1 // ignore-tidy-filelength
2 // This file almost exclusively consists of the definition of `Iterator`. We
3 // can't split that into multiple files.
5 use crate::cmp::{self, Ordering};
6 use crate::ops::{Add, ControlFlow, Try};
8 use super::super::TrustedRandomAccess;
9 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
10 use super::super::{FlatMap, Flatten};
11 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
13 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
16 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
18 /// An interface for dealing with iterators.
20 /// This is the main iterator trait. For more about the concept of iterators
21 /// generally, please see the [module-level documentation]. In particular, you
22 /// may want to know how to [implement `Iterator`][impl].
24 /// [module-level documentation]: crate::iter
25 /// [impl]: crate::iter#implementing-iterator
26 #[stable(feature = "rust1", since = "1.0.0")]
27 #[rustc_on_unimplemented(
29 _Self = "[std::ops::Range<Idx>; 1]",
30 label = "if you meant to iterate between two values, remove the square brackets",
31 note = "`[start..end]` is an array of one `Range`; you might have meant to have a `Range` \
32 without the brackets: `start..end`"
35 _Self = "[std::ops::RangeFrom<Idx>; 1]",
36 label = "if you meant to iterate from a value onwards, remove the square brackets",
37 note = "`[start..]` is an array of one `RangeFrom`; you might have meant to have a \
38 `RangeFrom` without the brackets: `start..`, keeping in mind that iterating over an \
39 unbounded iterator will run forever unless you `break` or `return` from within the \
43 _Self = "[std::ops::RangeTo<Idx>; 1]",
44 label = "if you meant to iterate until a value, remove the square brackets and add a \
46 note = "`[..end]` is an array of one `RangeTo`; you might have meant to have a bounded \
47 `Range` without the brackets: `0..end`"
50 _Self = "[std::ops::RangeInclusive<Idx>; 1]",
51 label = "if you meant to iterate between two values, remove the square brackets",
52 note = "`[start..=end]` is an array of one `RangeInclusive`; you might have meant to have a \
53 `RangeInclusive` without the brackets: `start..=end`"
56 _Self = "[std::ops::RangeToInclusive<Idx>; 1]",
57 label = "if you meant to iterate until a value (including it), remove the square brackets \
58 and add a starting value",
59 note = "`[..=end]` is an array of one `RangeToInclusive`; you might have meant to have a \
60 bounded `RangeInclusive` without the brackets: `0..=end`"
63 _Self = "std::ops::RangeTo<Idx>",
64 label = "if you meant to iterate until a value, add a starting value",
65 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
66 bounded `Range`: `0..end`"
69 _Self = "std::ops::RangeToInclusive<Idx>",
70 label = "if you meant to iterate until a value (including it), add a starting value",
71 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
72 to have a bounded `RangeInclusive`: `0..=end`"
76 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
79 _Self = "std::string::String",
80 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
84 label = "borrow the array with `&` or call `.iter()` on it to iterate over it",
85 note = "arrays are not iterators, but slices like the following are: `&[1, 2, 3]`"
89 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
90 syntax `start..end` or the inclusive range syntax `start..=end`"
92 label = "`{Self}` is not an iterator",
93 message = "`{Self}` is not an iterator"
96 #[must_use = "iterators are lazy and do nothing unless consumed"]
98 /// The type of the elements being iterated over.
99 #[stable(feature = "rust1", since = "1.0.0")]
102 /// Advances the iterator and returns the next value.
104 /// Returns [`None`] when iteration is finished. Individual iterator
105 /// implementations may choose to resume iteration, and so calling `next()`
106 /// again may or may not eventually start returning [`Some(Item)`] again at some
109 /// [`Some(Item)`]: Some
116 /// let a = [1, 2, 3];
118 /// let mut iter = a.iter();
120 /// // A call to next() returns the next value...
121 /// assert_eq!(Some(&1), iter.next());
122 /// assert_eq!(Some(&2), iter.next());
123 /// assert_eq!(Some(&3), iter.next());
125 /// // ... and then None once it's over.
126 /// assert_eq!(None, iter.next());
128 /// // More calls may or may not return `None`. Here, they always will.
129 /// assert_eq!(None, iter.next());
130 /// assert_eq!(None, iter.next());
133 #[stable(feature = "rust1", since = "1.0.0")]
134 fn next(&mut self) -> Option<Self::Item>;
136 /// Returns the bounds on the remaining length of the iterator.
138 /// Specifically, `size_hint()` returns a tuple where the first element
139 /// is the lower bound, and the second element is the upper bound.
141 /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
142 /// A [`None`] here means that either there is no known upper bound, or the
143 /// upper bound is larger than [`usize`].
145 /// # Implementation notes
147 /// It is not enforced that an iterator implementation yields the declared
148 /// number of elements. A buggy iterator may yield less than the lower bound
149 /// or more than the upper bound of elements.
151 /// `size_hint()` is primarily intended to be used for optimizations such as
152 /// reserving space for the elements of the iterator, but must not be
153 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
154 /// implementation of `size_hint()` should not lead to memory safety
157 /// That said, the implementation should provide a correct estimation,
158 /// because otherwise it would be a violation of the trait's protocol.
160 /// The default implementation returns `(0, `[`None`]`)` which is correct for any
163 /// [`usize`]: type@usize
170 /// let a = [1, 2, 3];
171 /// let iter = a.iter();
173 /// assert_eq!((3, Some(3)), iter.size_hint());
176 /// A more complex example:
179 /// // The even numbers from zero to ten.
180 /// let iter = (0..10).filter(|x| x % 2 == 0);
182 /// // We might iterate from zero to ten times. Knowing that it's five
183 /// // exactly wouldn't be possible without executing filter().
184 /// assert_eq!((0, Some(10)), iter.size_hint());
186 /// // Let's add five more numbers with chain()
187 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
189 /// // now both bounds are increased by five
190 /// assert_eq!((5, Some(15)), iter.size_hint());
193 /// Returning `None` for an upper bound:
196 /// // an infinite iterator has no upper bound
197 /// // and the maximum possible lower bound
200 /// assert_eq!((usize::MAX, None), iter.size_hint());
203 #[stable(feature = "rust1", since = "1.0.0")]
204 fn size_hint(&self) -> (usize, Option<usize>) {
208 /// Consumes the iterator, counting the number of iterations and returning it.
210 /// This method will call [`next`] repeatedly until [`None`] is encountered,
211 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
212 /// called at least once even if the iterator does not have any elements.
214 /// [`next`]: Iterator::next
216 /// # Overflow Behavior
218 /// The method does no guarding against overflows, so counting elements of
219 /// an iterator with more than [`usize::MAX`] elements either produces the
220 /// wrong result or panics. If debug assertions are enabled, a panic is
225 /// This function might panic if the iterator has more than [`usize::MAX`]
228 /// [`usize::MAX`]: crate::usize::MAX
235 /// let a = [1, 2, 3];
236 /// assert_eq!(a.iter().count(), 3);
238 /// let a = [1, 2, 3, 4, 5];
239 /// assert_eq!(a.iter().count(), 5);
242 #[stable(feature = "rust1", since = "1.0.0")]
243 fn count(self) -> usize
248 fn add1<T>(count: usize, _: T) -> usize {
256 /// Consumes the iterator, returning the last element.
258 /// This method will evaluate the iterator until it returns [`None`]. While
259 /// doing so, it keeps track of the current element. After [`None`] is
260 /// returned, `last()` will then return the last element it saw.
267 /// let a = [1, 2, 3];
268 /// assert_eq!(a.iter().last(), Some(&3));
270 /// let a = [1, 2, 3, 4, 5];
271 /// assert_eq!(a.iter().last(), Some(&5));
274 #[stable(feature = "rust1", since = "1.0.0")]
275 fn last(self) -> Option<Self::Item>
280 fn some<T>(_: Option<T>, x: T) -> Option<T> {
284 self.fold(None, some)
287 /// Advances the iterator by `n` elements.
289 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
290 /// times until [`None`] is encountered.
292 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
293 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
294 /// of elements the iterator is advanced by before running out of elements (i.e. the
295 /// length of the iterator). Note that `k` is always less than `n`.
297 /// Calling `advance_by(0)` does not consume any elements and always returns [`Ok(())`][Ok].
299 /// [`next`]: Iterator::next
306 /// #![feature(iter_advance_by)]
308 /// let a = [1, 2, 3, 4];
309 /// let mut iter = a.iter();
311 /// assert_eq!(iter.advance_by(2), Ok(()));
312 /// assert_eq!(iter.next(), Some(&3));
313 /// assert_eq!(iter.advance_by(0), Ok(()));
314 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
317 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
318 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
320 self.next().ok_or(i)?;
325 /// Returns the `n`th element of the iterator.
327 /// Like most indexing operations, the count starts from zero, so `nth(0)`
328 /// returns the first value, `nth(1)` the second, and so on.
330 /// Note that all preceding elements, as well as the returned element, will be
331 /// consumed from the iterator. That means that the preceding elements will be
332 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
333 /// will return different elements.
335 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
343 /// let a = [1, 2, 3];
344 /// assert_eq!(a.iter().nth(1), Some(&2));
347 /// Calling `nth()` multiple times doesn't rewind the iterator:
350 /// let a = [1, 2, 3];
352 /// let mut iter = a.iter();
354 /// assert_eq!(iter.nth(1), Some(&2));
355 /// assert_eq!(iter.nth(1), None);
358 /// Returning `None` if there are less than `n + 1` elements:
361 /// let a = [1, 2, 3];
362 /// assert_eq!(a.iter().nth(10), None);
365 #[stable(feature = "rust1", since = "1.0.0")]
366 fn nth(&mut self, n: usize) -> Option<Self::Item> {
367 self.advance_by(n).ok()?;
371 /// Creates an iterator starting at the same point, but stepping by
372 /// the given amount at each iteration.
374 /// Note 1: The first element of the iterator will always be returned,
375 /// regardless of the step given.
377 /// Note 2: The time at which ignored elements are pulled is not fixed.
378 /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
379 /// but is also free to behave like the sequence
380 /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
381 /// Which way is used may change for some iterators for performance reasons.
382 /// The second way will advance the iterator earlier and may consume more items.
384 /// `advance_n_and_return_first` is the equivalent of:
386 /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
390 /// let next = iter.next();
391 /// if total_step > 1 {
392 /// iter.nth(total_step-2);
400 /// The method will panic if the given step is `0`.
407 /// let a = [0, 1, 2, 3, 4, 5];
408 /// let mut iter = a.iter().step_by(2);
410 /// assert_eq!(iter.next(), Some(&0));
411 /// assert_eq!(iter.next(), Some(&2));
412 /// assert_eq!(iter.next(), Some(&4));
413 /// assert_eq!(iter.next(), None);
416 #[stable(feature = "iterator_step_by", since = "1.28.0")]
417 fn step_by(self, step: usize) -> StepBy<Self>
421 StepBy::new(self, step)
424 /// Takes two iterators and creates a new iterator over both in sequence.
426 /// `chain()` will return a new iterator which will first iterate over
427 /// values from the first iterator and then over values from the second
430 /// In other words, it links two iterators together, in a chain. 🔗
432 /// [`once`] is commonly used to adapt a single value into a chain of
433 /// other kinds of iteration.
440 /// let a1 = [1, 2, 3];
441 /// let a2 = [4, 5, 6];
443 /// let mut iter = a1.iter().chain(a2.iter());
445 /// assert_eq!(iter.next(), Some(&1));
446 /// assert_eq!(iter.next(), Some(&2));
447 /// assert_eq!(iter.next(), Some(&3));
448 /// assert_eq!(iter.next(), Some(&4));
449 /// assert_eq!(iter.next(), Some(&5));
450 /// assert_eq!(iter.next(), Some(&6));
451 /// assert_eq!(iter.next(), None);
454 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
455 /// anything that can be converted into an [`Iterator`], not just an
456 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
457 /// [`IntoIterator`], and so can be passed to `chain()` directly:
460 /// let s1 = &[1, 2, 3];
461 /// let s2 = &[4, 5, 6];
463 /// let mut iter = s1.iter().chain(s2);
465 /// assert_eq!(iter.next(), Some(&1));
466 /// assert_eq!(iter.next(), Some(&2));
467 /// assert_eq!(iter.next(), Some(&3));
468 /// assert_eq!(iter.next(), Some(&4));
469 /// assert_eq!(iter.next(), Some(&5));
470 /// assert_eq!(iter.next(), Some(&6));
471 /// assert_eq!(iter.next(), None);
474 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
478 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
479 /// use std::os::windows::ffi::OsStrExt;
480 /// s.encode_wide().chain(std::iter::once(0)).collect()
484 /// [`once`]: crate::iter::once
485 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
487 #[stable(feature = "rust1", since = "1.0.0")]
488 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
491 U: IntoIterator<Item = Self::Item>,
493 Chain::new(self, other.into_iter())
496 /// 'Zips up' two iterators into a single iterator of pairs.
498 /// `zip()` returns a new iterator that will iterate over two other
499 /// iterators, returning a tuple where the first element comes from the
500 /// first iterator, and the second element comes from the second iterator.
502 /// In other words, it zips two iterators together, into a single one.
504 /// If either iterator returns [`None`], [`next`] from the zipped iterator
505 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
506 /// short-circuit and `next` will not be called on the second iterator.
513 /// let a1 = [1, 2, 3];
514 /// let a2 = [4, 5, 6];
516 /// let mut iter = a1.iter().zip(a2.iter());
518 /// assert_eq!(iter.next(), Some((&1, &4)));
519 /// assert_eq!(iter.next(), Some((&2, &5)));
520 /// assert_eq!(iter.next(), Some((&3, &6)));
521 /// assert_eq!(iter.next(), None);
524 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
525 /// anything that can be converted into an [`Iterator`], not just an
526 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
527 /// [`IntoIterator`], and so can be passed to `zip()` directly:
530 /// let s1 = &[1, 2, 3];
531 /// let s2 = &[4, 5, 6];
533 /// let mut iter = s1.iter().zip(s2);
535 /// assert_eq!(iter.next(), Some((&1, &4)));
536 /// assert_eq!(iter.next(), Some((&2, &5)));
537 /// assert_eq!(iter.next(), Some((&3, &6)));
538 /// assert_eq!(iter.next(), None);
541 /// `zip()` is often used to zip an infinite iterator to a finite one.
542 /// This works because the finite iterator will eventually return [`None`],
543 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
546 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
548 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
550 /// assert_eq!((0, 'f'), enumerate[0]);
551 /// assert_eq!((0, 'f'), zipper[0]);
553 /// assert_eq!((1, 'o'), enumerate[1]);
554 /// assert_eq!((1, 'o'), zipper[1]);
556 /// assert_eq!((2, 'o'), enumerate[2]);
557 /// assert_eq!((2, 'o'), zipper[2]);
560 /// [`enumerate`]: Iterator::enumerate
561 /// [`next`]: Iterator::next
563 #[stable(feature = "rust1", since = "1.0.0")]
564 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
569 Zip::new(self, other.into_iter())
572 /// Creates a new iterator which places a copy of `separator` between adjacent
573 /// items of the original iterator.
575 /// In case `separator` does not implement [`Clone`] or needs to be
576 /// computed every time, use [`intersperse_with`].
583 /// #![feature(iter_intersperse)]
585 /// let mut a = [0, 1, 2].iter().intersperse(&100);
586 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
587 /// assert_eq!(a.next(), Some(&100)); // The separator.
588 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
589 /// assert_eq!(a.next(), Some(&100)); // The separator.
590 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
591 /// assert_eq!(a.next(), None); // The iterator is finished.
594 /// `intersperse` can be very useful to join an iterator's items using a common element:
596 /// #![feature(iter_intersperse)]
598 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
599 /// assert_eq!(hello, "Hello World !");
602 /// [`Clone`]: crate::clone::Clone
603 /// [`intersperse_with`]: Iterator::intersperse_with
605 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
606 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
611 Intersperse::new(self, separator)
614 /// Creates a new iterator which places an item generated by `separator`
615 /// between adjacent items of the original iterator.
617 /// The closure will be called exactly once each time an item is placed
618 /// between two adjacent items from the underlying iterator; specifically,
619 /// the closure is not called if the underlying iterator yields less than
620 /// two items and after the last item is yielded.
622 /// If the iterator's item implements [`Clone`], it may be easier to use
630 /// #![feature(iter_intersperse)]
632 /// #[derive(PartialEq, Debug)]
633 /// struct NotClone(usize);
635 /// let v = vec![NotClone(0), NotClone(1), NotClone(2)];
636 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
638 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
639 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
640 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
641 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
642 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
643 /// assert_eq!(it.next(), None); // The iterator is finished.
646 /// `intersperse_with` can be used in situations where the separator needs
649 /// #![feature(iter_intersperse)]
651 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
653 /// // The closure mutably borrows it's context to generate an item.
654 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
655 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
657 /// let result = src.intersperse_with(separator).collect::<String>();
658 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
660 /// [`Clone`]: crate::clone::Clone
661 /// [`intersperse`]: Iterator::intersperse
663 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
664 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
667 G: FnMut() -> Self::Item,
669 IntersperseWith::new(self, separator)
672 /// Takes a closure and creates an iterator which calls that closure on each
675 /// `map()` transforms one iterator into another, by means of its argument:
676 /// something that implements [`FnMut`]. It produces a new iterator which
677 /// calls this closure on each element of the original iterator.
679 /// If you are good at thinking in types, you can think of `map()` like this:
680 /// If you have an iterator that gives you elements of some type `A`, and
681 /// you want an iterator of some other type `B`, you can use `map()`,
682 /// passing a closure that takes an `A` and returns a `B`.
684 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
685 /// lazy, it is best used when you're already working with other iterators.
686 /// If you're doing some sort of looping for a side effect, it's considered
687 /// more idiomatic to use [`for`] than `map()`.
689 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
690 /// [`FnMut`]: crate::ops::FnMut
697 /// let a = [1, 2, 3];
699 /// let mut iter = a.iter().map(|x| 2 * x);
701 /// assert_eq!(iter.next(), Some(2));
702 /// assert_eq!(iter.next(), Some(4));
703 /// assert_eq!(iter.next(), Some(6));
704 /// assert_eq!(iter.next(), None);
707 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
710 /// # #![allow(unused_must_use)]
711 /// // don't do this:
712 /// (0..5).map(|x| println!("{}", x));
714 /// // it won't even execute, as it is lazy. Rust will warn you about this.
716 /// // Instead, use for:
718 /// println!("{}", x);
722 #[stable(feature = "rust1", since = "1.0.0")]
723 fn map<B, F>(self, f: F) -> Map<Self, F>
726 F: FnMut(Self::Item) -> B,
731 /// Calls a closure on each element of an iterator.
733 /// This is equivalent to using a [`for`] loop on the iterator, although
734 /// `break` and `continue` are not possible from a closure. It's generally
735 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
736 /// when processing items at the end of longer iterator chains. In some
737 /// cases `for_each` may also be faster than a loop, because it will use
738 /// internal iteration on adaptors like `Chain`.
740 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
747 /// use std::sync::mpsc::channel;
749 /// let (tx, rx) = channel();
750 /// (0..5).map(|x| x * 2 + 1)
751 /// .for_each(move |x| tx.send(x).unwrap());
753 /// let v: Vec<_> = rx.iter().collect();
754 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
757 /// For such a small example, a `for` loop may be cleaner, but `for_each`
758 /// might be preferable to keep a functional style with longer iterators:
761 /// (0..5).flat_map(|x| x * 100 .. x * 110)
763 /// .filter(|&(i, x)| (i + x) % 3 == 0)
764 /// .for_each(|(i, x)| println!("{}:{}", i, x));
767 #[stable(feature = "iterator_for_each", since = "1.21.0")]
768 fn for_each<F>(self, f: F)
771 F: FnMut(Self::Item),
774 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
775 move |(), item| f(item)
778 self.fold((), call(f));
781 /// Creates an iterator which uses a closure to determine if an element
782 /// should be yielded.
784 /// Given an element the closure must return `true` or `false`. The returned
785 /// iterator will yield only the elements for which the closure returns
793 /// let a = [0i32, 1, 2];
795 /// let mut iter = a.iter().filter(|x| x.is_positive());
797 /// assert_eq!(iter.next(), Some(&1));
798 /// assert_eq!(iter.next(), Some(&2));
799 /// assert_eq!(iter.next(), None);
802 /// Because the closure passed to `filter()` takes a reference, and many
803 /// iterators iterate over references, this leads to a possibly confusing
804 /// situation, where the type of the closure is a double reference:
807 /// let a = [0, 1, 2];
809 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
811 /// assert_eq!(iter.next(), Some(&2));
812 /// assert_eq!(iter.next(), None);
815 /// It's common to instead use destructuring on the argument to strip away
819 /// let a = [0, 1, 2];
821 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
823 /// assert_eq!(iter.next(), Some(&2));
824 /// assert_eq!(iter.next(), None);
830 /// let a = [0, 1, 2];
832 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
834 /// assert_eq!(iter.next(), Some(&2));
835 /// assert_eq!(iter.next(), None);
840 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
842 #[stable(feature = "rust1", since = "1.0.0")]
843 fn filter<P>(self, predicate: P) -> Filter<Self, P>
846 P: FnMut(&Self::Item) -> bool,
848 Filter::new(self, predicate)
851 /// Creates an iterator that both filters and maps.
853 /// The returned iterator yields only the `value`s for which the supplied
854 /// closure returns `Some(value)`.
856 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
857 /// concise. The example below shows how a `map().filter().map()` can be
858 /// shortened to a single call to `filter_map`.
860 /// [`filter`]: Iterator::filter
861 /// [`map`]: Iterator::map
868 /// let a = ["1", "two", "NaN", "four", "5"];
870 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
872 /// assert_eq!(iter.next(), Some(1));
873 /// assert_eq!(iter.next(), Some(5));
874 /// assert_eq!(iter.next(), None);
877 /// Here's the same example, but with [`filter`] and [`map`]:
880 /// let a = ["1", "two", "NaN", "four", "5"];
881 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
882 /// assert_eq!(iter.next(), Some(1));
883 /// assert_eq!(iter.next(), Some(5));
884 /// assert_eq!(iter.next(), None);
887 #[stable(feature = "rust1", since = "1.0.0")]
888 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
891 F: FnMut(Self::Item) -> Option<B>,
893 FilterMap::new(self, f)
896 /// Creates an iterator which gives the current iteration count as well as
899 /// The iterator returned yields pairs `(i, val)`, where `i` is the
900 /// current index of iteration and `val` is the value returned by the
903 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
904 /// different sized integer, the [`zip`] function provides similar
907 /// # Overflow Behavior
909 /// The method does no guarding against overflows, so enumerating more than
910 /// [`usize::MAX`] elements either produces the wrong result or panics. If
911 /// debug assertions are enabled, a panic is guaranteed.
915 /// The returned iterator might panic if the to-be-returned index would
916 /// overflow a [`usize`].
918 /// [`usize`]: type@usize
919 /// [`usize::MAX`]: crate::usize::MAX
920 /// [`zip`]: Iterator::zip
925 /// let a = ['a', 'b', 'c'];
927 /// let mut iter = a.iter().enumerate();
929 /// assert_eq!(iter.next(), Some((0, &'a')));
930 /// assert_eq!(iter.next(), Some((1, &'b')));
931 /// assert_eq!(iter.next(), Some((2, &'c')));
932 /// assert_eq!(iter.next(), None);
935 #[stable(feature = "rust1", since = "1.0.0")]
936 fn enumerate(self) -> Enumerate<Self>
943 /// Creates an iterator which can use [`peek`] to look at the next element of
944 /// the iterator without consuming it.
946 /// Adds a [`peek`] method to an iterator. See its documentation for
947 /// more information.
949 /// Note that the underlying iterator is still advanced when [`peek`] is
950 /// called for the first time: In order to retrieve the next element,
951 /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
952 /// anything other than fetching the next value) of the [`next`] method
955 /// [`peek`]: Peekable::peek
956 /// [`next`]: Iterator::next
963 /// let xs = [1, 2, 3];
965 /// let mut iter = xs.iter().peekable();
967 /// // peek() lets us see into the future
968 /// assert_eq!(iter.peek(), Some(&&1));
969 /// assert_eq!(iter.next(), Some(&1));
971 /// assert_eq!(iter.next(), Some(&2));
973 /// // we can peek() multiple times, the iterator won't advance
974 /// assert_eq!(iter.peek(), Some(&&3));
975 /// assert_eq!(iter.peek(), Some(&&3));
977 /// assert_eq!(iter.next(), Some(&3));
979 /// // after the iterator is finished, so is peek()
980 /// assert_eq!(iter.peek(), None);
981 /// assert_eq!(iter.next(), None);
984 #[stable(feature = "rust1", since = "1.0.0")]
985 fn peekable(self) -> Peekable<Self>
992 /// Creates an iterator that [`skip`]s elements based on a predicate.
994 /// [`skip`]: Iterator::skip
996 /// `skip_while()` takes a closure as an argument. It will call this
997 /// closure on each element of the iterator, and ignore elements
998 /// until it returns `false`.
1000 /// After `false` is returned, `skip_while()`'s job is over, and the
1001 /// rest of the elements are yielded.
1008 /// let a = [-1i32, 0, 1];
1010 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1012 /// assert_eq!(iter.next(), Some(&0));
1013 /// assert_eq!(iter.next(), Some(&1));
1014 /// assert_eq!(iter.next(), None);
1017 /// Because the closure passed to `skip_while()` takes a reference, and many
1018 /// iterators iterate over references, this leads to a possibly confusing
1019 /// situation, where the type of the closure is a double reference:
1022 /// let a = [-1, 0, 1];
1024 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1026 /// assert_eq!(iter.next(), Some(&0));
1027 /// assert_eq!(iter.next(), Some(&1));
1028 /// assert_eq!(iter.next(), None);
1031 /// Stopping after an initial `false`:
1034 /// let a = [-1, 0, 1, -2];
1036 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1038 /// assert_eq!(iter.next(), Some(&0));
1039 /// assert_eq!(iter.next(), Some(&1));
1041 /// // while this would have been false, since we already got a false,
1042 /// // skip_while() isn't used any more
1043 /// assert_eq!(iter.next(), Some(&-2));
1045 /// assert_eq!(iter.next(), None);
1048 #[stable(feature = "rust1", since = "1.0.0")]
1049 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1052 P: FnMut(&Self::Item) -> bool,
1054 SkipWhile::new(self, predicate)
1057 /// Creates an iterator that yields elements based on a predicate.
1059 /// `take_while()` takes a closure as an argument. It will call this
1060 /// closure on each element of the iterator, and yield elements
1061 /// while it returns `true`.
1063 /// After `false` is returned, `take_while()`'s job is over, and the
1064 /// rest of the elements are ignored.
1071 /// let a = [-1i32, 0, 1];
1073 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1075 /// assert_eq!(iter.next(), Some(&-1));
1076 /// assert_eq!(iter.next(), None);
1079 /// Because the closure passed to `take_while()` takes a reference, and many
1080 /// iterators iterate over references, this leads to a possibly confusing
1081 /// situation, where the type of the closure is a double reference:
1084 /// let a = [-1, 0, 1];
1086 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1088 /// assert_eq!(iter.next(), Some(&-1));
1089 /// assert_eq!(iter.next(), None);
1092 /// Stopping after an initial `false`:
1095 /// let a = [-1, 0, 1, -2];
1097 /// let mut iter = a.iter().take_while(|x| **x < 0);
1099 /// assert_eq!(iter.next(), Some(&-1));
1101 /// // We have more elements that are less than zero, but since we already
1102 /// // got a false, take_while() isn't used any more
1103 /// assert_eq!(iter.next(), None);
1106 /// Because `take_while()` needs to look at the value in order to see if it
1107 /// should be included or not, consuming iterators will see that it is
1111 /// let a = [1, 2, 3, 4];
1112 /// let mut iter = a.iter();
1114 /// let result: Vec<i32> = iter.by_ref()
1115 /// .take_while(|n| **n != 3)
1119 /// assert_eq!(result, &[1, 2]);
1121 /// let result: Vec<i32> = iter.cloned().collect();
1123 /// assert_eq!(result, &[4]);
1126 /// The `3` is no longer there, because it was consumed in order to see if
1127 /// the iteration should stop, but wasn't placed back into the iterator.
1129 #[stable(feature = "rust1", since = "1.0.0")]
1130 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1133 P: FnMut(&Self::Item) -> bool,
1135 TakeWhile::new(self, predicate)
1138 /// Creates an iterator that both yields elements based on a predicate and maps.
1140 /// `map_while()` takes a closure as an argument. It will call this
1141 /// closure on each element of the iterator, and yield elements
1142 /// while it returns [`Some(_)`][`Some`].
1149 /// #![feature(iter_map_while)]
1150 /// let a = [-1i32, 4, 0, 1];
1152 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1154 /// assert_eq!(iter.next(), Some(-16));
1155 /// assert_eq!(iter.next(), Some(4));
1156 /// assert_eq!(iter.next(), None);
1159 /// Here's the same example, but with [`take_while`] and [`map`]:
1161 /// [`take_while`]: Iterator::take_while
1162 /// [`map`]: Iterator::map
1165 /// let a = [-1i32, 4, 0, 1];
1167 /// let mut iter = a.iter()
1168 /// .map(|x| 16i32.checked_div(*x))
1169 /// .take_while(|x| x.is_some())
1170 /// .map(|x| x.unwrap());
1172 /// assert_eq!(iter.next(), Some(-16));
1173 /// assert_eq!(iter.next(), Some(4));
1174 /// assert_eq!(iter.next(), None);
1177 /// Stopping after an initial [`None`]:
1180 /// #![feature(iter_map_while)]
1181 /// use std::convert::TryFrom;
1183 /// let a = [0, 1, 2, -3, 4, 5, -6];
1185 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1186 /// let vec = iter.collect::<Vec<_>>();
1188 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1189 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1190 /// assert_eq!(vec, vec![0, 1, 2]);
1193 /// Because `map_while()` needs to look at the value in order to see if it
1194 /// should be included or not, consuming iterators will see that it is
1198 /// #![feature(iter_map_while)]
1199 /// use std::convert::TryFrom;
1201 /// let a = [1, 2, -3, 4];
1202 /// let mut iter = a.iter();
1204 /// let result: Vec<u32> = iter.by_ref()
1205 /// .map_while(|n| u32::try_from(*n).ok())
1208 /// assert_eq!(result, &[1, 2]);
1210 /// let result: Vec<i32> = iter.cloned().collect();
1212 /// assert_eq!(result, &[4]);
1215 /// The `-3` is no longer there, because it was consumed in order to see if
1216 /// the iteration should stop, but wasn't placed back into the iterator.
1218 /// Note that unlike [`take_while`] this iterator is **not** fused.
1219 /// It is also not specified what this iterator returns after the first` None` is returned.
1220 /// If you need fused iterator, use [`fuse`].
1222 /// [`fuse`]: Iterator::fuse
1224 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1225 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1228 P: FnMut(Self::Item) -> Option<B>,
1230 MapWhile::new(self, predicate)
1233 /// Creates an iterator that skips the first `n` elements.
1235 /// After they have been consumed, the rest of the elements are yielded.
1236 /// Rather than overriding this method directly, instead override the `nth` method.
1243 /// let a = [1, 2, 3];
1245 /// let mut iter = a.iter().skip(2);
1247 /// assert_eq!(iter.next(), Some(&3));
1248 /// assert_eq!(iter.next(), None);
1251 #[stable(feature = "rust1", since = "1.0.0")]
1252 fn skip(self, n: usize) -> Skip<Self>
1259 /// Creates an iterator that yields its first `n` elements.
1266 /// let a = [1, 2, 3];
1268 /// let mut iter = a.iter().take(2);
1270 /// assert_eq!(iter.next(), Some(&1));
1271 /// assert_eq!(iter.next(), Some(&2));
1272 /// assert_eq!(iter.next(), None);
1275 /// `take()` is often used with an infinite iterator, to make it finite:
1278 /// let mut iter = (0..).take(3);
1280 /// assert_eq!(iter.next(), Some(0));
1281 /// assert_eq!(iter.next(), Some(1));
1282 /// assert_eq!(iter.next(), Some(2));
1283 /// assert_eq!(iter.next(), None);
1286 /// If less than `n` elements are available,
1287 /// `take` will limit itself to the size of the underlying iterator:
1290 /// let v = vec![1, 2];
1291 /// let mut iter = v.into_iter().take(5);
1292 /// assert_eq!(iter.next(), Some(1));
1293 /// assert_eq!(iter.next(), Some(2));
1294 /// assert_eq!(iter.next(), None);
1297 #[stable(feature = "rust1", since = "1.0.0")]
1298 fn take(self, n: usize) -> Take<Self>
1305 /// An iterator adaptor similar to [`fold`] that holds internal state and
1306 /// produces a new iterator.
1308 /// [`fold`]: Iterator::fold
1310 /// `scan()` takes two arguments: an initial value which seeds the internal
1311 /// state, and a closure with two arguments, the first being a mutable
1312 /// reference to the internal state and the second an iterator element.
1313 /// The closure can assign to the internal state to share state between
1316 /// On iteration, the closure will be applied to each element of the
1317 /// iterator and the return value from the closure, an [`Option`], is
1318 /// yielded by the iterator.
1325 /// let a = [1, 2, 3];
1327 /// let mut iter = a.iter().scan(1, |state, &x| {
1328 /// // each iteration, we'll multiply the state by the element
1329 /// *state = *state * x;
1331 /// // then, we'll yield the negation of the state
1335 /// assert_eq!(iter.next(), Some(-1));
1336 /// assert_eq!(iter.next(), Some(-2));
1337 /// assert_eq!(iter.next(), Some(-6));
1338 /// assert_eq!(iter.next(), None);
1341 #[stable(feature = "rust1", since = "1.0.0")]
1342 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1345 F: FnMut(&mut St, Self::Item) -> Option<B>,
1347 Scan::new(self, initial_state, f)
1350 /// Creates an iterator that works like map, but flattens nested structure.
1352 /// The [`map`] adapter is very useful, but only when the closure
1353 /// argument produces values. If it produces an iterator instead, there's
1354 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1357 /// You can think of `flat_map(f)` as the semantic equivalent
1358 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1360 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1361 /// one item for each element, and `flat_map()`'s closure returns an
1362 /// iterator for each element.
1364 /// [`map`]: Iterator::map
1365 /// [`flatten`]: Iterator::flatten
1372 /// let words = ["alpha", "beta", "gamma"];
1374 /// // chars() returns an iterator
1375 /// let merged: String = words.iter()
1376 /// .flat_map(|s| s.chars())
1378 /// assert_eq!(merged, "alphabetagamma");
1381 #[stable(feature = "rust1", since = "1.0.0")]
1382 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1386 F: FnMut(Self::Item) -> U,
1388 FlatMap::new(self, f)
1391 /// Creates an iterator that flattens nested structure.
1393 /// This is useful when you have an iterator of iterators or an iterator of
1394 /// things that can be turned into iterators and you want to remove one
1395 /// level of indirection.
1402 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1403 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1404 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1407 /// Mapping and then flattening:
1410 /// let words = ["alpha", "beta", "gamma"];
1412 /// // chars() returns an iterator
1413 /// let merged: String = words.iter()
1414 /// .map(|s| s.chars())
1417 /// assert_eq!(merged, "alphabetagamma");
1420 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1421 /// in this case since it conveys intent more clearly:
1424 /// let words = ["alpha", "beta", "gamma"];
1426 /// // chars() returns an iterator
1427 /// let merged: String = words.iter()
1428 /// .flat_map(|s| s.chars())
1430 /// assert_eq!(merged, "alphabetagamma");
1433 /// Flattening only removes one level of nesting at a time:
1436 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1438 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1439 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1441 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1442 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1445 /// Here we see that `flatten()` does not perform a "deep" flatten.
1446 /// Instead, only one level of nesting is removed. That is, if you
1447 /// `flatten()` a three-dimensional array, the result will be
1448 /// two-dimensional and not one-dimensional. To get a one-dimensional
1449 /// structure, you have to `flatten()` again.
1451 /// [`flat_map()`]: Iterator::flat_map
1453 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1454 fn flatten(self) -> Flatten<Self>
1457 Self::Item: IntoIterator,
1462 /// Creates an iterator which ends after the first [`None`].
1464 /// After an iterator returns [`None`], future calls may or may not yield
1465 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1466 /// [`None`] is given, it will always return [`None`] forever.
1468 /// [`Some(T)`]: Some
1475 /// // an iterator which alternates between Some and None
1476 /// struct Alternate {
1480 /// impl Iterator for Alternate {
1481 /// type Item = i32;
1483 /// fn next(&mut self) -> Option<i32> {
1484 /// let val = self.state;
1485 /// self.state = self.state + 1;
1487 /// // if it's even, Some(i32), else None
1488 /// if val % 2 == 0 {
1496 /// let mut iter = Alternate { state: 0 };
1498 /// // we can see our iterator going back and forth
1499 /// assert_eq!(iter.next(), Some(0));
1500 /// assert_eq!(iter.next(), None);
1501 /// assert_eq!(iter.next(), Some(2));
1502 /// assert_eq!(iter.next(), None);
1504 /// // however, once we fuse it...
1505 /// let mut iter = iter.fuse();
1507 /// assert_eq!(iter.next(), Some(4));
1508 /// assert_eq!(iter.next(), None);
1510 /// // it will always return `None` after the first time.
1511 /// assert_eq!(iter.next(), None);
1512 /// assert_eq!(iter.next(), None);
1513 /// assert_eq!(iter.next(), None);
1516 #[stable(feature = "rust1", since = "1.0.0")]
1517 fn fuse(self) -> Fuse<Self>
1524 /// Does something with each element of an iterator, passing the value on.
1526 /// When using iterators, you'll often chain several of them together.
1527 /// While working on such code, you might want to check out what's
1528 /// happening at various parts in the pipeline. To do that, insert
1529 /// a call to `inspect()`.
1531 /// It's more common for `inspect()` to be used as a debugging tool than to
1532 /// exist in your final code, but applications may find it useful in certain
1533 /// situations when errors need to be logged before being discarded.
1540 /// let a = [1, 4, 2, 3];
1542 /// // this iterator sequence is complex.
1543 /// let sum = a.iter()
1545 /// .filter(|x| x % 2 == 0)
1546 /// .fold(0, |sum, i| sum + i);
1548 /// println!("{}", sum);
1550 /// // let's add some inspect() calls to investigate what's happening
1551 /// let sum = a.iter()
1553 /// .inspect(|x| println!("about to filter: {}", x))
1554 /// .filter(|x| x % 2 == 0)
1555 /// .inspect(|x| println!("made it through filter: {}", x))
1556 /// .fold(0, |sum, i| sum + i);
1558 /// println!("{}", sum);
1561 /// This will print:
1565 /// about to filter: 1
1566 /// about to filter: 4
1567 /// made it through filter: 4
1568 /// about to filter: 2
1569 /// made it through filter: 2
1570 /// about to filter: 3
1574 /// Logging errors before discarding them:
1577 /// let lines = ["1", "2", "a"];
1579 /// let sum: i32 = lines
1581 /// .map(|line| line.parse::<i32>())
1582 /// .inspect(|num| {
1583 /// if let Err(ref e) = *num {
1584 /// println!("Parsing error: {}", e);
1587 /// .filter_map(Result::ok)
1590 /// println!("Sum: {}", sum);
1593 /// This will print:
1596 /// Parsing error: invalid digit found in string
1600 #[stable(feature = "rust1", since = "1.0.0")]
1601 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1604 F: FnMut(&Self::Item),
1606 Inspect::new(self, f)
1609 /// Borrows an iterator, rather than consuming it.
1611 /// This is useful to allow applying iterator adaptors while still
1612 /// retaining ownership of the original iterator.
1619 /// let a = [1, 2, 3];
1621 /// let iter = a.iter();
1623 /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
1625 /// assert_eq!(sum, 6);
1627 /// // if we try to use iter again, it won't work. The following line
1628 /// // gives "error: use of moved value: `iter`
1629 /// // assert_eq!(iter.next(), None);
1631 /// // let's try that again
1632 /// let a = [1, 2, 3];
1634 /// let mut iter = a.iter();
1636 /// // instead, we add in a .by_ref()
1637 /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
1639 /// assert_eq!(sum, 3);
1641 /// // now this is just fine:
1642 /// assert_eq!(iter.next(), Some(&3));
1643 /// assert_eq!(iter.next(), None);
1645 #[stable(feature = "rust1", since = "1.0.0")]
1646 fn by_ref(&mut self) -> &mut Self
1653 /// Transforms an iterator into a collection.
1655 /// `collect()` can take anything iterable, and turn it into a relevant
1656 /// collection. This is one of the more powerful methods in the standard
1657 /// library, used in a variety of contexts.
1659 /// The most basic pattern in which `collect()` is used is to turn one
1660 /// collection into another. You take a collection, call [`iter`] on it,
1661 /// do a bunch of transformations, and then `collect()` at the end.
1663 /// `collect()` can also create instances of types that are not typical
1664 /// collections. For example, a [`String`] can be built from [`char`]s,
1665 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1666 /// into `Result<Collection<T>, E>`. See the examples below for more.
1668 /// Because `collect()` is so general, it can cause problems with type
1669 /// inference. As such, `collect()` is one of the few times you'll see
1670 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1671 /// helps the inference algorithm understand specifically which collection
1672 /// you're trying to collect into.
1679 /// let a = [1, 2, 3];
1681 /// let doubled: Vec<i32> = a.iter()
1682 /// .map(|&x| x * 2)
1685 /// assert_eq!(vec![2, 4, 6], doubled);
1688 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1689 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1691 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1694 /// use std::collections::VecDeque;
1696 /// let a = [1, 2, 3];
1698 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1700 /// assert_eq!(2, doubled[0]);
1701 /// assert_eq!(4, doubled[1]);
1702 /// assert_eq!(6, doubled[2]);
1705 /// Using the 'turbofish' instead of annotating `doubled`:
1708 /// let a = [1, 2, 3];
1710 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1712 /// assert_eq!(vec![2, 4, 6], doubled);
1715 /// Because `collect()` only cares about what you're collecting into, you can
1716 /// still use a partial type hint, `_`, with the turbofish:
1719 /// let a = [1, 2, 3];
1721 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1723 /// assert_eq!(vec![2, 4, 6], doubled);
1726 /// Using `collect()` to make a [`String`]:
1729 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1731 /// let hello: String = chars.iter()
1732 /// .map(|&x| x as u8)
1733 /// .map(|x| (x + 1) as char)
1736 /// assert_eq!("hello", hello);
1739 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1740 /// see if any of them failed:
1743 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1745 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1747 /// // gives us the first error
1748 /// assert_eq!(Err("nope"), result);
1750 /// let results = [Ok(1), Ok(3)];
1752 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1754 /// // gives us the list of answers
1755 /// assert_eq!(Ok(vec![1, 3]), result);
1758 /// [`iter`]: Iterator::next
1759 /// [`String`]: ../../std/string/struct.String.html
1760 /// [`char`]: type@char
1762 #[stable(feature = "rust1", since = "1.0.0")]
1763 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1764 fn collect<B: FromIterator<Self::Item>>(self) -> B
1768 FromIterator::from_iter(self)
1771 /// Consumes an iterator, creating two collections from it.
1773 /// The predicate passed to `partition()` can return `true`, or `false`.
1774 /// `partition()` returns a pair, all of the elements for which it returned
1775 /// `true`, and all of the elements for which it returned `false`.
1777 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1779 /// [`is_partitioned()`]: Iterator::is_partitioned
1780 /// [`partition_in_place()`]: Iterator::partition_in_place
1787 /// let a = [1, 2, 3];
1789 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1791 /// .partition(|&n| n % 2 == 0);
1793 /// assert_eq!(even, vec![2]);
1794 /// assert_eq!(odd, vec![1, 3]);
1796 #[stable(feature = "rust1", since = "1.0.0")]
1797 fn partition<B, F>(self, f: F) -> (B, B)
1800 B: Default + Extend<Self::Item>,
1801 F: FnMut(&Self::Item) -> bool,
1804 fn extend<'a, T, B: Extend<T>>(
1805 mut f: impl FnMut(&T) -> bool + 'a,
1808 ) -> impl FnMut((), T) + 'a {
1813 right.extend_one(x);
1818 let mut left: B = Default::default();
1819 let mut right: B = Default::default();
1821 self.fold((), extend(f, &mut left, &mut right));
1826 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1827 /// such that all those that return `true` precede all those that return `false`.
1828 /// Returns the number of `true` elements found.
1830 /// The relative order of partitioned items is not maintained.
1832 /// See also [`is_partitioned()`] and [`partition()`].
1834 /// [`is_partitioned()`]: Iterator::is_partitioned
1835 /// [`partition()`]: Iterator::partition
1840 /// #![feature(iter_partition_in_place)]
1842 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1844 /// // Partition in-place between evens and odds
1845 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1847 /// assert_eq!(i, 3);
1848 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1849 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1851 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1852 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1854 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1855 P: FnMut(&T) -> bool,
1857 // FIXME: should we worry about the count overflowing? The only way to have more than
1858 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1860 // These closure "factory" functions exist to avoid genericity in `Self`.
1864 predicate: &'a mut impl FnMut(&T) -> bool,
1865 true_count: &'a mut usize,
1866 ) -> impl FnMut(&&mut T) -> bool + 'a {
1868 let p = predicate(&**x);
1869 *true_count += p as usize;
1875 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1876 move |x| predicate(&**x)
1879 // Repeatedly find the first `false` and swap it with the last `true`.
1880 let mut true_count = 0;
1881 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1882 if let Some(tail) = self.rfind(is_true(predicate)) {
1883 crate::mem::swap(head, tail);
1892 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1893 /// such that all those that return `true` precede all those that return `false`.
1895 /// See also [`partition()`] and [`partition_in_place()`].
1897 /// [`partition()`]: Iterator::partition
1898 /// [`partition_in_place()`]: Iterator::partition_in_place
1903 /// #![feature(iter_is_partitioned)]
1905 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1906 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1908 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1909 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1912 P: FnMut(Self::Item) -> bool,
1914 // Either all items test `true`, or the first clause stops at `false`
1915 // and we check that there are no more `true` items after that.
1916 self.all(&mut predicate) || !self.any(predicate)
1919 /// An iterator method that applies a function as long as it returns
1920 /// successfully, producing a single, final value.
1922 /// `try_fold()` takes two arguments: an initial value, and a closure with
1923 /// two arguments: an 'accumulator', and an element. The closure either
1924 /// returns successfully, with the value that the accumulator should have
1925 /// for the next iteration, or it returns failure, with an error value that
1926 /// is propagated back to the caller immediately (short-circuiting).
1928 /// The initial value is the value the accumulator will have on the first
1929 /// call. If applying the closure succeeded against every element of the
1930 /// iterator, `try_fold()` returns the final accumulator as success.
1932 /// Folding is useful whenever you have a collection of something, and want
1933 /// to produce a single value from it.
1935 /// # Note to Implementors
1937 /// Several of the other (forward) methods have default implementations in
1938 /// terms of this one, so try to implement this explicitly if it can
1939 /// do something better than the default `for` loop implementation.
1941 /// In particular, try to have this call `try_fold()` on the internal parts
1942 /// from which this iterator is composed. If multiple calls are needed,
1943 /// the `?` operator may be convenient for chaining the accumulator value
1944 /// along, but beware any invariants that need to be upheld before those
1945 /// early returns. This is a `&mut self` method, so iteration needs to be
1946 /// resumable after hitting an error here.
1953 /// let a = [1, 2, 3];
1955 /// // the checked sum of all of the elements of the array
1956 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1958 /// assert_eq!(sum, Some(6));
1961 /// Short-circuiting:
1964 /// let a = [10, 20, 30, 100, 40, 50];
1965 /// let mut it = a.iter();
1967 /// // This sum overflows when adding the 100 element
1968 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1969 /// assert_eq!(sum, None);
1971 /// // Because it short-circuited, the remaining elements are still
1972 /// // available through the iterator.
1973 /// assert_eq!(it.len(), 2);
1974 /// assert_eq!(it.next(), Some(&40));
1977 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1978 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1981 F: FnMut(B, Self::Item) -> R,
1984 let mut accum = init;
1985 while let Some(x) = self.next() {
1986 accum = f(accum, x)?;
1991 /// An iterator method that applies a fallible function to each item in the
1992 /// iterator, stopping at the first error and returning that error.
1994 /// This can also be thought of as the fallible form of [`for_each()`]
1995 /// or as the stateless version of [`try_fold()`].
1997 /// [`for_each()`]: Iterator::for_each
1998 /// [`try_fold()`]: Iterator::try_fold
2003 /// use std::fs::rename;
2004 /// use std::io::{stdout, Write};
2005 /// use std::path::Path;
2007 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2009 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2010 /// assert!(res.is_ok());
2012 /// let mut it = data.iter().cloned();
2013 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2014 /// assert!(res.is_err());
2015 /// // It short-circuited, so the remaining items are still in the iterator:
2016 /// assert_eq!(it.next(), Some("stale_bread.json"));
2019 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2020 fn try_for_each<F, R>(&mut self, f: F) -> R
2023 F: FnMut(Self::Item) -> R,
2027 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2031 self.try_fold((), call(f))
2034 /// An iterator method that applies a function, producing a single, final value.
2036 /// `fold()` takes two arguments: an initial value, and a closure with two
2037 /// arguments: an 'accumulator', and an element. The closure returns the value that
2038 /// the accumulator should have for the next iteration.
2040 /// The initial value is the value the accumulator will have on the first
2043 /// After applying this closure to every element of the iterator, `fold()`
2044 /// returns the accumulator.
2046 /// This operation is sometimes called 'reduce' or 'inject'.
2048 /// Folding is useful whenever you have a collection of something, and want
2049 /// to produce a single value from it.
2051 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2052 /// may not terminate for infinite iterators, even on traits for which a
2053 /// result is determinable in finite time.
2055 /// # Note to Implementors
2057 /// Several of the other (forward) methods have default implementations in
2058 /// terms of this one, so try to implement this explicitly if it can
2059 /// do something better than the default `for` loop implementation.
2061 /// In particular, try to have this call `fold()` on the internal parts
2062 /// from which this iterator is composed.
2069 /// let a = [1, 2, 3];
2071 /// // the sum of all of the elements of the array
2072 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2074 /// assert_eq!(sum, 6);
2077 /// Let's walk through each step of the iteration here:
2079 /// | element | acc | x | result |
2080 /// |---------|-----|---|--------|
2082 /// | 1 | 0 | 1 | 1 |
2083 /// | 2 | 1 | 2 | 3 |
2084 /// | 3 | 3 | 3 | 6 |
2086 /// And so, our final result, `6`.
2088 /// It's common for people who haven't used iterators a lot to
2089 /// use a `for` loop with a list of things to build up a result. Those
2090 /// can be turned into `fold()`s:
2092 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2095 /// let numbers = [1, 2, 3, 4, 5];
2097 /// let mut result = 0;
2100 /// for i in &numbers {
2101 /// result = result + i;
2105 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2107 /// // they're the same
2108 /// assert_eq!(result, result2);
2110 #[doc(alias = "reduce")]
2111 #[doc(alias = "inject")]
2113 #[stable(feature = "rust1", since = "1.0.0")]
2114 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2117 F: FnMut(B, Self::Item) -> B,
2119 let mut accum = init;
2120 while let Some(x) = self.next() {
2121 accum = f(accum, x);
2126 /// The same as [`fold()`], but uses the first element in the
2127 /// iterator as the initial value, folding every subsequent element into it.
2128 /// If the iterator is empty, return [`None`]; otherwise, return the result
2131 /// [`fold()`]: Iterator::fold
2135 /// Find the maximum value:
2138 /// #![feature(iterator_fold_self)]
2140 /// fn find_max<I>(iter: I) -> Option<I::Item>
2141 /// where I: Iterator,
2144 /// iter.fold_first(|a, b| {
2145 /// if a >= b { a } else { b }
2148 /// let a = [10, 20, 5, -23, 0];
2149 /// let b: [u32; 0] = [];
2151 /// assert_eq!(find_max(a.iter()), Some(&20));
2152 /// assert_eq!(find_max(b.iter()), None);
2155 #[unstable(feature = "iterator_fold_self", issue = "68125")]
2156 fn fold_first<F>(mut self, f: F) -> Option<Self::Item>
2159 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2161 let first = self.next()?;
2162 Some(self.fold(first, f))
2165 /// Tests if every element of the iterator matches a predicate.
2167 /// `all()` takes a closure that returns `true` or `false`. It applies
2168 /// this closure to each element of the iterator, and if they all return
2169 /// `true`, then so does `all()`. If any of them return `false`, it
2170 /// returns `false`.
2172 /// `all()` is short-circuiting; in other words, it will stop processing
2173 /// as soon as it finds a `false`, given that no matter what else happens,
2174 /// the result will also be `false`.
2176 /// An empty iterator returns `true`.
2183 /// let a = [1, 2, 3];
2185 /// assert!(a.iter().all(|&x| x > 0));
2187 /// assert!(!a.iter().all(|&x| x > 2));
2190 /// Stopping at the first `false`:
2193 /// let a = [1, 2, 3];
2195 /// let mut iter = a.iter();
2197 /// assert!(!iter.all(|&x| x != 2));
2199 /// // we can still use `iter`, as there are more elements.
2200 /// assert_eq!(iter.next(), Some(&3));
2203 #[stable(feature = "rust1", since = "1.0.0")]
2204 fn all<F>(&mut self, f: F) -> bool
2207 F: FnMut(Self::Item) -> bool,
2210 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2212 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2215 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2218 /// Tests if any element of the iterator matches a predicate.
2220 /// `any()` takes a closure that returns `true` or `false`. It applies
2221 /// this closure to each element of the iterator, and if any of them return
2222 /// `true`, then so does `any()`. If they all return `false`, it
2223 /// returns `false`.
2225 /// `any()` is short-circuiting; in other words, it will stop processing
2226 /// as soon as it finds a `true`, given that no matter what else happens,
2227 /// the result will also be `true`.
2229 /// An empty iterator returns `false`.
2236 /// let a = [1, 2, 3];
2238 /// assert!(a.iter().any(|&x| x > 0));
2240 /// assert!(!a.iter().any(|&x| x > 5));
2243 /// Stopping at the first `true`:
2246 /// let a = [1, 2, 3];
2248 /// let mut iter = a.iter();
2250 /// assert!(iter.any(|&x| x != 2));
2252 /// // we can still use `iter`, as there are more elements.
2253 /// assert_eq!(iter.next(), Some(&2));
2256 #[stable(feature = "rust1", since = "1.0.0")]
2257 fn any<F>(&mut self, f: F) -> bool
2260 F: FnMut(Self::Item) -> bool,
2263 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2265 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2269 self.try_fold((), check(f)) == ControlFlow::BREAK
2272 /// Searches for an element of an iterator that satisfies a predicate.
2274 /// `find()` takes a closure that returns `true` or `false`. It applies
2275 /// this closure to each element of the iterator, and if any of them return
2276 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2277 /// `false`, it returns [`None`].
2279 /// `find()` is short-circuiting; in other words, it will stop processing
2280 /// as soon as the closure returns `true`.
2282 /// Because `find()` takes a reference, and many iterators iterate over
2283 /// references, this leads to a possibly confusing situation where the
2284 /// argument is a double reference. You can see this effect in the
2285 /// examples below, with `&&x`.
2287 /// [`Some(element)`]: Some
2294 /// let a = [1, 2, 3];
2296 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2298 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2301 /// Stopping at the first `true`:
2304 /// let a = [1, 2, 3];
2306 /// let mut iter = a.iter();
2308 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2310 /// // we can still use `iter`, as there are more elements.
2311 /// assert_eq!(iter.next(), Some(&3));
2314 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2316 #[stable(feature = "rust1", since = "1.0.0")]
2317 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2320 P: FnMut(&Self::Item) -> bool,
2323 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2325 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2329 self.try_fold((), check(predicate)).break_value()
2332 /// Applies function to the elements of iterator and returns
2333 /// the first non-none result.
2335 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2340 /// let a = ["lol", "NaN", "2", "5"];
2342 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2344 /// assert_eq!(first_number, Some(2));
2347 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2348 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2351 F: FnMut(Self::Item) -> Option<B>,
2354 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2355 move |(), x| match f(x) {
2356 Some(x) => ControlFlow::Break(x),
2357 None => ControlFlow::CONTINUE,
2361 self.try_fold((), check(f)).break_value()
2364 /// Applies function to the elements of iterator and returns
2365 /// the first true result or the first error.
2370 /// #![feature(try_find)]
2372 /// let a = ["1", "2", "lol", "NaN", "5"];
2374 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2375 /// Ok(s.parse::<i32>()? == search)
2378 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2379 /// assert_eq!(result, Ok(Some(&"2")));
2381 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2382 /// assert!(result.is_err());
2385 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2386 fn try_find<F, R>(&mut self, f: F) -> Result<Option<Self::Item>, R::Error>
2389 F: FnMut(&Self::Item) -> R,
2393 fn check<F, T, R>(mut f: F) -> impl FnMut((), T) -> ControlFlow<Result<T, R::Error>>
2398 move |(), x| match f(&x).into_result() {
2399 Ok(false) => ControlFlow::CONTINUE,
2400 Ok(true) => ControlFlow::Break(Ok(x)),
2401 Err(x) => ControlFlow::Break(Err(x)),
2405 self.try_fold((), check(f)).break_value().transpose()
2408 /// Searches for an element in an iterator, returning its index.
2410 /// `position()` takes a closure that returns `true` or `false`. It applies
2411 /// this closure to each element of the iterator, and if one of them
2412 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2413 /// them return `false`, it returns [`None`].
2415 /// `position()` is short-circuiting; in other words, it will stop
2416 /// processing as soon as it finds a `true`.
2418 /// # Overflow Behavior
2420 /// The method does no guarding against overflows, so if there are more
2421 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2422 /// result or panics. If debug assertions are enabled, a panic is
2427 /// This function might panic if the iterator has more than `usize::MAX`
2428 /// non-matching elements.
2430 /// [`Some(index)`]: Some
2431 /// [`usize::MAX`]: crate::usize::MAX
2438 /// let a = [1, 2, 3];
2440 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2442 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2445 /// Stopping at the first `true`:
2448 /// let a = [1, 2, 3, 4];
2450 /// let mut iter = a.iter();
2452 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2454 /// // we can still use `iter`, as there are more elements.
2455 /// assert_eq!(iter.next(), Some(&3));
2457 /// // The returned index depends on iterator state
2458 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2462 #[stable(feature = "rust1", since = "1.0.0")]
2463 fn position<P>(&mut self, predicate: P) -> Option<usize>
2466 P: FnMut(Self::Item) -> bool,
2470 mut predicate: impl FnMut(T) -> bool,
2471 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2472 // The addition might panic on overflow
2475 ControlFlow::Break(i)
2477 ControlFlow::Continue(Add::add(i, 1))
2482 self.try_fold(0, check(predicate)).break_value()
2485 /// Searches for an element in an iterator from the right, returning its
2488 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2489 /// this closure to each element of the iterator, starting from the end,
2490 /// and if one of them returns `true`, then `rposition()` returns
2491 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2493 /// `rposition()` is short-circuiting; in other words, it will stop
2494 /// processing as soon as it finds a `true`.
2496 /// [`Some(index)`]: Some
2503 /// let a = [1, 2, 3];
2505 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2507 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2510 /// Stopping at the first `true`:
2513 /// let a = [1, 2, 3];
2515 /// let mut iter = a.iter();
2517 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2519 /// // we can still use `iter`, as there are more elements.
2520 /// assert_eq!(iter.next(), Some(&1));
2523 #[stable(feature = "rust1", since = "1.0.0")]
2524 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2526 P: FnMut(Self::Item) -> bool,
2527 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2529 // No need for an overflow check here, because `ExactSizeIterator`
2530 // implies that the number of elements fits into a `usize`.
2533 mut predicate: impl FnMut(T) -> bool,
2534 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2537 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2542 self.try_rfold(n, check(predicate)).break_value()
2545 /// Returns the maximum element of an iterator.
2547 /// If several elements are equally maximum, the last element is
2548 /// returned. If the iterator is empty, [`None`] is returned.
2555 /// let a = [1, 2, 3];
2556 /// let b: Vec<u32> = Vec::new();
2558 /// assert_eq!(a.iter().max(), Some(&3));
2559 /// assert_eq!(b.iter().max(), None);
2562 #[stable(feature = "rust1", since = "1.0.0")]
2563 fn max(self) -> Option<Self::Item>
2568 self.max_by(Ord::cmp)
2571 /// Returns the minimum element of an iterator.
2573 /// If several elements are equally minimum, the first element is
2574 /// returned. If the iterator is empty, [`None`] is returned.
2581 /// let a = [1, 2, 3];
2582 /// let b: Vec<u32> = Vec::new();
2584 /// assert_eq!(a.iter().min(), Some(&1));
2585 /// assert_eq!(b.iter().min(), None);
2588 #[stable(feature = "rust1", since = "1.0.0")]
2589 fn min(self) -> Option<Self::Item>
2594 self.min_by(Ord::cmp)
2597 /// Returns the element that gives the maximum value from the
2598 /// specified function.
2600 /// If several elements are equally maximum, the last element is
2601 /// returned. If the iterator is empty, [`None`] is returned.
2606 /// let a = [-3_i32, 0, 1, 5, -10];
2607 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2610 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2611 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2614 F: FnMut(&Self::Item) -> B,
2617 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2622 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2626 let (_, x) = self.map(key(f)).max_by(compare)?;
2630 /// Returns the element that gives the maximum value with respect to the
2631 /// specified comparison function.
2633 /// If several elements are equally maximum, the last element is
2634 /// returned. If the iterator is empty, [`None`] is returned.
2639 /// let a = [-3_i32, 0, 1, 5, -10];
2640 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2643 #[stable(feature = "iter_max_by", since = "1.15.0")]
2644 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2647 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2650 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2651 move |x, y| cmp::max_by(x, y, &mut compare)
2654 self.fold_first(fold(compare))
2657 /// Returns the element that gives the minimum value from the
2658 /// specified function.
2660 /// If several elements are equally minimum, the first element is
2661 /// returned. If the iterator is empty, [`None`] is returned.
2666 /// let a = [-3_i32, 0, 1, 5, -10];
2667 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2670 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2671 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2674 F: FnMut(&Self::Item) -> B,
2677 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2682 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2686 let (_, x) = self.map(key(f)).min_by(compare)?;
2690 /// Returns the element that gives the minimum value with respect to the
2691 /// specified comparison function.
2693 /// If several elements are equally minimum, the first element is
2694 /// returned. If the iterator is empty, [`None`] is returned.
2699 /// let a = [-3_i32, 0, 1, 5, -10];
2700 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2703 #[stable(feature = "iter_min_by", since = "1.15.0")]
2704 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2707 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2710 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2711 move |x, y| cmp::min_by(x, y, &mut compare)
2714 self.fold_first(fold(compare))
2717 /// Reverses an iterator's direction.
2719 /// Usually, iterators iterate from left to right. After using `rev()`,
2720 /// an iterator will instead iterate from right to left.
2722 /// This is only possible if the iterator has an end, so `rev()` only
2723 /// works on [`DoubleEndedIterator`]s.
2728 /// let a = [1, 2, 3];
2730 /// let mut iter = a.iter().rev();
2732 /// assert_eq!(iter.next(), Some(&3));
2733 /// assert_eq!(iter.next(), Some(&2));
2734 /// assert_eq!(iter.next(), Some(&1));
2736 /// assert_eq!(iter.next(), None);
2739 #[stable(feature = "rust1", since = "1.0.0")]
2740 fn rev(self) -> Rev<Self>
2742 Self: Sized + DoubleEndedIterator,
2747 /// Converts an iterator of pairs into a pair of containers.
2749 /// `unzip()` consumes an entire iterator of pairs, producing two
2750 /// collections: one from the left elements of the pairs, and one
2751 /// from the right elements.
2753 /// This function is, in some sense, the opposite of [`zip`].
2755 /// [`zip`]: Iterator::zip
2762 /// let a = [(1, 2), (3, 4)];
2764 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2766 /// assert_eq!(left, [1, 3]);
2767 /// assert_eq!(right, [2, 4]);
2769 #[stable(feature = "rust1", since = "1.0.0")]
2770 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2772 FromA: Default + Extend<A>,
2773 FromB: Default + Extend<B>,
2774 Self: Sized + Iterator<Item = (A, B)>,
2776 fn extend<'a, A, B>(
2777 ts: &'a mut impl Extend<A>,
2778 us: &'a mut impl Extend<B>,
2779 ) -> impl FnMut((), (A, B)) + 'a {
2786 let mut ts: FromA = Default::default();
2787 let mut us: FromB = Default::default();
2789 let (lower_bound, _) = self.size_hint();
2790 if lower_bound > 0 {
2791 ts.extend_reserve(lower_bound);
2792 us.extend_reserve(lower_bound);
2795 self.fold((), extend(&mut ts, &mut us));
2800 /// Creates an iterator which copies all of its elements.
2802 /// This is useful when you have an iterator over `&T`, but you need an
2803 /// iterator over `T`.
2810 /// let a = [1, 2, 3];
2812 /// let v_copied: Vec<_> = a.iter().copied().collect();
2814 /// // copied is the same as .map(|&x| x)
2815 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2817 /// assert_eq!(v_copied, vec![1, 2, 3]);
2818 /// assert_eq!(v_map, vec![1, 2, 3]);
2820 #[stable(feature = "iter_copied", since = "1.36.0")]
2821 fn copied<'a, T: 'a>(self) -> Copied<Self>
2823 Self: Sized + Iterator<Item = &'a T>,
2829 /// Creates an iterator which [`clone`]s all of its elements.
2831 /// This is useful when you have an iterator over `&T`, but you need an
2832 /// iterator over `T`.
2834 /// [`clone`]: Clone::clone
2841 /// let a = [1, 2, 3];
2843 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2845 /// // cloned is the same as .map(|&x| x), for integers
2846 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2848 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2849 /// assert_eq!(v_map, vec![1, 2, 3]);
2851 #[stable(feature = "rust1", since = "1.0.0")]
2852 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2854 Self: Sized + Iterator<Item = &'a T>,
2860 /// Repeats an iterator endlessly.
2862 /// Instead of stopping at [`None`], the iterator will instead start again,
2863 /// from the beginning. After iterating again, it will start at the
2864 /// beginning again. And again. And again. Forever.
2871 /// let a = [1, 2, 3];
2873 /// let mut it = a.iter().cycle();
2875 /// assert_eq!(it.next(), Some(&1));
2876 /// assert_eq!(it.next(), Some(&2));
2877 /// assert_eq!(it.next(), Some(&3));
2878 /// assert_eq!(it.next(), Some(&1));
2879 /// assert_eq!(it.next(), Some(&2));
2880 /// assert_eq!(it.next(), Some(&3));
2881 /// assert_eq!(it.next(), Some(&1));
2883 #[stable(feature = "rust1", since = "1.0.0")]
2885 fn cycle(self) -> Cycle<Self>
2887 Self: Sized + Clone,
2892 /// Sums the elements of an iterator.
2894 /// Takes each element, adds them together, and returns the result.
2896 /// An empty iterator returns the zero value of the type.
2900 /// When calling `sum()` and a primitive integer type is being returned, this
2901 /// method will panic if the computation overflows and debug assertions are
2909 /// let a = [1, 2, 3];
2910 /// let sum: i32 = a.iter().sum();
2912 /// assert_eq!(sum, 6);
2914 #[stable(feature = "iter_arith", since = "1.11.0")]
2915 fn sum<S>(self) -> S
2923 /// Iterates over the entire iterator, multiplying all the elements
2925 /// An empty iterator returns the one value of the type.
2929 /// When calling `product()` and a primitive integer type is being returned,
2930 /// method will panic if the computation overflows and debug assertions are
2936 /// fn factorial(n: u32) -> u32 {
2937 /// (1..=n).product()
2939 /// assert_eq!(factorial(0), 1);
2940 /// assert_eq!(factorial(1), 1);
2941 /// assert_eq!(factorial(5), 120);
2943 #[stable(feature = "iter_arith", since = "1.11.0")]
2944 fn product<P>(self) -> P
2947 P: Product<Self::Item>,
2949 Product::product(self)
2952 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2958 /// use std::cmp::Ordering;
2960 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
2961 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
2962 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
2964 #[stable(feature = "iter_order", since = "1.5.0")]
2965 fn cmp<I>(self, other: I) -> Ordering
2967 I: IntoIterator<Item = Self::Item>,
2971 self.cmp_by(other, |x, y| x.cmp(&y))
2974 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2975 /// of another with respect to the specified comparison function.
2982 /// #![feature(iter_order_by)]
2984 /// use std::cmp::Ordering;
2986 /// let xs = [1, 2, 3, 4];
2987 /// let ys = [1, 4, 9, 16];
2989 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
2990 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
2991 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
2993 #[unstable(feature = "iter_order_by", issue = "64295")]
2994 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
2998 F: FnMut(Self::Item, I::Item) -> Ordering,
3000 let mut other = other.into_iter();
3003 let x = match self.next() {
3005 if other.next().is_none() {
3006 return Ordering::Equal;
3008 return Ordering::Less;
3014 let y = match other.next() {
3015 None => return Ordering::Greater,
3020 Ordering::Equal => (),
3021 non_eq => return non_eq,
3026 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3032 /// use std::cmp::Ordering;
3034 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3035 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3036 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3038 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3040 #[stable(feature = "iter_order", since = "1.5.0")]
3041 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3044 Self::Item: PartialOrd<I::Item>,
3047 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3050 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3051 /// of another with respect to the specified comparison function.
3058 /// #![feature(iter_order_by)]
3060 /// use std::cmp::Ordering;
3062 /// let xs = [1.0, 2.0, 3.0, 4.0];
3063 /// let ys = [1.0, 4.0, 9.0, 16.0];
3066 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3067 /// Some(Ordering::Less)
3070 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3071 /// Some(Ordering::Equal)
3074 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3075 /// Some(Ordering::Greater)
3078 #[unstable(feature = "iter_order_by", issue = "64295")]
3079 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3083 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3085 let mut other = other.into_iter();
3088 let x = match self.next() {
3090 if other.next().is_none() {
3091 return Some(Ordering::Equal);
3093 return Some(Ordering::Less);
3099 let y = match other.next() {
3100 None => return Some(Ordering::Greater),
3104 match partial_cmp(x, y) {
3105 Some(Ordering::Equal) => (),
3106 non_eq => return non_eq,
3111 /// Determines if the elements of this [`Iterator`] are equal to those of
3117 /// assert_eq!([1].iter().eq([1].iter()), true);
3118 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3120 #[stable(feature = "iter_order", since = "1.5.0")]
3121 fn eq<I>(self, other: I) -> bool
3124 Self::Item: PartialEq<I::Item>,
3127 self.eq_by(other, |x, y| x == y)
3130 /// Determines if the elements of this [`Iterator`] are equal to those of
3131 /// another with respect to the specified equality function.
3138 /// #![feature(iter_order_by)]
3140 /// let xs = [1, 2, 3, 4];
3141 /// let ys = [1, 4, 9, 16];
3143 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3145 #[unstable(feature = "iter_order_by", issue = "64295")]
3146 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3150 F: FnMut(Self::Item, I::Item) -> bool,
3152 let mut other = other.into_iter();
3155 let x = match self.next() {
3156 None => return other.next().is_none(),
3160 let y = match other.next() {
3161 None => return false,
3171 /// Determines if the elements of this [`Iterator`] are unequal to those of
3177 /// assert_eq!([1].iter().ne([1].iter()), false);
3178 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3180 #[stable(feature = "iter_order", since = "1.5.0")]
3181 fn ne<I>(self, other: I) -> bool
3184 Self::Item: PartialEq<I::Item>,
3190 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3191 /// less than those of another.
3196 /// assert_eq!([1].iter().lt([1].iter()), false);
3197 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3198 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3199 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3201 #[stable(feature = "iter_order", since = "1.5.0")]
3202 fn lt<I>(self, other: I) -> bool
3205 Self::Item: PartialOrd<I::Item>,
3208 self.partial_cmp(other) == Some(Ordering::Less)
3211 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3212 /// less or equal to those of another.
3217 /// assert_eq!([1].iter().le([1].iter()), true);
3218 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3219 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3220 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3222 #[stable(feature = "iter_order", since = "1.5.0")]
3223 fn le<I>(self, other: I) -> bool
3226 Self::Item: PartialOrd<I::Item>,
3229 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3232 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3233 /// greater than those of another.
3238 /// assert_eq!([1].iter().gt([1].iter()), false);
3239 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3240 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3241 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3243 #[stable(feature = "iter_order", since = "1.5.0")]
3244 fn gt<I>(self, other: I) -> bool
3247 Self::Item: PartialOrd<I::Item>,
3250 self.partial_cmp(other) == Some(Ordering::Greater)
3253 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3254 /// greater than or equal to those of another.
3259 /// assert_eq!([1].iter().ge([1].iter()), true);
3260 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3261 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3262 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3264 #[stable(feature = "iter_order", since = "1.5.0")]
3265 fn ge<I>(self, other: I) -> bool
3268 Self::Item: PartialOrd<I::Item>,
3271 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3274 /// Checks if the elements of this iterator are sorted.
3276 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3277 /// iterator yields exactly zero or one element, `true` is returned.
3279 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3280 /// implies that this function returns `false` if any two consecutive items are not
3286 /// #![feature(is_sorted)]
3288 /// assert!([1, 2, 2, 9].iter().is_sorted());
3289 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3290 /// assert!([0].iter().is_sorted());
3291 /// assert!(std::iter::empty::<i32>().is_sorted());
3292 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3295 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3296 fn is_sorted(self) -> bool
3299 Self::Item: PartialOrd,
3301 self.is_sorted_by(PartialOrd::partial_cmp)
3304 /// Checks if the elements of this iterator are sorted using the given comparator function.
3306 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3307 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3308 /// [`is_sorted`]; see its documentation for more information.
3313 /// #![feature(is_sorted)]
3315 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3316 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3317 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3318 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3319 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3322 /// [`is_sorted`]: Iterator::is_sorted
3323 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3324 fn is_sorted_by<F>(mut self, mut compare: F) -> bool
3327 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3329 let mut last = match self.next() {
3331 None => return true,
3334 while let Some(curr) = self.next() {
3335 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3344 /// Checks if the elements of this iterator are sorted using the given key extraction
3347 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3348 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3349 /// its documentation for more information.
3351 /// [`is_sorted`]: Iterator::is_sorted
3356 /// #![feature(is_sorted)]
3358 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3359 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3362 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3363 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3366 F: FnMut(Self::Item) -> K,
3369 self.map(f).is_sorted()
3372 /// See [TrustedRandomAccess]
3373 // The unusual name is to avoid name collisions in method resolution
3377 #[unstable(feature = "trusted_random_access", issue = "none")]
3378 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3380 Self: TrustedRandomAccess,
3382 unreachable!("Always specialized");
3386 #[stable(feature = "rust1", since = "1.0.0")]
3387 impl<I: Iterator + ?Sized> Iterator for &mut I {
3388 type Item = I::Item;
3389 fn next(&mut self) -> Option<I::Item> {
3392 fn size_hint(&self) -> (usize, Option<usize>) {
3393 (**self).size_hint()
3395 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3396 (**self).advance_by(n)
3398 fn nth(&mut self, n: usize) -> Option<Self::Item> {