1 use crate::cmp::{self, Ordering};
2 use crate::ops::{ControlFlow, Try};
4 use super::super::TrustedRandomAccessNoCoerce;
5 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
6 use super::super::{FlatMap, Flatten};
7 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
9 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
12 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
14 /// An interface for dealing with iterators.
16 /// This is the main iterator trait. For more about the concept of iterators
17 /// generally, please see the [module-level documentation]. In particular, you
18 /// may want to know how to [implement `Iterator`][impl].
20 /// [module-level documentation]: crate::iter
21 /// [impl]: crate::iter#implementing-iterator
22 #[stable(feature = "rust1", since = "1.0.0")]
23 #[rustc_on_unimplemented(
25 _Self = "std::ops::RangeTo<Idx>",
26 label = "if you meant to iterate until a value, add a starting value",
27 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
28 bounded `Range`: `0..end`"
31 _Self = "std::ops::RangeToInclusive<Idx>",
32 label = "if you meant to iterate until a value (including it), add a starting value",
33 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
34 to have a bounded `RangeInclusive`: `0..=end`"
38 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
41 _Self = "std::string::String",
42 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
46 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
47 syntax `start..end` or the inclusive range syntax `start..=end`"
49 label = "`{Self}` is not an iterator",
50 message = "`{Self}` is not an iterator"
53 #[rustc_diagnostic_item = "Iterator"]
54 #[must_use = "iterators are lazy and do nothing unless consumed"]
56 /// The type of the elements being iterated over.
57 #[stable(feature = "rust1", since = "1.0.0")]
60 /// Advances the iterator and returns the next value.
62 /// Returns [`None`] when iteration is finished. Individual iterator
63 /// implementations may choose to resume iteration, and so calling `next()`
64 /// again may or may not eventually start returning [`Some(Item)`] again at some
67 /// [`Some(Item)`]: Some
74 /// let a = [1, 2, 3];
76 /// let mut iter = a.iter();
78 /// // A call to next() returns the next value...
79 /// assert_eq!(Some(&1), iter.next());
80 /// assert_eq!(Some(&2), iter.next());
81 /// assert_eq!(Some(&3), iter.next());
83 /// // ... and then None once it's over.
84 /// assert_eq!(None, iter.next());
86 /// // More calls may or may not return `None`. Here, they always will.
87 /// assert_eq!(None, iter.next());
88 /// assert_eq!(None, iter.next());
91 #[stable(feature = "rust1", since = "1.0.0")]
92 fn next(&mut self) -> Option<Self::Item>;
94 /// Returns the bounds on the remaining length of the iterator.
96 /// Specifically, `size_hint()` returns a tuple where the first element
97 /// is the lower bound, and the second element is the upper bound.
99 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
100 /// A [`None`] here means that either there is no known upper bound, or the
101 /// upper bound is larger than [`usize`].
103 /// # Implementation notes
105 /// It is not enforced that an iterator implementation yields the declared
106 /// number of elements. A buggy iterator may yield less than the lower bound
107 /// or more than the upper bound of elements.
109 /// `size_hint()` is primarily intended to be used for optimizations such as
110 /// reserving space for the elements of the iterator, but must not be
111 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
112 /// implementation of `size_hint()` should not lead to memory safety
115 /// That said, the implementation should provide a correct estimation,
116 /// because otherwise it would be a violation of the trait's protocol.
118 /// The default implementation returns <code>(0, [None])</code> which is correct for any
126 /// let a = [1, 2, 3];
127 /// let iter = a.iter();
129 /// assert_eq!((3, Some(3)), iter.size_hint());
132 /// A more complex example:
135 /// // The even numbers in the range of zero to nine.
136 /// let iter = (0..10).filter(|x| x % 2 == 0);
138 /// // We might iterate from zero to ten times. Knowing that it's five
139 /// // exactly wouldn't be possible without executing filter().
140 /// assert_eq!((0, Some(10)), iter.size_hint());
142 /// // Let's add five more numbers with chain()
143 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
145 /// // now both bounds are increased by five
146 /// assert_eq!((5, Some(15)), iter.size_hint());
149 /// Returning `None` for an upper bound:
152 /// // an infinite iterator has no upper bound
153 /// // and the maximum possible lower bound
156 /// assert_eq!((usize::MAX, None), iter.size_hint());
159 #[stable(feature = "rust1", since = "1.0.0")]
160 fn size_hint(&self) -> (usize, Option<usize>) {
164 /// Consumes the iterator, counting the number of iterations and returning it.
166 /// This method will call [`next`] repeatedly until [`None`] is encountered,
167 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
168 /// called at least once even if the iterator does not have any elements.
170 /// [`next`]: Iterator::next
172 /// # Overflow Behavior
174 /// The method does no guarding against overflows, so counting elements of
175 /// an iterator with more than [`usize::MAX`] elements either produces the
176 /// wrong result or panics. If debug assertions are enabled, a panic is
181 /// This function might panic if the iterator has more than [`usize::MAX`]
189 /// let a = [1, 2, 3];
190 /// assert_eq!(a.iter().count(), 3);
192 /// let a = [1, 2, 3, 4, 5];
193 /// assert_eq!(a.iter().count(), 5);
196 #[stable(feature = "rust1", since = "1.0.0")]
197 fn count(self) -> usize
203 #[rustc_inherit_overflow_checks]
204 |count, _| count + 1,
208 /// Consumes the iterator, returning the last element.
210 /// This method will evaluate the iterator until it returns [`None`]. While
211 /// doing so, it keeps track of the current element. After [`None`] is
212 /// returned, `last()` will then return the last element it saw.
219 /// let a = [1, 2, 3];
220 /// assert_eq!(a.iter().last(), Some(&3));
222 /// let a = [1, 2, 3, 4, 5];
223 /// assert_eq!(a.iter().last(), Some(&5));
226 #[stable(feature = "rust1", since = "1.0.0")]
227 fn last(self) -> Option<Self::Item>
232 fn some<T>(_: Option<T>, x: T) -> Option<T> {
236 self.fold(None, some)
239 /// Advances the iterator by `n` elements.
241 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
242 /// times until [`None`] is encountered.
244 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
245 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
246 /// of elements the iterator is advanced by before running out of elements (i.e. the
247 /// length of the iterator). Note that `k` is always less than `n`.
249 /// Calling `advance_by(0)` does not consume any elements and always returns [`Ok(())`][Ok].
251 /// [`next`]: Iterator::next
258 /// #![feature(iter_advance_by)]
260 /// let a = [1, 2, 3, 4];
261 /// let mut iter = a.iter();
263 /// assert_eq!(iter.advance_by(2), Ok(()));
264 /// assert_eq!(iter.next(), Some(&3));
265 /// assert_eq!(iter.advance_by(0), Ok(()));
266 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
269 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
270 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
272 self.next().ok_or(i)?;
277 /// Returns the `n`th element of the iterator.
279 /// Like most indexing operations, the count starts from zero, so `nth(0)`
280 /// returns the first value, `nth(1)` the second, and so on.
282 /// Note that all preceding elements, as well as the returned element, will be
283 /// consumed from the iterator. That means that the preceding elements will be
284 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
285 /// will return different elements.
287 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
295 /// let a = [1, 2, 3];
296 /// assert_eq!(a.iter().nth(1), Some(&2));
299 /// Calling `nth()` multiple times doesn't rewind the iterator:
302 /// let a = [1, 2, 3];
304 /// let mut iter = a.iter();
306 /// assert_eq!(iter.nth(1), Some(&2));
307 /// assert_eq!(iter.nth(1), None);
310 /// Returning `None` if there are less than `n + 1` elements:
313 /// let a = [1, 2, 3];
314 /// assert_eq!(a.iter().nth(10), None);
317 #[stable(feature = "rust1", since = "1.0.0")]
318 fn nth(&mut self, n: usize) -> Option<Self::Item> {
319 self.advance_by(n).ok()?;
323 /// Creates an iterator starting at the same point, but stepping by
324 /// the given amount at each iteration.
326 /// Note 1: The first element of the iterator will always be returned,
327 /// regardless of the step given.
329 /// Note 2: The time at which ignored elements are pulled is not fixed.
330 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
331 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
332 /// `advance_n_and_return_first(&mut self, step)`,
333 /// `advance_n_and_return_first(&mut self, step)`, …
334 /// Which way is used may change for some iterators for performance reasons.
335 /// The second way will advance the iterator earlier and may consume more items.
337 /// `advance_n_and_return_first` is the equivalent of:
339 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
343 /// let next = iter.next();
353 /// The method will panic if the given step is `0`.
360 /// let a = [0, 1, 2, 3, 4, 5];
361 /// let mut iter = a.iter().step_by(2);
363 /// assert_eq!(iter.next(), Some(&0));
364 /// assert_eq!(iter.next(), Some(&2));
365 /// assert_eq!(iter.next(), Some(&4));
366 /// assert_eq!(iter.next(), None);
369 #[stable(feature = "iterator_step_by", since = "1.28.0")]
370 fn step_by(self, step: usize) -> StepBy<Self>
374 StepBy::new(self, step)
377 /// Takes two iterators and creates a new iterator over both in sequence.
379 /// `chain()` will return a new iterator which will first iterate over
380 /// values from the first iterator and then over values from the second
383 /// In other words, it links two iterators together, in a chain. 🔗
385 /// [`once`] is commonly used to adapt a single value into a chain of
386 /// other kinds of iteration.
393 /// let a1 = [1, 2, 3];
394 /// let a2 = [4, 5, 6];
396 /// let mut iter = a1.iter().chain(a2.iter());
398 /// assert_eq!(iter.next(), Some(&1));
399 /// assert_eq!(iter.next(), Some(&2));
400 /// assert_eq!(iter.next(), Some(&3));
401 /// assert_eq!(iter.next(), Some(&4));
402 /// assert_eq!(iter.next(), Some(&5));
403 /// assert_eq!(iter.next(), Some(&6));
404 /// assert_eq!(iter.next(), None);
407 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
408 /// anything that can be converted into an [`Iterator`], not just an
409 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
410 /// [`IntoIterator`], and so can be passed to `chain()` directly:
413 /// let s1 = &[1, 2, 3];
414 /// let s2 = &[4, 5, 6];
416 /// let mut iter = s1.iter().chain(s2);
418 /// assert_eq!(iter.next(), Some(&1));
419 /// assert_eq!(iter.next(), Some(&2));
420 /// assert_eq!(iter.next(), Some(&3));
421 /// assert_eq!(iter.next(), Some(&4));
422 /// assert_eq!(iter.next(), Some(&5));
423 /// assert_eq!(iter.next(), Some(&6));
424 /// assert_eq!(iter.next(), None);
427 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
431 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
432 /// use std::os::windows::ffi::OsStrExt;
433 /// s.encode_wide().chain(std::iter::once(0)).collect()
437 /// [`once`]: crate::iter::once
438 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
440 #[stable(feature = "rust1", since = "1.0.0")]
441 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
444 U: IntoIterator<Item = Self::Item>,
446 Chain::new(self, other.into_iter())
449 /// 'Zips up' two iterators into a single iterator of pairs.
451 /// `zip()` returns a new iterator that will iterate over two other
452 /// iterators, returning a tuple where the first element comes from the
453 /// first iterator, and the second element comes from the second iterator.
455 /// In other words, it zips two iterators together, into a single one.
457 /// If either iterator returns [`None`], [`next`] from the zipped iterator
458 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
459 /// short-circuit and `next` will not be called on the second iterator.
466 /// let a1 = [1, 2, 3];
467 /// let a2 = [4, 5, 6];
469 /// let mut iter = a1.iter().zip(a2.iter());
471 /// assert_eq!(iter.next(), Some((&1, &4)));
472 /// assert_eq!(iter.next(), Some((&2, &5)));
473 /// assert_eq!(iter.next(), Some((&3, &6)));
474 /// assert_eq!(iter.next(), None);
477 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
478 /// anything that can be converted into an [`Iterator`], not just an
479 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
480 /// [`IntoIterator`], and so can be passed to `zip()` directly:
483 /// let s1 = &[1, 2, 3];
484 /// let s2 = &[4, 5, 6];
486 /// let mut iter = s1.iter().zip(s2);
488 /// assert_eq!(iter.next(), Some((&1, &4)));
489 /// assert_eq!(iter.next(), Some((&2, &5)));
490 /// assert_eq!(iter.next(), Some((&3, &6)));
491 /// assert_eq!(iter.next(), None);
494 /// `zip()` is often used to zip an infinite iterator to a finite one.
495 /// This works because the finite iterator will eventually return [`None`],
496 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
499 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
501 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
503 /// assert_eq!((0, 'f'), enumerate[0]);
504 /// assert_eq!((0, 'f'), zipper[0]);
506 /// assert_eq!((1, 'o'), enumerate[1]);
507 /// assert_eq!((1, 'o'), zipper[1]);
509 /// assert_eq!((2, 'o'), enumerate[2]);
510 /// assert_eq!((2, 'o'), zipper[2]);
513 /// [`enumerate`]: Iterator::enumerate
514 /// [`next`]: Iterator::next
516 #[stable(feature = "rust1", since = "1.0.0")]
517 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
522 Zip::new(self, other.into_iter())
525 /// Creates a new iterator which places a copy of `separator` between adjacent
526 /// items of the original iterator.
528 /// In case `separator` does not implement [`Clone`] or needs to be
529 /// computed every time, use [`intersperse_with`].
536 /// let mut a = [0, 1, 2].iter().intersperse(&100);
537 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
538 /// assert_eq!(a.next(), Some(&100)); // The separator.
539 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
540 /// assert_eq!(a.next(), Some(&100)); // The separator.
541 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
542 /// assert_eq!(a.next(), None); // The iterator is finished.
545 /// `intersperse` can be very useful to join an iterator's items using a common element:
548 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
549 /// assert_eq!(hello, "Hello World !");
552 /// [`Clone`]: crate::clone::Clone
553 /// [`intersperse_with`]: Iterator::intersperse_with
555 #[stable(feature = "iter_intersperse", since = "1.56.0")]
556 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
561 Intersperse::new(self, separator)
564 /// Creates a new iterator which places an item generated by `separator`
565 /// between adjacent items of the original iterator.
567 /// The closure will be called exactly once each time an item is placed
568 /// between two adjacent items from the underlying iterator; specifically,
569 /// the closure is not called if the underlying iterator yields less than
570 /// two items and after the last item is yielded.
572 /// If the iterator's item implements [`Clone`], it may be easier to use
580 /// #[derive(PartialEq, Debug)]
581 /// struct NotClone(usize);
583 /// let v = vec![NotClone(0), NotClone(1), NotClone(2)];
584 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
586 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
587 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
588 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
589 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
590 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
591 /// assert_eq!(it.next(), None); // The iterator is finished.
594 /// `intersperse_with` can be used in situations where the separator needs
598 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
600 /// // The closure mutably borrows its context to generate an item.
601 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
602 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
604 /// let result = src.intersperse_with(separator).collect::<String>();
605 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
607 /// [`Clone`]: crate::clone::Clone
608 /// [`intersperse`]: Iterator::intersperse
610 #[stable(feature = "iter_intersperse", since = "1.56.0")]
611 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
614 G: FnMut() -> Self::Item,
616 IntersperseWith::new(self, separator)
619 /// Takes a closure and creates an iterator which calls that closure on each
622 /// `map()` transforms one iterator into another, by means of its argument:
623 /// something that implements [`FnMut`]. It produces a new iterator which
624 /// calls this closure on each element of the original iterator.
626 /// If you are good at thinking in types, you can think of `map()` like this:
627 /// If you have an iterator that gives you elements of some type `A`, and
628 /// you want an iterator of some other type `B`, you can use `map()`,
629 /// passing a closure that takes an `A` and returns a `B`.
631 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
632 /// lazy, it is best used when you're already working with other iterators.
633 /// If you're doing some sort of looping for a side effect, it's considered
634 /// more idiomatic to use [`for`] than `map()`.
636 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
637 /// [`FnMut`]: crate::ops::FnMut
644 /// let a = [1, 2, 3];
646 /// let mut iter = a.iter().map(|x| 2 * x);
648 /// assert_eq!(iter.next(), Some(2));
649 /// assert_eq!(iter.next(), Some(4));
650 /// assert_eq!(iter.next(), Some(6));
651 /// assert_eq!(iter.next(), None);
654 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
657 /// # #![allow(unused_must_use)]
658 /// // don't do this:
659 /// (0..5).map(|x| println!("{}", x));
661 /// // it won't even execute, as it is lazy. Rust will warn you about this.
663 /// // Instead, use for:
665 /// println!("{}", x);
669 #[stable(feature = "rust1", since = "1.0.0")]
670 fn map<B, F>(self, f: F) -> Map<Self, F>
673 F: FnMut(Self::Item) -> B,
678 /// Calls a closure on each element of an iterator.
680 /// This is equivalent to using a [`for`] loop on the iterator, although
681 /// `break` and `continue` are not possible from a closure. It's generally
682 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
683 /// when processing items at the end of longer iterator chains. In some
684 /// cases `for_each` may also be faster than a loop, because it will use
685 /// internal iteration on adapters like `Chain`.
687 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
694 /// use std::sync::mpsc::channel;
696 /// let (tx, rx) = channel();
697 /// (0..5).map(|x| x * 2 + 1)
698 /// .for_each(move |x| tx.send(x).unwrap());
700 /// let v: Vec<_> = rx.iter().collect();
701 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
704 /// For such a small example, a `for` loop may be cleaner, but `for_each`
705 /// might be preferable to keep a functional style with longer iterators:
708 /// (0..5).flat_map(|x| x * 100 .. x * 110)
710 /// .filter(|&(i, x)| (i + x) % 3 == 0)
711 /// .for_each(|(i, x)| println!("{}:{}", i, x));
714 #[stable(feature = "iterator_for_each", since = "1.21.0")]
715 fn for_each<F>(self, f: F)
718 F: FnMut(Self::Item),
721 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
722 move |(), item| f(item)
725 self.fold((), call(f));
728 /// Creates an iterator which uses a closure to determine if an element
729 /// should be yielded.
731 /// Given an element the closure must return `true` or `false`. The returned
732 /// iterator will yield only the elements for which the closure returns
740 /// let a = [0i32, 1, 2];
742 /// let mut iter = a.iter().filter(|x| x.is_positive());
744 /// assert_eq!(iter.next(), Some(&1));
745 /// assert_eq!(iter.next(), Some(&2));
746 /// assert_eq!(iter.next(), None);
749 /// Because the closure passed to `filter()` takes a reference, and many
750 /// iterators iterate over references, this leads to a possibly confusing
751 /// situation, where the type of the closure is a double reference:
754 /// let a = [0, 1, 2];
756 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
758 /// assert_eq!(iter.next(), Some(&2));
759 /// assert_eq!(iter.next(), None);
762 /// It's common to instead use destructuring on the argument to strip away
766 /// let a = [0, 1, 2];
768 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
770 /// assert_eq!(iter.next(), Some(&2));
771 /// assert_eq!(iter.next(), None);
777 /// let a = [0, 1, 2];
779 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
781 /// assert_eq!(iter.next(), Some(&2));
782 /// assert_eq!(iter.next(), None);
787 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
789 #[stable(feature = "rust1", since = "1.0.0")]
790 fn filter<P>(self, predicate: P) -> Filter<Self, P>
793 P: FnMut(&Self::Item) -> bool,
795 Filter::new(self, predicate)
798 /// Creates an iterator that both filters and maps.
800 /// The returned iterator yields only the `value`s for which the supplied
801 /// closure returns `Some(value)`.
803 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
804 /// concise. The example below shows how a `map().filter().map()` can be
805 /// shortened to a single call to `filter_map`.
807 /// [`filter`]: Iterator::filter
808 /// [`map`]: Iterator::map
815 /// let a = ["1", "two", "NaN", "four", "5"];
817 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
819 /// assert_eq!(iter.next(), Some(1));
820 /// assert_eq!(iter.next(), Some(5));
821 /// assert_eq!(iter.next(), None);
824 /// Here's the same example, but with [`filter`] and [`map`]:
827 /// let a = ["1", "two", "NaN", "four", "5"];
828 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
829 /// assert_eq!(iter.next(), Some(1));
830 /// assert_eq!(iter.next(), Some(5));
831 /// assert_eq!(iter.next(), None);
834 #[stable(feature = "rust1", since = "1.0.0")]
835 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
838 F: FnMut(Self::Item) -> Option<B>,
840 FilterMap::new(self, f)
843 /// Creates an iterator which gives the current iteration count as well as
846 /// The iterator returned yields pairs `(i, val)`, where `i` is the
847 /// current index of iteration and `val` is the value returned by the
850 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
851 /// different sized integer, the [`zip`] function provides similar
854 /// # Overflow Behavior
856 /// The method does no guarding against overflows, so enumerating more than
857 /// [`usize::MAX`] elements either produces the wrong result or panics. If
858 /// debug assertions are enabled, a panic is guaranteed.
862 /// The returned iterator might panic if the to-be-returned index would
863 /// overflow a [`usize`].
865 /// [`zip`]: Iterator::zip
870 /// let a = ['a', 'b', 'c'];
872 /// let mut iter = a.iter().enumerate();
874 /// assert_eq!(iter.next(), Some((0, &'a')));
875 /// assert_eq!(iter.next(), Some((1, &'b')));
876 /// assert_eq!(iter.next(), Some((2, &'c')));
877 /// assert_eq!(iter.next(), None);
880 #[stable(feature = "rust1", since = "1.0.0")]
881 fn enumerate(self) -> Enumerate<Self>
888 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
889 /// to look at the next element of the iterator without consuming it. See
890 /// their documentation for more information.
892 /// Note that the underlying iterator is still advanced when [`peek`] or
893 /// [`peek_mut`] are called for the first time: In order to retrieve the
894 /// next element, [`next`] is called on the underlying iterator, hence any
895 /// side effects (i.e. anything other than fetching the next value) of
896 /// the [`next`] method will occur.
904 /// let xs = [1, 2, 3];
906 /// let mut iter = xs.iter().peekable();
908 /// // peek() lets us see into the future
909 /// assert_eq!(iter.peek(), Some(&&1));
910 /// assert_eq!(iter.next(), Some(&1));
912 /// assert_eq!(iter.next(), Some(&2));
914 /// // we can peek() multiple times, the iterator won't advance
915 /// assert_eq!(iter.peek(), Some(&&3));
916 /// assert_eq!(iter.peek(), Some(&&3));
918 /// assert_eq!(iter.next(), Some(&3));
920 /// // after the iterator is finished, so is peek()
921 /// assert_eq!(iter.peek(), None);
922 /// assert_eq!(iter.next(), None);
925 /// Using [`peek_mut`] to mutate the next item without advancing the
929 /// let xs = [1, 2, 3];
931 /// let mut iter = xs.iter().peekable();
933 /// // `peek_mut()` lets us see into the future
934 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
935 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
936 /// assert_eq!(iter.next(), Some(&1));
938 /// if let Some(mut p) = iter.peek_mut() {
939 /// assert_eq!(*p, &2);
940 /// // put a value into the iterator
944 /// // The value reappears as the iterator continues
945 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
947 /// [`peek`]: Peekable::peek
948 /// [`peek_mut`]: Peekable::peek_mut
949 /// [`next`]: Iterator::next
951 #[stable(feature = "rust1", since = "1.0.0")]
952 fn peekable(self) -> Peekable<Self>
959 /// Creates an iterator that [`skip`]s elements based on a predicate.
961 /// [`skip`]: Iterator::skip
963 /// `skip_while()` takes a closure as an argument. It will call this
964 /// closure on each element of the iterator, and ignore elements
965 /// until it returns `false`.
967 /// After `false` is returned, `skip_while()`'s job is over, and the
968 /// rest of the elements are yielded.
975 /// let a = [-1i32, 0, 1];
977 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
979 /// assert_eq!(iter.next(), Some(&0));
980 /// assert_eq!(iter.next(), Some(&1));
981 /// assert_eq!(iter.next(), None);
984 /// Because the closure passed to `skip_while()` takes a reference, and many
985 /// iterators iterate over references, this leads to a possibly confusing
986 /// situation, where the type of the closure argument is a double reference:
989 /// let a = [-1, 0, 1];
991 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
993 /// assert_eq!(iter.next(), Some(&0));
994 /// assert_eq!(iter.next(), Some(&1));
995 /// assert_eq!(iter.next(), None);
998 /// Stopping after an initial `false`:
1001 /// let a = [-1, 0, 1, -2];
1003 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1005 /// assert_eq!(iter.next(), Some(&0));
1006 /// assert_eq!(iter.next(), Some(&1));
1008 /// // while this would have been false, since we already got a false,
1009 /// // skip_while() isn't used any more
1010 /// assert_eq!(iter.next(), Some(&-2));
1012 /// assert_eq!(iter.next(), None);
1015 #[stable(feature = "rust1", since = "1.0.0")]
1016 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1019 P: FnMut(&Self::Item) -> bool,
1021 SkipWhile::new(self, predicate)
1024 /// Creates an iterator that yields elements based on a predicate.
1026 /// `take_while()` takes a closure as an argument. It will call this
1027 /// closure on each element of the iterator, and yield elements
1028 /// while it returns `true`.
1030 /// After `false` is returned, `take_while()`'s job is over, and the
1031 /// rest of the elements are ignored.
1038 /// let a = [-1i32, 0, 1];
1040 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1042 /// assert_eq!(iter.next(), Some(&-1));
1043 /// assert_eq!(iter.next(), None);
1046 /// Because the closure passed to `take_while()` takes a reference, and many
1047 /// iterators iterate over references, this leads to a possibly confusing
1048 /// situation, where the type of the closure is a double reference:
1051 /// let a = [-1, 0, 1];
1053 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1055 /// assert_eq!(iter.next(), Some(&-1));
1056 /// assert_eq!(iter.next(), None);
1059 /// Stopping after an initial `false`:
1062 /// let a = [-1, 0, 1, -2];
1064 /// let mut iter = a.iter().take_while(|x| **x < 0);
1066 /// assert_eq!(iter.next(), Some(&-1));
1068 /// // We have more elements that are less than zero, but since we already
1069 /// // got a false, take_while() isn't used any more
1070 /// assert_eq!(iter.next(), None);
1073 /// Because `take_while()` needs to look at the value in order to see if it
1074 /// should be included or not, consuming iterators will see that it is
1078 /// let a = [1, 2, 3, 4];
1079 /// let mut iter = a.iter();
1081 /// let result: Vec<i32> = iter.by_ref()
1082 /// .take_while(|n| **n != 3)
1086 /// assert_eq!(result, &[1, 2]);
1088 /// let result: Vec<i32> = iter.cloned().collect();
1090 /// assert_eq!(result, &[4]);
1093 /// The `3` is no longer there, because it was consumed in order to see if
1094 /// the iteration should stop, but wasn't placed back into the iterator.
1096 #[stable(feature = "rust1", since = "1.0.0")]
1097 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1100 P: FnMut(&Self::Item) -> bool,
1102 TakeWhile::new(self, predicate)
1105 /// Creates an iterator that both yields elements based on a predicate and maps.
1107 /// `map_while()` takes a closure as an argument. It will call this
1108 /// closure on each element of the iterator, and yield elements
1109 /// while it returns [`Some(_)`][`Some`].
1116 /// let a = [-1i32, 4, 0, 1];
1118 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1120 /// assert_eq!(iter.next(), Some(-16));
1121 /// assert_eq!(iter.next(), Some(4));
1122 /// assert_eq!(iter.next(), None);
1125 /// Here's the same example, but with [`take_while`] and [`map`]:
1127 /// [`take_while`]: Iterator::take_while
1128 /// [`map`]: Iterator::map
1131 /// let a = [-1i32, 4, 0, 1];
1133 /// let mut iter = a.iter()
1134 /// .map(|x| 16i32.checked_div(*x))
1135 /// .take_while(|x| x.is_some())
1136 /// .map(|x| x.unwrap());
1138 /// assert_eq!(iter.next(), Some(-16));
1139 /// assert_eq!(iter.next(), Some(4));
1140 /// assert_eq!(iter.next(), None);
1143 /// Stopping after an initial [`None`]:
1146 /// use std::convert::TryFrom;
1148 /// let a = [0, 1, 2, -3, 4, 5, -6];
1150 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1151 /// let vec = iter.collect::<Vec<_>>();
1153 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1154 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1155 /// assert_eq!(vec, vec![0, 1, 2]);
1158 /// Because `map_while()` needs to look at the value in order to see if it
1159 /// should be included or not, consuming iterators will see that it is
1163 /// use std::convert::TryFrom;
1165 /// let a = [1, 2, -3, 4];
1166 /// let mut iter = a.iter();
1168 /// let result: Vec<u32> = iter.by_ref()
1169 /// .map_while(|n| u32::try_from(*n).ok())
1172 /// assert_eq!(result, &[1, 2]);
1174 /// let result: Vec<i32> = iter.cloned().collect();
1176 /// assert_eq!(result, &[4]);
1179 /// The `-3` is no longer there, because it was consumed in order to see if
1180 /// the iteration should stop, but wasn't placed back into the iterator.
1182 /// Note that unlike [`take_while`] this iterator is **not** fused.
1183 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1184 /// If you need fused iterator, use [`fuse`].
1186 /// [`fuse`]: Iterator::fuse
1188 #[stable(feature = "iter_map_while", since = "1.57.0")]
1189 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1192 P: FnMut(Self::Item) -> Option<B>,
1194 MapWhile::new(self, predicate)
1197 /// Creates an iterator that skips the first `n` elements.
1199 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1200 /// iterator is reached (whichever happens first). After that, all the remaining
1201 /// elements are yielded. In particular, if the original iterator is too short,
1202 /// then the returned iterator is empty.
1204 /// Rather than overriding this method directly, instead override the `nth` method.
1211 /// let a = [1, 2, 3];
1213 /// let mut iter = a.iter().skip(2);
1215 /// assert_eq!(iter.next(), Some(&3));
1216 /// assert_eq!(iter.next(), None);
1219 #[stable(feature = "rust1", since = "1.0.0")]
1220 fn skip(self, n: usize) -> Skip<Self>
1227 /// Creates an iterator that yields the first `n` elements, or fewer
1228 /// if the underlying iterator ends sooner.
1230 /// `take(n)` yields elements until `n` elements are yielded or the end of
1231 /// the iterator is reached (whichever happens first).
1232 /// The returned iterator is a prefix of length `n` if the original iterator
1233 /// contains at least `n` elements, otherwise it contains all of the
1234 /// (fewer than `n`) elements of the original iterator.
1241 /// let a = [1, 2, 3];
1243 /// let mut iter = a.iter().take(2);
1245 /// assert_eq!(iter.next(), Some(&1));
1246 /// assert_eq!(iter.next(), Some(&2));
1247 /// assert_eq!(iter.next(), None);
1250 /// `take()` is often used with an infinite iterator, to make it finite:
1253 /// let mut iter = (0..).take(3);
1255 /// assert_eq!(iter.next(), Some(0));
1256 /// assert_eq!(iter.next(), Some(1));
1257 /// assert_eq!(iter.next(), Some(2));
1258 /// assert_eq!(iter.next(), None);
1261 /// If less than `n` elements are available,
1262 /// `take` will limit itself to the size of the underlying iterator:
1265 /// let v = vec![1, 2];
1266 /// let mut iter = v.into_iter().take(5);
1267 /// assert_eq!(iter.next(), Some(1));
1268 /// assert_eq!(iter.next(), Some(2));
1269 /// assert_eq!(iter.next(), None);
1272 #[stable(feature = "rust1", since = "1.0.0")]
1273 fn take(self, n: usize) -> Take<Self>
1280 /// An iterator adapter similar to [`fold`] that holds internal state and
1281 /// produces a new iterator.
1283 /// [`fold`]: Iterator::fold
1285 /// `scan()` takes two arguments: an initial value which seeds the internal
1286 /// state, and a closure with two arguments, the first being a mutable
1287 /// reference to the internal state and the second an iterator element.
1288 /// The closure can assign to the internal state to share state between
1291 /// On iteration, the closure will be applied to each element of the
1292 /// iterator and the return value from the closure, an [`Option`], is
1293 /// yielded by the iterator.
1300 /// let a = [1, 2, 3];
1302 /// let mut iter = a.iter().scan(1, |state, &x| {
1303 /// // each iteration, we'll multiply the state by the element
1304 /// *state = *state * x;
1306 /// // then, we'll yield the negation of the state
1310 /// assert_eq!(iter.next(), Some(-1));
1311 /// assert_eq!(iter.next(), Some(-2));
1312 /// assert_eq!(iter.next(), Some(-6));
1313 /// assert_eq!(iter.next(), None);
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1320 F: FnMut(&mut St, Self::Item) -> Option<B>,
1322 Scan::new(self, initial_state, f)
1325 /// Creates an iterator that works like map, but flattens nested structure.
1327 /// The [`map`] adapter is very useful, but only when the closure
1328 /// argument produces values. If it produces an iterator instead, there's
1329 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1332 /// You can think of `flat_map(f)` as the semantic equivalent
1333 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1335 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1336 /// one item for each element, and `flat_map()`'s closure returns an
1337 /// iterator for each element.
1339 /// [`map`]: Iterator::map
1340 /// [`flatten`]: Iterator::flatten
1347 /// let words = ["alpha", "beta", "gamma"];
1349 /// // chars() returns an iterator
1350 /// let merged: String = words.iter()
1351 /// .flat_map(|s| s.chars())
1353 /// assert_eq!(merged, "alphabetagamma");
1356 #[stable(feature = "rust1", since = "1.0.0")]
1357 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1361 F: FnMut(Self::Item) -> U,
1363 FlatMap::new(self, f)
1366 /// Creates an iterator that flattens nested structure.
1368 /// This is useful when you have an iterator of iterators or an iterator of
1369 /// things that can be turned into iterators and you want to remove one
1370 /// level of indirection.
1377 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1378 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1379 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1382 /// Mapping and then flattening:
1385 /// let words = ["alpha", "beta", "gamma"];
1387 /// // chars() returns an iterator
1388 /// let merged: String = words.iter()
1389 /// .map(|s| s.chars())
1392 /// assert_eq!(merged, "alphabetagamma");
1395 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1396 /// in this case since it conveys intent more clearly:
1399 /// let words = ["alpha", "beta", "gamma"];
1401 /// // chars() returns an iterator
1402 /// let merged: String = words.iter()
1403 /// .flat_map(|s| s.chars())
1405 /// assert_eq!(merged, "alphabetagamma");
1408 /// Flattening only removes one level of nesting at a time:
1411 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1413 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1414 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1416 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1417 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1420 /// Here we see that `flatten()` does not perform a "deep" flatten.
1421 /// Instead, only one level of nesting is removed. That is, if you
1422 /// `flatten()` a three-dimensional array, the result will be
1423 /// two-dimensional and not one-dimensional. To get a one-dimensional
1424 /// structure, you have to `flatten()` again.
1426 /// [`flat_map()`]: Iterator::flat_map
1428 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1429 fn flatten(self) -> Flatten<Self>
1432 Self::Item: IntoIterator,
1437 /// Creates an iterator which ends after the first [`None`].
1439 /// After an iterator returns [`None`], future calls may or may not yield
1440 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1441 /// [`None`] is given, it will always return [`None`] forever.
1443 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1444 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1445 /// if the [`FusedIterator`] trait is improperly implemented.
1447 /// [`Some(T)`]: Some
1448 /// [`FusedIterator`]: crate::iter::FusedIterator
1455 /// // an iterator which alternates between Some and None
1456 /// struct Alternate {
1460 /// impl Iterator for Alternate {
1461 /// type Item = i32;
1463 /// fn next(&mut self) -> Option<i32> {
1464 /// let val = self.state;
1465 /// self.state = self.state + 1;
1467 /// // if it's even, Some(i32), else None
1468 /// if val % 2 == 0 {
1476 /// let mut iter = Alternate { state: 0 };
1478 /// // we can see our iterator going back and forth
1479 /// assert_eq!(iter.next(), Some(0));
1480 /// assert_eq!(iter.next(), None);
1481 /// assert_eq!(iter.next(), Some(2));
1482 /// assert_eq!(iter.next(), None);
1484 /// // however, once we fuse it...
1485 /// let mut iter = iter.fuse();
1487 /// assert_eq!(iter.next(), Some(4));
1488 /// assert_eq!(iter.next(), None);
1490 /// // it will always return `None` after the first time.
1491 /// assert_eq!(iter.next(), None);
1492 /// assert_eq!(iter.next(), None);
1493 /// assert_eq!(iter.next(), None);
1496 #[stable(feature = "rust1", since = "1.0.0")]
1497 fn fuse(self) -> Fuse<Self>
1504 /// Does something with each element of an iterator, passing the value on.
1506 /// When using iterators, you'll often chain several of them together.
1507 /// While working on such code, you might want to check out what's
1508 /// happening at various parts in the pipeline. To do that, insert
1509 /// a call to `inspect()`.
1511 /// It's more common for `inspect()` to be used as a debugging tool than to
1512 /// exist in your final code, but applications may find it useful in certain
1513 /// situations when errors need to be logged before being discarded.
1520 /// let a = [1, 4, 2, 3];
1522 /// // this iterator sequence is complex.
1523 /// let sum = a.iter()
1525 /// .filter(|x| x % 2 == 0)
1526 /// .fold(0, |sum, i| sum + i);
1528 /// println!("{}", sum);
1530 /// // let's add some inspect() calls to investigate what's happening
1531 /// let sum = a.iter()
1533 /// .inspect(|x| println!("about to filter: {}", x))
1534 /// .filter(|x| x % 2 == 0)
1535 /// .inspect(|x| println!("made it through filter: {}", x))
1536 /// .fold(0, |sum, i| sum + i);
1538 /// println!("{}", sum);
1541 /// This will print:
1545 /// about to filter: 1
1546 /// about to filter: 4
1547 /// made it through filter: 4
1548 /// about to filter: 2
1549 /// made it through filter: 2
1550 /// about to filter: 3
1554 /// Logging errors before discarding them:
1557 /// let lines = ["1", "2", "a"];
1559 /// let sum: i32 = lines
1561 /// .map(|line| line.parse::<i32>())
1562 /// .inspect(|num| {
1563 /// if let Err(ref e) = *num {
1564 /// println!("Parsing error: {}", e);
1567 /// .filter_map(Result::ok)
1570 /// println!("Sum: {}", sum);
1573 /// This will print:
1576 /// Parsing error: invalid digit found in string
1580 #[stable(feature = "rust1", since = "1.0.0")]
1581 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1584 F: FnMut(&Self::Item),
1586 Inspect::new(self, f)
1589 /// Borrows an iterator, rather than consuming it.
1591 /// This is useful to allow applying iterator adapters while still
1592 /// retaining ownership of the original iterator.
1599 /// let mut words = vec!["hello", "world", "of", "Rust"].into_iter();
1601 /// // Take the first two words.
1602 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1603 /// assert_eq!(hello_world, vec!["hello", "world"]);
1605 /// // Collect the rest of the words.
1606 /// // We can only do this because we used `by_ref` earlier.
1607 /// let of_rust: Vec<_> = words.collect();
1608 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1610 #[stable(feature = "rust1", since = "1.0.0")]
1611 fn by_ref(&mut self) -> &mut Self
1618 /// Transforms an iterator into a collection.
1620 /// `collect()` can take anything iterable, and turn it into a relevant
1621 /// collection. This is one of the more powerful methods in the standard
1622 /// library, used in a variety of contexts.
1624 /// The most basic pattern in which `collect()` is used is to turn one
1625 /// collection into another. You take a collection, call [`iter`] on it,
1626 /// do a bunch of transformations, and then `collect()` at the end.
1628 /// `collect()` can also create instances of types that are not typical
1629 /// collections. For example, a [`String`] can be built from [`char`]s,
1630 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1631 /// into `Result<Collection<T>, E>`. See the examples below for more.
1633 /// Because `collect()` is so general, it can cause problems with type
1634 /// inference. As such, `collect()` is one of the few times you'll see
1635 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1636 /// helps the inference algorithm understand specifically which collection
1637 /// you're trying to collect into.
1644 /// let a = [1, 2, 3];
1646 /// let doubled: Vec<i32> = a.iter()
1647 /// .map(|&x| x * 2)
1650 /// assert_eq!(vec![2, 4, 6], doubled);
1653 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1654 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1656 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1659 /// use std::collections::VecDeque;
1661 /// let a = [1, 2, 3];
1663 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1665 /// assert_eq!(2, doubled[0]);
1666 /// assert_eq!(4, doubled[1]);
1667 /// assert_eq!(6, doubled[2]);
1670 /// Using the 'turbofish' instead of annotating `doubled`:
1673 /// let a = [1, 2, 3];
1675 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1677 /// assert_eq!(vec![2, 4, 6], doubled);
1680 /// Because `collect()` only cares about what you're collecting into, you can
1681 /// still use a partial type hint, `_`, with the turbofish:
1684 /// let a = [1, 2, 3];
1686 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1688 /// assert_eq!(vec![2, 4, 6], doubled);
1691 /// Using `collect()` to make a [`String`]:
1694 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1696 /// let hello: String = chars.iter()
1697 /// .map(|&x| x as u8)
1698 /// .map(|x| (x + 1) as char)
1701 /// assert_eq!("hello", hello);
1704 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1705 /// see if any of them failed:
1708 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1710 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1712 /// // gives us the first error
1713 /// assert_eq!(Err("nope"), result);
1715 /// let results = [Ok(1), Ok(3)];
1717 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1719 /// // gives us the list of answers
1720 /// assert_eq!(Ok(vec![1, 3]), result);
1723 /// [`iter`]: Iterator::next
1724 /// [`String`]: ../../std/string/struct.String.html
1725 /// [`char`]: type@char
1727 #[stable(feature = "rust1", since = "1.0.0")]
1728 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1729 fn collect<B: FromIterator<Self::Item>>(self) -> B
1733 FromIterator::from_iter(self)
1736 /// Consumes an iterator, creating two collections from it.
1738 /// The predicate passed to `partition()` can return `true`, or `false`.
1739 /// `partition()` returns a pair, all of the elements for which it returned
1740 /// `true`, and all of the elements for which it returned `false`.
1742 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1744 /// [`is_partitioned()`]: Iterator::is_partitioned
1745 /// [`partition_in_place()`]: Iterator::partition_in_place
1752 /// let a = [1, 2, 3];
1754 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1756 /// .partition(|&n| n % 2 == 0);
1758 /// assert_eq!(even, vec![2]);
1759 /// assert_eq!(odd, vec![1, 3]);
1761 #[stable(feature = "rust1", since = "1.0.0")]
1762 fn partition<B, F>(self, f: F) -> (B, B)
1765 B: Default + Extend<Self::Item>,
1766 F: FnMut(&Self::Item) -> bool,
1769 fn extend<'a, T, B: Extend<T>>(
1770 mut f: impl FnMut(&T) -> bool + 'a,
1773 ) -> impl FnMut((), T) + 'a {
1778 right.extend_one(x);
1783 let mut left: B = Default::default();
1784 let mut right: B = Default::default();
1786 self.fold((), extend(f, &mut left, &mut right));
1791 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1792 /// such that all those that return `true` precede all those that return `false`.
1793 /// Returns the number of `true` elements found.
1795 /// The relative order of partitioned items is not maintained.
1797 /// # Current implementation
1798 /// Current algorithms tries finding the first element for which the predicate evaluates
1799 /// to false, and the last element for which it evaluates to true and repeatedly swaps them.
1801 /// Time Complexity: *O*(*N*)
1803 /// See also [`is_partitioned()`] and [`partition()`].
1805 /// [`is_partitioned()`]: Iterator::is_partitioned
1806 /// [`partition()`]: Iterator::partition
1811 /// #![feature(iter_partition_in_place)]
1813 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1815 /// // Partition in-place between evens and odds
1816 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1818 /// assert_eq!(i, 3);
1819 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1820 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1822 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1823 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1825 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1826 P: FnMut(&T) -> bool,
1828 // FIXME: should we worry about the count overflowing? The only way to have more than
1829 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1831 // These closure "factory" functions exist to avoid genericity in `Self`.
1835 predicate: &'a mut impl FnMut(&T) -> bool,
1836 true_count: &'a mut usize,
1837 ) -> impl FnMut(&&mut T) -> bool + 'a {
1839 let p = predicate(&**x);
1840 *true_count += p as usize;
1846 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1847 move |x| predicate(&**x)
1850 // Repeatedly find the first `false` and swap it with the last `true`.
1851 let mut true_count = 0;
1852 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1853 if let Some(tail) = self.rfind(is_true(predicate)) {
1854 crate::mem::swap(head, tail);
1863 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1864 /// such that all those that return `true` precede all those that return `false`.
1866 /// See also [`partition()`] and [`partition_in_place()`].
1868 /// [`partition()`]: Iterator::partition
1869 /// [`partition_in_place()`]: Iterator::partition_in_place
1874 /// #![feature(iter_is_partitioned)]
1876 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1877 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1879 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1880 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1883 P: FnMut(Self::Item) -> bool,
1885 // Either all items test `true`, or the first clause stops at `false`
1886 // and we check that there are no more `true` items after that.
1887 self.all(&mut predicate) || !self.any(predicate)
1890 /// An iterator method that applies a function as long as it returns
1891 /// successfully, producing a single, final value.
1893 /// `try_fold()` takes two arguments: an initial value, and a closure with
1894 /// two arguments: an 'accumulator', and an element. The closure either
1895 /// returns successfully, with the value that the accumulator should have
1896 /// for the next iteration, or it returns failure, with an error value that
1897 /// is propagated back to the caller immediately (short-circuiting).
1899 /// The initial value is the value the accumulator will have on the first
1900 /// call. If applying the closure succeeded against every element of the
1901 /// iterator, `try_fold()` returns the final accumulator as success.
1903 /// Folding is useful whenever you have a collection of something, and want
1904 /// to produce a single value from it.
1906 /// # Note to Implementors
1908 /// Several of the other (forward) methods have default implementations in
1909 /// terms of this one, so try to implement this explicitly if it can
1910 /// do something better than the default `for` loop implementation.
1912 /// In particular, try to have this call `try_fold()` on the internal parts
1913 /// from which this iterator is composed. If multiple calls are needed,
1914 /// the `?` operator may be convenient for chaining the accumulator value
1915 /// along, but beware any invariants that need to be upheld before those
1916 /// early returns. This is a `&mut self` method, so iteration needs to be
1917 /// resumable after hitting an error here.
1924 /// let a = [1, 2, 3];
1926 /// // the checked sum of all of the elements of the array
1927 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1929 /// assert_eq!(sum, Some(6));
1932 /// Short-circuiting:
1935 /// let a = [10, 20, 30, 100, 40, 50];
1936 /// let mut it = a.iter();
1938 /// // This sum overflows when adding the 100 element
1939 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1940 /// assert_eq!(sum, None);
1942 /// // Because it short-circuited, the remaining elements are still
1943 /// // available through the iterator.
1944 /// assert_eq!(it.len(), 2);
1945 /// assert_eq!(it.next(), Some(&40));
1948 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
1952 /// use std::ops::ControlFlow;
1954 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
1955 /// if let Some(next) = prev.checked_add(x) {
1956 /// ControlFlow::Continue(next)
1958 /// ControlFlow::Break(prev)
1961 /// assert_eq!(triangular, ControlFlow::Break(120));
1963 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
1964 /// if let Some(next) = prev.checked_add(x) {
1965 /// ControlFlow::Continue(next)
1967 /// ControlFlow::Break(prev)
1970 /// assert_eq!(triangular, ControlFlow::Continue(435));
1973 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1974 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1977 F: FnMut(B, Self::Item) -> R,
1980 let mut accum = init;
1981 while let Some(x) = self.next() {
1982 accum = f(accum, x)?;
1987 /// An iterator method that applies a fallible function to each item in the
1988 /// iterator, stopping at the first error and returning that error.
1990 /// This can also be thought of as the fallible form of [`for_each()`]
1991 /// or as the stateless version of [`try_fold()`].
1993 /// [`for_each()`]: Iterator::for_each
1994 /// [`try_fold()`]: Iterator::try_fold
1999 /// use std::fs::rename;
2000 /// use std::io::{stdout, Write};
2001 /// use std::path::Path;
2003 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2005 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2006 /// assert!(res.is_ok());
2008 /// let mut it = data.iter().cloned();
2009 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2010 /// assert!(res.is_err());
2011 /// // It short-circuited, so the remaining items are still in the iterator:
2012 /// assert_eq!(it.next(), Some("stale_bread.json"));
2015 /// The [`ControlFlow`] type can be used with this method for the situations
2016 /// in which you'd use `break` and `continue` in a normal loop:
2019 /// use std::ops::ControlFlow;
2021 /// let r = (2..100).try_for_each(|x| {
2022 /// if 323 % x == 0 {
2023 /// return ControlFlow::Break(x)
2026 /// ControlFlow::Continue(())
2028 /// assert_eq!(r, ControlFlow::Break(17));
2031 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2032 fn try_for_each<F, R>(&mut self, f: F) -> R
2035 F: FnMut(Self::Item) -> R,
2036 R: Try<Output = ()>,
2039 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2043 self.try_fold((), call(f))
2046 /// Folds every element into an accumulator by applying an operation,
2047 /// returning the final result.
2049 /// `fold()` takes two arguments: an initial value, and a closure with two
2050 /// arguments: an 'accumulator', and an element. The closure returns the value that
2051 /// the accumulator should have for the next iteration.
2053 /// The initial value is the value the accumulator will have on the first
2056 /// After applying this closure to every element of the iterator, `fold()`
2057 /// returns the accumulator.
2059 /// This operation is sometimes called 'reduce' or 'inject'.
2061 /// Folding is useful whenever you have a collection of something, and want
2062 /// to produce a single value from it.
2064 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2065 /// might not terminate for infinite iterators, even on traits for which a
2066 /// result is determinable in finite time.
2068 /// Note: [`reduce()`] can be used to use the first element as the initial
2069 /// value, if the accumulator type and item type is the same.
2071 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2072 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2073 /// operators like `-` the order will affect the final result.
2074 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2076 /// # Note to Implementors
2078 /// Several of the other (forward) methods have default implementations in
2079 /// terms of this one, so try to implement this explicitly if it can
2080 /// do something better than the default `for` loop implementation.
2082 /// In particular, try to have this call `fold()` on the internal parts
2083 /// from which this iterator is composed.
2090 /// let a = [1, 2, 3];
2092 /// // the sum of all of the elements of the array
2093 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2095 /// assert_eq!(sum, 6);
2098 /// Let's walk through each step of the iteration here:
2100 /// | element | acc | x | result |
2101 /// |---------|-----|---|--------|
2103 /// | 1 | 0 | 1 | 1 |
2104 /// | 2 | 1 | 2 | 3 |
2105 /// | 3 | 3 | 3 | 6 |
2107 /// And so, our final result, `6`.
2109 /// This example demonstrates the left-associative nature of `fold()`:
2110 /// it builds a string, starting with an initial value
2111 /// and continuing with each element from the front until the back:
2114 /// let numbers = [1, 2, 3, 4, 5];
2116 /// let zero = "0".to_string();
2118 /// let result = numbers.iter().fold(zero, |acc, &x| {
2119 /// format!("({} + {})", acc, x)
2122 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2124 /// It's common for people who haven't used iterators a lot to
2125 /// use a `for` loop with a list of things to build up a result. Those
2126 /// can be turned into `fold()`s:
2128 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2131 /// let numbers = [1, 2, 3, 4, 5];
2133 /// let mut result = 0;
2136 /// for i in &numbers {
2137 /// result = result + i;
2141 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2143 /// // they're the same
2144 /// assert_eq!(result, result2);
2147 /// [`reduce()`]: Iterator::reduce
2148 #[doc(alias = "inject", alias = "foldl")]
2150 #[stable(feature = "rust1", since = "1.0.0")]
2151 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2154 F: FnMut(B, Self::Item) -> B,
2156 let mut accum = init;
2157 while let Some(x) = self.next() {
2158 accum = f(accum, x);
2163 /// Reduces the elements to a single one, by repeatedly applying a reducing
2166 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2167 /// result of the reduction.
2169 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2170 /// For iterators with at least one element, this is the same as [`fold()`]
2171 /// with the first element of the iterator as the initial accumulator value, folding
2172 /// every subsequent element into it.
2174 /// [`fold()`]: Iterator::fold
2178 /// Find the maximum value:
2181 /// fn find_max<I>(iter: I) -> Option<I::Item>
2182 /// where I: Iterator,
2185 /// iter.reduce(|accum, item| {
2186 /// if accum >= item { accum } else { item }
2189 /// let a = [10, 20, 5, -23, 0];
2190 /// let b: [u32; 0] = [];
2192 /// assert_eq!(find_max(a.iter()), Some(&20));
2193 /// assert_eq!(find_max(b.iter()), None);
2196 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2197 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2200 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2202 let first = self.next()?;
2203 Some(self.fold(first, f))
2206 /// Tests if every element of the iterator matches a predicate.
2208 /// `all()` takes a closure that returns `true` or `false`. It applies
2209 /// this closure to each element of the iterator, and if they all return
2210 /// `true`, then so does `all()`. If any of them return `false`, it
2211 /// returns `false`.
2213 /// `all()` is short-circuiting; in other words, it will stop processing
2214 /// as soon as it finds a `false`, given that no matter what else happens,
2215 /// the result will also be `false`.
2217 /// An empty iterator returns `true`.
2224 /// let a = [1, 2, 3];
2226 /// assert!(a.iter().all(|&x| x > 0));
2228 /// assert!(!a.iter().all(|&x| x > 2));
2231 /// Stopping at the first `false`:
2234 /// let a = [1, 2, 3];
2236 /// let mut iter = a.iter();
2238 /// assert!(!iter.all(|&x| x != 2));
2240 /// // we can still use `iter`, as there are more elements.
2241 /// assert_eq!(iter.next(), Some(&3));
2244 #[stable(feature = "rust1", since = "1.0.0")]
2245 fn all<F>(&mut self, f: F) -> bool
2248 F: FnMut(Self::Item) -> bool,
2251 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2253 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2256 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2259 /// Tests if any element of the iterator matches a predicate.
2261 /// `any()` takes a closure that returns `true` or `false`. It applies
2262 /// this closure to each element of the iterator, and if any of them return
2263 /// `true`, then so does `any()`. If they all return `false`, it
2264 /// returns `false`.
2266 /// `any()` is short-circuiting; in other words, it will stop processing
2267 /// as soon as it finds a `true`, given that no matter what else happens,
2268 /// the result will also be `true`.
2270 /// An empty iterator returns `false`.
2277 /// let a = [1, 2, 3];
2279 /// assert!(a.iter().any(|&x| x > 0));
2281 /// assert!(!a.iter().any(|&x| x > 5));
2284 /// Stopping at the first `true`:
2287 /// let a = [1, 2, 3];
2289 /// let mut iter = a.iter();
2291 /// assert!(iter.any(|&x| x != 2));
2293 /// // we can still use `iter`, as there are more elements.
2294 /// assert_eq!(iter.next(), Some(&2));
2297 #[stable(feature = "rust1", since = "1.0.0")]
2298 fn any<F>(&mut self, f: F) -> bool
2301 F: FnMut(Self::Item) -> bool,
2304 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2306 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2310 self.try_fold((), check(f)) == ControlFlow::BREAK
2313 /// Searches for an element of an iterator that satisfies a predicate.
2315 /// `find()` takes a closure that returns `true` or `false`. It applies
2316 /// this closure to each element of the iterator, and if any of them return
2317 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2318 /// `false`, it returns [`None`].
2320 /// `find()` is short-circuiting; in other words, it will stop processing
2321 /// as soon as the closure returns `true`.
2323 /// Because `find()` takes a reference, and many iterators iterate over
2324 /// references, this leads to a possibly confusing situation where the
2325 /// argument is a double reference. You can see this effect in the
2326 /// examples below, with `&&x`.
2328 /// [`Some(element)`]: Some
2335 /// let a = [1, 2, 3];
2337 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2339 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2342 /// Stopping at the first `true`:
2345 /// let a = [1, 2, 3];
2347 /// let mut iter = a.iter();
2349 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2351 /// // we can still use `iter`, as there are more elements.
2352 /// assert_eq!(iter.next(), Some(&3));
2355 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2357 #[stable(feature = "rust1", since = "1.0.0")]
2358 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2361 P: FnMut(&Self::Item) -> bool,
2364 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2366 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2370 self.try_fold((), check(predicate)).break_value()
2373 /// Applies function to the elements of iterator and returns
2374 /// the first non-none result.
2376 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2381 /// let a = ["lol", "NaN", "2", "5"];
2383 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2385 /// assert_eq!(first_number, Some(2));
2388 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2389 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2392 F: FnMut(Self::Item) -> Option<B>,
2395 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2396 move |(), x| match f(x) {
2397 Some(x) => ControlFlow::Break(x),
2398 None => ControlFlow::CONTINUE,
2402 self.try_fold((), check(f)).break_value()
2405 /// Applies function to the elements of iterator and returns
2406 /// the first true result or the first error.
2411 /// #![feature(try_find)]
2413 /// let a = ["1", "2", "lol", "NaN", "5"];
2415 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2416 /// Ok(s.parse::<i32>()? == search)
2419 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2420 /// assert_eq!(result, Ok(Some(&"2")));
2422 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2423 /// assert!(result.is_err());
2426 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2427 fn try_find<F, R, E>(&mut self, f: F) -> Result<Option<Self::Item>, E>
2430 F: FnMut(&Self::Item) -> R,
2431 R: Try<Output = bool>,
2432 // FIXME: This bound is rather strange, but means minimal breakage on nightly.
2433 // See #85115 for the issue tracking a holistic solution for this and try_map.
2434 R: Try<Residual = Result<crate::convert::Infallible, E>>,
2437 fn check<F, T, R, E>(mut f: F) -> impl FnMut((), T) -> ControlFlow<Result<T, E>>
2440 R: Try<Output = bool>,
2441 R: Try<Residual = Result<crate::convert::Infallible, E>>,
2443 move |(), x| match f(&x).branch() {
2444 ControlFlow::Continue(false) => ControlFlow::CONTINUE,
2445 ControlFlow::Continue(true) => ControlFlow::Break(Ok(x)),
2446 ControlFlow::Break(Err(x)) => ControlFlow::Break(Err(x)),
2450 self.try_fold((), check(f)).break_value().transpose()
2453 /// Searches for an element in an iterator, returning its index.
2455 /// `position()` takes a closure that returns `true` or `false`. It applies
2456 /// this closure to each element of the iterator, and if one of them
2457 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2458 /// them return `false`, it returns [`None`].
2460 /// `position()` is short-circuiting; in other words, it will stop
2461 /// processing as soon as it finds a `true`.
2463 /// # Overflow Behavior
2465 /// The method does no guarding against overflows, so if there are more
2466 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2467 /// result or panics. If debug assertions are enabled, a panic is
2472 /// This function might panic if the iterator has more than `usize::MAX`
2473 /// non-matching elements.
2475 /// [`Some(index)`]: Some
2482 /// let a = [1, 2, 3];
2484 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2486 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2489 /// Stopping at the first `true`:
2492 /// let a = [1, 2, 3, 4];
2494 /// let mut iter = a.iter();
2496 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2498 /// // we can still use `iter`, as there are more elements.
2499 /// assert_eq!(iter.next(), Some(&3));
2501 /// // The returned index depends on iterator state
2502 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2506 #[stable(feature = "rust1", since = "1.0.0")]
2507 fn position<P>(&mut self, predicate: P) -> Option<usize>
2510 P: FnMut(Self::Item) -> bool,
2514 mut predicate: impl FnMut(T) -> bool,
2515 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2516 #[rustc_inherit_overflow_checks]
2518 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2522 self.try_fold(0, check(predicate)).break_value()
2525 /// Searches for an element in an iterator from the right, returning its
2528 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2529 /// this closure to each element of the iterator, starting from the end,
2530 /// and if one of them returns `true`, then `rposition()` returns
2531 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2533 /// `rposition()` is short-circuiting; in other words, it will stop
2534 /// processing as soon as it finds a `true`.
2536 /// [`Some(index)`]: Some
2543 /// let a = [1, 2, 3];
2545 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2547 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2550 /// Stopping at the first `true`:
2553 /// let a = [1, 2, 3];
2555 /// let mut iter = a.iter();
2557 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2559 /// // we can still use `iter`, as there are more elements.
2560 /// assert_eq!(iter.next(), Some(&1));
2563 #[stable(feature = "rust1", since = "1.0.0")]
2564 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2566 P: FnMut(Self::Item) -> bool,
2567 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2569 // No need for an overflow check here, because `ExactSizeIterator`
2570 // implies that the number of elements fits into a `usize`.
2573 mut predicate: impl FnMut(T) -> bool,
2574 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2577 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2582 self.try_rfold(n, check(predicate)).break_value()
2585 /// Returns the maximum element of an iterator.
2587 /// If several elements are equally maximum, the last element is
2588 /// returned. If the iterator is empty, [`None`] is returned.
2590 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2591 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2594 /// vec![2.4, f32::NAN, 1.3]
2596 /// .reduce(f32::max)
2607 /// let a = [1, 2, 3];
2608 /// let b: Vec<u32> = Vec::new();
2610 /// assert_eq!(a.iter().max(), Some(&3));
2611 /// assert_eq!(b.iter().max(), None);
2614 #[stable(feature = "rust1", since = "1.0.0")]
2615 fn max(self) -> Option<Self::Item>
2620 self.max_by(Ord::cmp)
2623 /// Returns the minimum element of an iterator.
2625 /// If several elements are equally minimum, the first element is returned.
2626 /// If the iterator is empty, [`None`] is returned.
2628 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2629 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2632 /// vec![2.4, f32::NAN, 1.3]
2634 /// .reduce(f32::min)
2645 /// let a = [1, 2, 3];
2646 /// let b: Vec<u32> = Vec::new();
2648 /// assert_eq!(a.iter().min(), Some(&1));
2649 /// assert_eq!(b.iter().min(), None);
2652 #[stable(feature = "rust1", since = "1.0.0")]
2653 fn min(self) -> Option<Self::Item>
2658 self.min_by(Ord::cmp)
2661 /// Returns the element that gives the maximum value from the
2662 /// specified function.
2664 /// If several elements are equally maximum, the last element is
2665 /// returned. If the iterator is empty, [`None`] is returned.
2670 /// let a = [-3_i32, 0, 1, 5, -10];
2671 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2674 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2675 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2678 F: FnMut(&Self::Item) -> B,
2681 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2686 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2690 let (_, x) = self.map(key(f)).max_by(compare)?;
2694 /// Returns the element that gives the maximum value with respect to the
2695 /// specified comparison function.
2697 /// If several elements are equally maximum, the last element is
2698 /// returned. If the iterator is empty, [`None`] is returned.
2703 /// let a = [-3_i32, 0, 1, 5, -10];
2704 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2707 #[stable(feature = "iter_max_by", since = "1.15.0")]
2708 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2711 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2714 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2715 move |x, y| cmp::max_by(x, y, &mut compare)
2718 self.reduce(fold(compare))
2721 /// Returns the element that gives the minimum value from the
2722 /// specified function.
2724 /// If several elements are equally minimum, the first element is
2725 /// returned. If the iterator is empty, [`None`] is returned.
2730 /// let a = [-3_i32, 0, 1, 5, -10];
2731 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2734 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2735 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2738 F: FnMut(&Self::Item) -> B,
2741 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2746 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2750 let (_, x) = self.map(key(f)).min_by(compare)?;
2754 /// Returns the element that gives the minimum value with respect to the
2755 /// specified comparison function.
2757 /// If several elements are equally minimum, the first element is
2758 /// returned. If the iterator is empty, [`None`] is returned.
2763 /// let a = [-3_i32, 0, 1, 5, -10];
2764 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2767 #[stable(feature = "iter_min_by", since = "1.15.0")]
2768 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2771 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2774 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2775 move |x, y| cmp::min_by(x, y, &mut compare)
2778 self.reduce(fold(compare))
2781 /// Reverses an iterator's direction.
2783 /// Usually, iterators iterate from left to right. After using `rev()`,
2784 /// an iterator will instead iterate from right to left.
2786 /// This is only possible if the iterator has an end, so `rev()` only
2787 /// works on [`DoubleEndedIterator`]s.
2792 /// let a = [1, 2, 3];
2794 /// let mut iter = a.iter().rev();
2796 /// assert_eq!(iter.next(), Some(&3));
2797 /// assert_eq!(iter.next(), Some(&2));
2798 /// assert_eq!(iter.next(), Some(&1));
2800 /// assert_eq!(iter.next(), None);
2803 #[doc(alias = "reverse")]
2804 #[stable(feature = "rust1", since = "1.0.0")]
2805 fn rev(self) -> Rev<Self>
2807 Self: Sized + DoubleEndedIterator,
2812 /// Converts an iterator of pairs into a pair of containers.
2814 /// `unzip()` consumes an entire iterator of pairs, producing two
2815 /// collections: one from the left elements of the pairs, and one
2816 /// from the right elements.
2818 /// This function is, in some sense, the opposite of [`zip`].
2820 /// [`zip`]: Iterator::zip
2827 /// let a = [(1, 2), (3, 4)];
2829 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2831 /// assert_eq!(left, [1, 3]);
2832 /// assert_eq!(right, [2, 4]);
2834 /// // you can also unzip multiple nested tuples at once
2835 /// let a = [(1, (2, 3)), (4, (5, 6))];
2837 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
2838 /// assert_eq!(x, [1, 4]);
2839 /// assert_eq!(y, [2, 5]);
2840 /// assert_eq!(z, [3, 6]);
2842 #[stable(feature = "rust1", since = "1.0.0")]
2843 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2845 FromA: Default + Extend<A>,
2846 FromB: Default + Extend<B>,
2847 Self: Sized + Iterator<Item = (A, B)>,
2849 let mut unzipped: (FromA, FromB) = Default::default();
2850 unzipped.extend(self);
2854 /// Creates an iterator which copies all of its elements.
2856 /// This is useful when you have an iterator over `&T`, but you need an
2857 /// iterator over `T`.
2864 /// let a = [1, 2, 3];
2866 /// let v_copied: Vec<_> = a.iter().copied().collect();
2868 /// // copied is the same as .map(|&x| x)
2869 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2871 /// assert_eq!(v_copied, vec![1, 2, 3]);
2872 /// assert_eq!(v_map, vec![1, 2, 3]);
2874 #[stable(feature = "iter_copied", since = "1.36.0")]
2875 fn copied<'a, T: 'a>(self) -> Copied<Self>
2877 Self: Sized + Iterator<Item = &'a T>,
2883 /// Creates an iterator which [`clone`]s all of its elements.
2885 /// This is useful when you have an iterator over `&T`, but you need an
2886 /// iterator over `T`.
2888 /// [`clone`]: Clone::clone
2895 /// let a = [1, 2, 3];
2897 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2899 /// // cloned is the same as .map(|&x| x), for integers
2900 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2902 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2903 /// assert_eq!(v_map, vec![1, 2, 3]);
2905 #[stable(feature = "rust1", since = "1.0.0")]
2906 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2908 Self: Sized + Iterator<Item = &'a T>,
2914 /// Repeats an iterator endlessly.
2916 /// Instead of stopping at [`None`], the iterator will instead start again,
2917 /// from the beginning. After iterating again, it will start at the
2918 /// beginning again. And again. And again. Forever.
2925 /// let a = [1, 2, 3];
2927 /// let mut it = a.iter().cycle();
2929 /// assert_eq!(it.next(), Some(&1));
2930 /// assert_eq!(it.next(), Some(&2));
2931 /// assert_eq!(it.next(), Some(&3));
2932 /// assert_eq!(it.next(), Some(&1));
2933 /// assert_eq!(it.next(), Some(&2));
2934 /// assert_eq!(it.next(), Some(&3));
2935 /// assert_eq!(it.next(), Some(&1));
2937 #[stable(feature = "rust1", since = "1.0.0")]
2939 fn cycle(self) -> Cycle<Self>
2941 Self: Sized + Clone,
2946 /// Sums the elements of an iterator.
2948 /// Takes each element, adds them together, and returns the result.
2950 /// An empty iterator returns the zero value of the type.
2954 /// When calling `sum()` and a primitive integer type is being returned, this
2955 /// method will panic if the computation overflows and debug assertions are
2963 /// let a = [1, 2, 3];
2964 /// let sum: i32 = a.iter().sum();
2966 /// assert_eq!(sum, 6);
2968 #[stable(feature = "iter_arith", since = "1.11.0")]
2969 fn sum<S>(self) -> S
2977 /// Iterates over the entire iterator, multiplying all the elements
2979 /// An empty iterator returns the one value of the type.
2983 /// When calling `product()` and a primitive integer type is being returned,
2984 /// method will panic if the computation overflows and debug assertions are
2990 /// fn factorial(n: u32) -> u32 {
2991 /// (1..=n).product()
2993 /// assert_eq!(factorial(0), 1);
2994 /// assert_eq!(factorial(1), 1);
2995 /// assert_eq!(factorial(5), 120);
2997 #[stable(feature = "iter_arith", since = "1.11.0")]
2998 fn product<P>(self) -> P
3001 P: Product<Self::Item>,
3003 Product::product(self)
3006 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3012 /// use std::cmp::Ordering;
3014 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3015 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3016 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3018 #[stable(feature = "iter_order", since = "1.5.0")]
3019 fn cmp<I>(self, other: I) -> Ordering
3021 I: IntoIterator<Item = Self::Item>,
3025 self.cmp_by(other, |x, y| x.cmp(&y))
3028 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3029 /// of another with respect to the specified comparison function.
3036 /// #![feature(iter_order_by)]
3038 /// use std::cmp::Ordering;
3040 /// let xs = [1, 2, 3, 4];
3041 /// let ys = [1, 4, 9, 16];
3043 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3044 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3045 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3047 #[unstable(feature = "iter_order_by", issue = "64295")]
3048 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
3052 F: FnMut(Self::Item, I::Item) -> Ordering,
3054 let mut other = other.into_iter();
3057 let x = match self.next() {
3059 if other.next().is_none() {
3060 return Ordering::Equal;
3062 return Ordering::Less;
3068 let y = match other.next() {
3069 None => return Ordering::Greater,
3074 Ordering::Equal => (),
3075 non_eq => return non_eq,
3080 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3086 /// use std::cmp::Ordering;
3088 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3089 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3090 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3092 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3094 #[stable(feature = "iter_order", since = "1.5.0")]
3095 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3098 Self::Item: PartialOrd<I::Item>,
3101 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3104 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3105 /// of another with respect to the specified comparison function.
3112 /// #![feature(iter_order_by)]
3114 /// use std::cmp::Ordering;
3116 /// let xs = [1.0, 2.0, 3.0, 4.0];
3117 /// let ys = [1.0, 4.0, 9.0, 16.0];
3120 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3121 /// Some(Ordering::Less)
3124 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3125 /// Some(Ordering::Equal)
3128 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3129 /// Some(Ordering::Greater)
3132 #[unstable(feature = "iter_order_by", issue = "64295")]
3133 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3137 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3139 let mut other = other.into_iter();
3142 let x = match self.next() {
3144 if other.next().is_none() {
3145 return Some(Ordering::Equal);
3147 return Some(Ordering::Less);
3153 let y = match other.next() {
3154 None => return Some(Ordering::Greater),
3158 match partial_cmp(x, y) {
3159 Some(Ordering::Equal) => (),
3160 non_eq => return non_eq,
3165 /// Determines if the elements of this [`Iterator`] are equal to those of
3171 /// assert_eq!([1].iter().eq([1].iter()), true);
3172 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3174 #[stable(feature = "iter_order", since = "1.5.0")]
3175 fn eq<I>(self, other: I) -> bool
3178 Self::Item: PartialEq<I::Item>,
3181 self.eq_by(other, |x, y| x == y)
3184 /// Determines if the elements of this [`Iterator`] are equal to those of
3185 /// another with respect to the specified equality function.
3192 /// #![feature(iter_order_by)]
3194 /// let xs = [1, 2, 3, 4];
3195 /// let ys = [1, 4, 9, 16];
3197 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3199 #[unstable(feature = "iter_order_by", issue = "64295")]
3200 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3204 F: FnMut(Self::Item, I::Item) -> bool,
3206 let mut other = other.into_iter();
3209 let x = match self.next() {
3210 None => return other.next().is_none(),
3214 let y = match other.next() {
3215 None => return false,
3225 /// Determines if the elements of this [`Iterator`] are unequal to those of
3231 /// assert_eq!([1].iter().ne([1].iter()), false);
3232 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3234 #[stable(feature = "iter_order", since = "1.5.0")]
3235 fn ne<I>(self, other: I) -> bool
3238 Self::Item: PartialEq<I::Item>,
3244 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3245 /// less than those of another.
3250 /// assert_eq!([1].iter().lt([1].iter()), false);
3251 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3252 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3253 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3255 #[stable(feature = "iter_order", since = "1.5.0")]
3256 fn lt<I>(self, other: I) -> bool
3259 Self::Item: PartialOrd<I::Item>,
3262 self.partial_cmp(other) == Some(Ordering::Less)
3265 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3266 /// less or equal to those of another.
3271 /// assert_eq!([1].iter().le([1].iter()), true);
3272 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3273 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3274 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3276 #[stable(feature = "iter_order", since = "1.5.0")]
3277 fn le<I>(self, other: I) -> bool
3280 Self::Item: PartialOrd<I::Item>,
3283 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3286 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3287 /// greater than those of another.
3292 /// assert_eq!([1].iter().gt([1].iter()), false);
3293 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3294 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3295 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3297 #[stable(feature = "iter_order", since = "1.5.0")]
3298 fn gt<I>(self, other: I) -> bool
3301 Self::Item: PartialOrd<I::Item>,
3304 self.partial_cmp(other) == Some(Ordering::Greater)
3307 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3308 /// greater than or equal to those of another.
3313 /// assert_eq!([1].iter().ge([1].iter()), true);
3314 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3315 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3316 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3318 #[stable(feature = "iter_order", since = "1.5.0")]
3319 fn ge<I>(self, other: I) -> bool
3322 Self::Item: PartialOrd<I::Item>,
3325 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3328 /// Checks if the elements of this iterator are sorted.
3330 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3331 /// iterator yields exactly zero or one element, `true` is returned.
3333 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3334 /// implies that this function returns `false` if any two consecutive items are not
3340 /// #![feature(is_sorted)]
3342 /// assert!([1, 2, 2, 9].iter().is_sorted());
3343 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3344 /// assert!([0].iter().is_sorted());
3345 /// assert!(std::iter::empty::<i32>().is_sorted());
3346 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3349 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3350 fn is_sorted(self) -> bool
3353 Self::Item: PartialOrd,
3355 self.is_sorted_by(PartialOrd::partial_cmp)
3358 /// Checks if the elements of this iterator are sorted using the given comparator function.
3360 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3361 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3362 /// [`is_sorted`]; see its documentation for more information.
3367 /// #![feature(is_sorted)]
3369 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3370 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3371 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3372 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3373 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3376 /// [`is_sorted`]: Iterator::is_sorted
3377 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3378 fn is_sorted_by<F>(mut self, compare: F) -> bool
3381 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3386 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3387 ) -> impl FnMut(T) -> bool + 'a {
3389 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3397 let mut last = match self.next() {
3399 None => return true,
3402 self.all(check(&mut last, compare))
3405 /// Checks if the elements of this iterator are sorted using the given key extraction
3408 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3409 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3410 /// its documentation for more information.
3412 /// [`is_sorted`]: Iterator::is_sorted
3417 /// #![feature(is_sorted)]
3419 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3420 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3423 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3424 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3427 F: FnMut(Self::Item) -> K,
3430 self.map(f).is_sorted()
3433 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3434 // The unusual name is to avoid name collisions in method resolution
3438 #[unstable(feature = "trusted_random_access", issue = "none")]
3439 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3441 Self: TrustedRandomAccessNoCoerce,
3443 unreachable!("Always specialized");
3447 #[stable(feature = "rust1", since = "1.0.0")]
3448 impl<I: Iterator + ?Sized> Iterator for &mut I {
3449 type Item = I::Item;
3450 fn next(&mut self) -> Option<I::Item> {
3453 fn size_hint(&self) -> (usize, Option<usize>) {
3454 (**self).size_hint()
3456 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3457 (**self).advance_by(n)
3459 fn nth(&mut self, n: usize) -> Option<Self::Item> {