less)
3653 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3654 /// less or equal to those of another.
3659 /// assert_eq!([1].iter().le([1].iter()), true);
3660 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3661 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3662 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3664 #[stable(feature = "iter_order", since = "1.5.0")]
3665 fn le<I>(self, other: I) -> bool
3668 Self::Item: PartialOrd<I::Item>,
3671 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3674 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3675 /// greater than those of another.
3680 /// assert_eq!([1].iter().gt([1].iter()), false);
3681 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3682 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3683 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3685 #[stable(feature = "iter_order", since = "1.5.0")]
3686 fn gt<I>(self, other: I) -> bool
3689 Self::Item: PartialOrd<I::Item>,
3692 self.partial_cmp(other) == Some(Ordering::Greater)
3695 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3696 /// greater than or equal to those of another.
3701 /// assert_eq!([1].iter().ge([1].iter()), true);
3702 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3703 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3704 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3706 #[stable(feature = "iter_order", since = "1.5.0")]
3707 fn ge<I>(self, other: I) -> bool
3710 Self::Item: PartialOrd<I::Item>,
3713 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3716 /// Checks if the elements of this iterator are sorted.
3718 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3719 /// iterator yields exactly zero or one element, `true` is returned.
3721 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3722 /// implies that this function returns `false` if any two consecutive items are not
3728 /// #![feature(is_sorted)]
3730 /// assert!([1, 2, 2, 9].iter().is_sorted());
3731 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3732 /// assert!([0].iter().is_sorted());
3733 /// assert!(std::iter::empty::<i32>().is_sorted());
3734 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3737 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3738 fn is_sorted(self) -> bool
3741 Self::Item: PartialOrd,
3743 self.is_sorted_by(PartialOrd::partial_cmp)
3746 /// Checks if the elements of this iterator are sorted using the given comparator function.
3748 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3749 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3750 /// [`is_sorted`]; see its documentation for more information.
3755 /// #![feature(is_sorted)]
3757 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3758 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3759 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3760 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3761 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3764 /// [`is_sorted`]: Iterator::is_sorted
3765 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3766 fn is_sorted_by<F>(mut self, compare: F) -> bool
3769 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3774 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3775 ) -> impl FnMut(T) -> bool + 'a {
3777 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3785 let mut last = match self.next() {
3787 None => return true,
3790 self.all(check(&mut last, compare))
3793 /// Checks if the elements of this iterator are sorted using the given key extraction
3796 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3797 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3798 /// its documentation for more information.
3800 /// [`is_sorted`]: Iterator::is_sorted
3805 /// #![feature(is_sorted)]
3807 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3808 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3811 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3812 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3815 F: FnMut(Self::Item) -> K,
3818 self.map(f).is_sorted()
3821 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3822 // The unusual name is to avoid name collisions in method resolution
3826 #[unstable(feature = "trusted_random_access", issue = "none")]
3827 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3829 Self: TrustedRandomAccessNoCoerce,
3831 unreachable!("Always specialized");
3835 /// Compares two iterators element-wise using the given function.
3837 /// If `ControlFlow::CONTINUE` is returned from the function, the comparison moves on to the next
3838 /// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
3839 /// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
3840 /// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
3843 /// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
3844 /// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
3846 fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
3850 F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
3853 fn compare<'a, B, X, T>(
3855 mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
3856 ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
3860 move |x| match b.next() {
3861 None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
3862 Some(y) => f(x, y).map_break(ControlFlow::Break),
3866 match a.try_for_each(compare(&mut b, f)) {
3867 ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
3868 None => Ordering::Equal,
3869 Some(_) => Ordering::Less,
3871 ControlFlow::Break(x) => x,
3875 #[stable(feature = "rust1", since = "1.0.0")]
3876 impl<I: Iterator + ?Sized> Iterator for &mut I {
3877 type Item = I::Item;
3879 fn next(&mut self) -> Option<I::Item> {
3882 fn size_hint(&self) -> (usize, Option<usize>) {
3883 (**self).size_hint()
3885 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3886 (**self).advance_by(n)
3888 fn nth(&mut self, n: usize) -> Option<Self::Item> {