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, 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 /// Returns the `n`th element of the iterator.
289 /// Like most indexing operations, the count starts from zero, so `nth(0)`
290 /// returns the first value, `nth(1)` the second, and so on.
292 /// Note that all preceding elements, as well as the returned element, will be
293 /// consumed from the iterator. That means that the preceding elements will be
294 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
295 /// will return different elements.
297 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
305 /// let a = [1, 2, 3];
306 /// assert_eq!(a.iter().nth(1), Some(&2));
309 /// Calling `nth()` multiple times doesn't rewind the iterator:
312 /// let a = [1, 2, 3];
314 /// let mut iter = a.iter();
316 /// assert_eq!(iter.nth(1), Some(&2));
317 /// assert_eq!(iter.nth(1), None);
320 /// Returning `None` if there are less than `n + 1` elements:
323 /// let a = [1, 2, 3];
324 /// assert_eq!(a.iter().nth(10), None);
327 #[stable(feature = "rust1", since = "1.0.0")]
328 fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
329 while let Some(x) = self.next() {
338 /// Creates an iterator starting at the same point, but stepping by
339 /// the given amount at each iteration.
341 /// Note 1: The first element of the iterator will always be returned,
342 /// regardless of the step given.
344 /// Note 2: The time at which ignored elements are pulled is not fixed.
345 /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
346 /// but is also free to behave like the sequence
347 /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
348 /// Which way is used may change for some iterators for performance reasons.
349 /// The second way will advance the iterator earlier and may consume more items.
351 /// `advance_n_and_return_first` is the equivalent of:
353 /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
357 /// let next = iter.next();
358 /// if total_step > 1 {
359 /// iter.nth(total_step-2);
367 /// The method will panic if the given step is `0`.
374 /// let a = [0, 1, 2, 3, 4, 5];
375 /// let mut iter = a.iter().step_by(2);
377 /// assert_eq!(iter.next(), Some(&0));
378 /// assert_eq!(iter.next(), Some(&2));
379 /// assert_eq!(iter.next(), Some(&4));
380 /// assert_eq!(iter.next(), None);
383 #[stable(feature = "iterator_step_by", since = "1.28.0")]
384 fn step_by(self, step: usize) -> StepBy<Self>
388 StepBy::new(self, step)
391 /// Takes two iterators and creates a new iterator over both in sequence.
393 /// `chain()` will return a new iterator which will first iterate over
394 /// values from the first iterator and then over values from the second
397 /// In other words, it links two iterators together, in a chain. 🔗
399 /// [`once`] is commonly used to adapt a single value into a chain of
400 /// other kinds of iteration.
407 /// let a1 = [1, 2, 3];
408 /// let a2 = [4, 5, 6];
410 /// let mut iter = a1.iter().chain(a2.iter());
412 /// assert_eq!(iter.next(), Some(&1));
413 /// assert_eq!(iter.next(), Some(&2));
414 /// assert_eq!(iter.next(), Some(&3));
415 /// assert_eq!(iter.next(), Some(&4));
416 /// assert_eq!(iter.next(), Some(&5));
417 /// assert_eq!(iter.next(), Some(&6));
418 /// assert_eq!(iter.next(), None);
421 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
422 /// anything that can be converted into an [`Iterator`], not just an
423 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
424 /// [`IntoIterator`], and so can be passed to `chain()` directly:
427 /// let s1 = &[1, 2, 3];
428 /// let s2 = &[4, 5, 6];
430 /// let mut iter = s1.iter().chain(s2);
432 /// assert_eq!(iter.next(), Some(&1));
433 /// assert_eq!(iter.next(), Some(&2));
434 /// assert_eq!(iter.next(), Some(&3));
435 /// assert_eq!(iter.next(), Some(&4));
436 /// assert_eq!(iter.next(), Some(&5));
437 /// assert_eq!(iter.next(), Some(&6));
438 /// assert_eq!(iter.next(), None);
441 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
445 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
446 /// use std::os::windows::ffi::OsStrExt;
447 /// s.encode_wide().chain(std::iter::once(0)).collect()
451 /// [`once`]: crate::iter::once
452 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
454 #[stable(feature = "rust1", since = "1.0.0")]
455 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
458 U: IntoIterator<Item = Self::Item>,
460 Chain::new(self, other.into_iter())
463 /// 'Zips up' two iterators into a single iterator of pairs.
465 /// `zip()` returns a new iterator that will iterate over two other
466 /// iterators, returning a tuple where the first element comes from the
467 /// first iterator, and the second element comes from the second iterator.
469 /// In other words, it zips two iterators together, into a single one.
471 /// If either iterator returns [`None`], [`next`] from the zipped iterator
472 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
473 /// short-circuit and `next` will not be called on the second iterator.
480 /// let a1 = [1, 2, 3];
481 /// let a2 = [4, 5, 6];
483 /// let mut iter = a1.iter().zip(a2.iter());
485 /// assert_eq!(iter.next(), Some((&1, &4)));
486 /// assert_eq!(iter.next(), Some((&2, &5)));
487 /// assert_eq!(iter.next(), Some((&3, &6)));
488 /// assert_eq!(iter.next(), None);
491 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
492 /// anything that can be converted into an [`Iterator`], not just an
493 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
494 /// [`IntoIterator`], and so can be passed to `zip()` directly:
497 /// let s1 = &[1, 2, 3];
498 /// let s2 = &[4, 5, 6];
500 /// let mut iter = s1.iter().zip(s2);
502 /// assert_eq!(iter.next(), Some((&1, &4)));
503 /// assert_eq!(iter.next(), Some((&2, &5)));
504 /// assert_eq!(iter.next(), Some((&3, &6)));
505 /// assert_eq!(iter.next(), None);
508 /// `zip()` is often used to zip an infinite iterator to a finite one.
509 /// This works because the finite iterator will eventually return [`None`],
510 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
513 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
515 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
517 /// assert_eq!((0, 'f'), enumerate[0]);
518 /// assert_eq!((0, 'f'), zipper[0]);
520 /// assert_eq!((1, 'o'), enumerate[1]);
521 /// assert_eq!((1, 'o'), zipper[1]);
523 /// assert_eq!((2, 'o'), enumerate[2]);
524 /// assert_eq!((2, 'o'), zipper[2]);
527 /// [`enumerate`]: Iterator::enumerate
528 /// [`next`]: Iterator::next
530 #[stable(feature = "rust1", since = "1.0.0")]
531 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
536 Zip::new(self, other.into_iter())
539 /// Takes a closure and creates an iterator which calls that closure on each
542 /// `map()` transforms one iterator into another, by means of its argument:
543 /// something that implements [`FnMut`]. It produces a new iterator which
544 /// calls this closure on each element of the original iterator.
546 /// If you are good at thinking in types, you can think of `map()` like this:
547 /// If you have an iterator that gives you elements of some type `A`, and
548 /// you want an iterator of some other type `B`, you can use `map()`,
549 /// passing a closure that takes an `A` and returns a `B`.
551 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
552 /// lazy, it is best used when you're already working with other iterators.
553 /// If you're doing some sort of looping for a side effect, it's considered
554 /// more idiomatic to use [`for`] than `map()`.
556 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
557 /// [`FnMut`]: crate::ops::FnMut
564 /// let a = [1, 2, 3];
566 /// let mut iter = a.iter().map(|x| 2 * x);
568 /// assert_eq!(iter.next(), Some(2));
569 /// assert_eq!(iter.next(), Some(4));
570 /// assert_eq!(iter.next(), Some(6));
571 /// assert_eq!(iter.next(), None);
574 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
577 /// # #![allow(unused_must_use)]
578 /// // don't do this:
579 /// (0..5).map(|x| println!("{}", x));
581 /// // it won't even execute, as it is lazy. Rust will warn you about this.
583 /// // Instead, use for:
585 /// println!("{}", x);
589 #[stable(feature = "rust1", since = "1.0.0")]
590 fn map<B, F>(self, f: F) -> Map<Self, F>
593 F: FnMut(Self::Item) -> B,
598 /// Calls a closure on each element of an iterator.
600 /// This is equivalent to using a [`for`] loop on the iterator, although
601 /// `break` and `continue` are not possible from a closure. It's generally
602 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
603 /// when processing items at the end of longer iterator chains. In some
604 /// cases `for_each` may also be faster than a loop, because it will use
605 /// internal iteration on adaptors like `Chain`.
607 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
614 /// use std::sync::mpsc::channel;
616 /// let (tx, rx) = channel();
617 /// (0..5).map(|x| x * 2 + 1)
618 /// .for_each(move |x| tx.send(x).unwrap());
620 /// let v: Vec<_> = rx.iter().collect();
621 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
624 /// For such a small example, a `for` loop may be cleaner, but `for_each`
625 /// might be preferable to keep a functional style with longer iterators:
628 /// (0..5).flat_map(|x| x * 100 .. x * 110)
630 /// .filter(|&(i, x)| (i + x) % 3 == 0)
631 /// .for_each(|(i, x)| println!("{}:{}", i, x));
634 #[stable(feature = "iterator_for_each", since = "1.21.0")]
635 fn for_each<F>(self, f: F)
638 F: FnMut(Self::Item),
641 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
642 move |(), item| f(item)
645 self.fold((), call(f));
648 /// Creates an iterator which uses a closure to determine if an element
649 /// should be yielded.
651 /// Given an element the closure must return `true` or `false`. The returned
652 /// iterator will yield only the elements for which the closure returns
660 /// let a = [0i32, 1, 2];
662 /// let mut iter = a.iter().filter(|x| x.is_positive());
664 /// assert_eq!(iter.next(), Some(&1));
665 /// assert_eq!(iter.next(), Some(&2));
666 /// assert_eq!(iter.next(), None);
669 /// Because the closure passed to `filter()` takes a reference, and many
670 /// iterators iterate over references, this leads to a possibly confusing
671 /// situation, where the type of the closure is a double reference:
674 /// let a = [0, 1, 2];
676 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
678 /// assert_eq!(iter.next(), Some(&2));
679 /// assert_eq!(iter.next(), None);
682 /// It's common to instead use destructuring on the argument to strip away
686 /// let a = [0, 1, 2];
688 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
690 /// assert_eq!(iter.next(), Some(&2));
691 /// assert_eq!(iter.next(), None);
697 /// let a = [0, 1, 2];
699 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
701 /// assert_eq!(iter.next(), Some(&2));
702 /// assert_eq!(iter.next(), None);
707 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
709 #[stable(feature = "rust1", since = "1.0.0")]
710 fn filter<P>(self, predicate: P) -> Filter<Self, P>
713 P: FnMut(&Self::Item) -> bool,
715 Filter::new(self, predicate)
718 /// Creates an iterator that both filters and maps.
720 /// The returned iterator yields only the `value`s for which the supplied
721 /// closure returns `Some(value)`.
723 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
724 /// concise. The example below shows how a `map().filter().map()` can be
725 /// shortened to a single call to `filter_map`.
727 /// [`filter`]: Iterator::filter
728 /// [`map`]: Iterator::map
735 /// let a = ["1", "two", "NaN", "four", "5"];
737 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
739 /// assert_eq!(iter.next(), Some(1));
740 /// assert_eq!(iter.next(), Some(5));
741 /// assert_eq!(iter.next(), None);
744 /// Here's the same example, but with [`filter`] and [`map`]:
747 /// let a = ["1", "two", "NaN", "four", "5"];
748 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
749 /// assert_eq!(iter.next(), Some(1));
750 /// assert_eq!(iter.next(), Some(5));
751 /// assert_eq!(iter.next(), None);
754 /// [`Option<T>`]: Option
756 #[stable(feature = "rust1", since = "1.0.0")]
757 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
760 F: FnMut(Self::Item) -> Option<B>,
762 FilterMap::new(self, f)
765 /// Creates an iterator which gives the current iteration count as well as
768 /// The iterator returned yields pairs `(i, val)`, where `i` is the
769 /// current index of iteration and `val` is the value returned by the
772 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
773 /// different sized integer, the [`zip`] function provides similar
776 /// # Overflow Behavior
778 /// The method does no guarding against overflows, so enumerating more than
779 /// [`usize::MAX`] elements either produces the wrong result or panics. If
780 /// debug assertions are enabled, a panic is guaranteed.
784 /// The returned iterator might panic if the to-be-returned index would
785 /// overflow a [`usize`].
787 /// [`usize`]: type@usize
788 /// [`usize::MAX`]: crate::usize::MAX
789 /// [`zip`]: Iterator::zip
794 /// let a = ['a', 'b', 'c'];
796 /// let mut iter = a.iter().enumerate();
798 /// assert_eq!(iter.next(), Some((0, &'a')));
799 /// assert_eq!(iter.next(), Some((1, &'b')));
800 /// assert_eq!(iter.next(), Some((2, &'c')));
801 /// assert_eq!(iter.next(), None);
804 #[stable(feature = "rust1", since = "1.0.0")]
805 fn enumerate(self) -> Enumerate<Self>
812 /// Creates an iterator which can use [`peek`] to look at the next element of
813 /// the iterator without consuming it.
815 /// Adds a [`peek`] method to an iterator. See its documentation for
816 /// more information.
818 /// Note that the underlying iterator is still advanced when [`peek`] is
819 /// called for the first time: In order to retrieve the next element,
820 /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
821 /// anything other than fetching the next value) of the [`next`] method
824 /// [`peek`]: Peekable::peek
825 /// [`next`]: Iterator::next
832 /// let xs = [1, 2, 3];
834 /// let mut iter = xs.iter().peekable();
836 /// // peek() lets us see into the future
837 /// assert_eq!(iter.peek(), Some(&&1));
838 /// assert_eq!(iter.next(), Some(&1));
840 /// assert_eq!(iter.next(), Some(&2));
842 /// // we can peek() multiple times, the iterator won't advance
843 /// assert_eq!(iter.peek(), Some(&&3));
844 /// assert_eq!(iter.peek(), Some(&&3));
846 /// assert_eq!(iter.next(), Some(&3));
848 /// // after the iterator is finished, so is peek()
849 /// assert_eq!(iter.peek(), None);
850 /// assert_eq!(iter.next(), None);
853 #[stable(feature = "rust1", since = "1.0.0")]
854 fn peekable(self) -> Peekable<Self>
861 /// Creates an iterator that [`skip`]s elements based on a predicate.
863 /// [`skip`]: Iterator::skip
865 /// `skip_while()` takes a closure as an argument. It will call this
866 /// closure on each element of the iterator, and ignore elements
867 /// until it returns `false`.
869 /// After `false` is returned, `skip_while()`'s job is over, and the
870 /// rest of the elements are yielded.
877 /// let a = [-1i32, 0, 1];
879 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
881 /// assert_eq!(iter.next(), Some(&0));
882 /// assert_eq!(iter.next(), Some(&1));
883 /// assert_eq!(iter.next(), None);
886 /// Because the closure passed to `skip_while()` takes a reference, and many
887 /// iterators iterate over references, this leads to a possibly confusing
888 /// situation, where the type of the closure is a double reference:
891 /// let a = [-1, 0, 1];
893 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
895 /// assert_eq!(iter.next(), Some(&0));
896 /// assert_eq!(iter.next(), Some(&1));
897 /// assert_eq!(iter.next(), None);
900 /// Stopping after an initial `false`:
903 /// let a = [-1, 0, 1, -2];
905 /// let mut iter = a.iter().skip_while(|x| **x < 0);
907 /// assert_eq!(iter.next(), Some(&0));
908 /// assert_eq!(iter.next(), Some(&1));
910 /// // while this would have been false, since we already got a false,
911 /// // skip_while() isn't used any more
912 /// assert_eq!(iter.next(), Some(&-2));
914 /// assert_eq!(iter.next(), None);
917 #[stable(feature = "rust1", since = "1.0.0")]
918 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
921 P: FnMut(&Self::Item) -> bool,
923 SkipWhile::new(self, predicate)
926 /// Creates an iterator that yields elements based on a predicate.
928 /// `take_while()` takes a closure as an argument. It will call this
929 /// closure on each element of the iterator, and yield elements
930 /// while it returns `true`.
932 /// After `false` is returned, `take_while()`'s job is over, and the
933 /// rest of the elements are ignored.
940 /// let a = [-1i32, 0, 1];
942 /// let mut iter = a.iter().take_while(|x| x.is_negative());
944 /// assert_eq!(iter.next(), Some(&-1));
945 /// assert_eq!(iter.next(), None);
948 /// Because the closure passed to `take_while()` takes a reference, and many
949 /// iterators iterate over references, this leads to a possibly confusing
950 /// situation, where the type of the closure is a double reference:
953 /// let a = [-1, 0, 1];
955 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
957 /// assert_eq!(iter.next(), Some(&-1));
958 /// assert_eq!(iter.next(), None);
961 /// Stopping after an initial `false`:
964 /// let a = [-1, 0, 1, -2];
966 /// let mut iter = a.iter().take_while(|x| **x < 0);
968 /// assert_eq!(iter.next(), Some(&-1));
970 /// // We have more elements that are less than zero, but since we already
971 /// // got a false, take_while() isn't used any more
972 /// assert_eq!(iter.next(), None);
975 /// Because `take_while()` needs to look at the value in order to see if it
976 /// should be included or not, consuming iterators will see that it is
980 /// let a = [1, 2, 3, 4];
981 /// let mut iter = a.iter();
983 /// let result: Vec<i32> = iter.by_ref()
984 /// .take_while(|n| **n != 3)
988 /// assert_eq!(result, &[1, 2]);
990 /// let result: Vec<i32> = iter.cloned().collect();
992 /// assert_eq!(result, &[4]);
995 /// The `3` is no longer there, because it was consumed in order to see if
996 /// the iteration should stop, but wasn't placed back into the iterator.
998 #[stable(feature = "rust1", since = "1.0.0")]
999 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1002 P: FnMut(&Self::Item) -> bool,
1004 TakeWhile::new(self, predicate)
1007 /// Creates an iterator that both yields elements based on a predicate and maps.
1009 /// `map_while()` takes a closure as an argument. It will call this
1010 /// closure on each element of the iterator, and yield elements
1011 /// while it returns [`Some(_)`][`Some`].
1018 /// #![feature(iter_map_while)]
1019 /// let a = [-1i32, 4, 0, 1];
1021 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1023 /// assert_eq!(iter.next(), Some(-16));
1024 /// assert_eq!(iter.next(), Some(4));
1025 /// assert_eq!(iter.next(), None);
1028 /// Here's the same example, but with [`take_while`] and [`map`]:
1030 /// [`take_while`]: Iterator::take_while
1031 /// [`map`]: Iterator::map
1034 /// let a = [-1i32, 4, 0, 1];
1036 /// let mut iter = a.iter()
1037 /// .map(|x| 16i32.checked_div(*x))
1038 /// .take_while(|x| x.is_some())
1039 /// .map(|x| x.unwrap());
1041 /// assert_eq!(iter.next(), Some(-16));
1042 /// assert_eq!(iter.next(), Some(4));
1043 /// assert_eq!(iter.next(), None);
1046 /// Stopping after an initial [`None`]:
1049 /// #![feature(iter_map_while)]
1050 /// use std::convert::TryFrom;
1052 /// let a = [0, 1, 2, -3, 4, 5, -6];
1054 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1055 /// let vec = iter.collect::<Vec<_>>();
1057 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1058 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1059 /// assert_eq!(vec, vec![0, 1, 2]);
1062 /// Because `map_while()` needs to look at the value in order to see if it
1063 /// should be included or not, consuming iterators will see that it is
1067 /// #![feature(iter_map_while)]
1068 /// use std::convert::TryFrom;
1070 /// let a = [1, 2, -3, 4];
1071 /// let mut iter = a.iter();
1073 /// let result: Vec<u32> = iter.by_ref()
1074 /// .map_while(|n| u32::try_from(*n).ok())
1077 /// assert_eq!(result, &[1, 2]);
1079 /// let result: Vec<i32> = iter.cloned().collect();
1081 /// assert_eq!(result, &[4]);
1084 /// The `-3` is no longer there, because it was consumed in order to see if
1085 /// the iteration should stop, but wasn't placed back into the iterator.
1087 /// Note that unlike [`take_while`] this iterator is **not** fused.
1088 /// It is also not specified what this iterator returns after the first` None` is returned.
1089 /// If you need fused iterator, use [`fuse`].
1091 /// [`fuse`]: Iterator::fuse
1093 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1094 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1097 P: FnMut(Self::Item) -> Option<B>,
1099 MapWhile::new(self, predicate)
1102 /// Creates an iterator that skips the first `n` elements.
1104 /// After they have been consumed, the rest of the elements are yielded.
1105 /// Rather than overriding this method directly, instead override the `nth` method.
1112 /// let a = [1, 2, 3];
1114 /// let mut iter = a.iter().skip(2);
1116 /// assert_eq!(iter.next(), Some(&3));
1117 /// assert_eq!(iter.next(), None);
1120 #[stable(feature = "rust1", since = "1.0.0")]
1121 fn skip(self, n: usize) -> Skip<Self>
1128 /// Creates an iterator that yields its first `n` elements.
1135 /// let a = [1, 2, 3];
1137 /// let mut iter = a.iter().take(2);
1139 /// assert_eq!(iter.next(), Some(&1));
1140 /// assert_eq!(iter.next(), Some(&2));
1141 /// assert_eq!(iter.next(), None);
1144 /// `take()` is often used with an infinite iterator, to make it finite:
1147 /// let mut iter = (0..).take(3);
1149 /// assert_eq!(iter.next(), Some(0));
1150 /// assert_eq!(iter.next(), Some(1));
1151 /// assert_eq!(iter.next(), Some(2));
1152 /// assert_eq!(iter.next(), None);
1155 /// If less than `n` elements are available,
1156 /// `take` will limit itself to the size of the underlying iterator:
1159 /// let v = vec![1, 2];
1160 /// let mut iter = v.into_iter().take(5);
1161 /// assert_eq!(iter.next(), Some(1));
1162 /// assert_eq!(iter.next(), Some(2));
1163 /// assert_eq!(iter.next(), None);
1166 #[stable(feature = "rust1", since = "1.0.0")]
1167 fn take(self, n: usize) -> Take<Self>
1174 /// An iterator adaptor similar to [`fold`] that holds internal state and
1175 /// produces a new iterator.
1177 /// [`fold`]: Iterator::fold
1179 /// `scan()` takes two arguments: an initial value which seeds the internal
1180 /// state, and a closure with two arguments, the first being a mutable
1181 /// reference to the internal state and the second an iterator element.
1182 /// The closure can assign to the internal state to share state between
1185 /// On iteration, the closure will be applied to each element of the
1186 /// iterator and the return value from the closure, an [`Option`], is
1187 /// yielded by the iterator.
1194 /// let a = [1, 2, 3];
1196 /// let mut iter = a.iter().scan(1, |state, &x| {
1197 /// // each iteration, we'll multiply the state by the element
1198 /// *state = *state * x;
1200 /// // then, we'll yield the negation of the state
1204 /// assert_eq!(iter.next(), Some(-1));
1205 /// assert_eq!(iter.next(), Some(-2));
1206 /// assert_eq!(iter.next(), Some(-6));
1207 /// assert_eq!(iter.next(), None);
1210 #[stable(feature = "rust1", since = "1.0.0")]
1211 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1214 F: FnMut(&mut St, Self::Item) -> Option<B>,
1216 Scan::new(self, initial_state, f)
1219 /// Creates an iterator that works like map, but flattens nested structure.
1221 /// The [`map`] adapter is very useful, but only when the closure
1222 /// argument produces values. If it produces an iterator instead, there's
1223 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1226 /// You can think of `flat_map(f)` as the semantic equivalent
1227 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1229 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1230 /// one item for each element, and `flat_map()`'s closure returns an
1231 /// iterator for each element.
1233 /// [`map`]: Iterator::map
1234 /// [`flatten`]: Iterator::flatten
1241 /// let words = ["alpha", "beta", "gamma"];
1243 /// // chars() returns an iterator
1244 /// let merged: String = words.iter()
1245 /// .flat_map(|s| s.chars())
1247 /// assert_eq!(merged, "alphabetagamma");
1250 #[stable(feature = "rust1", since = "1.0.0")]
1251 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1255 F: FnMut(Self::Item) -> U,
1257 FlatMap::new(self, f)
1260 /// Creates an iterator that flattens nested structure.
1262 /// This is useful when you have an iterator of iterators or an iterator of
1263 /// things that can be turned into iterators and you want to remove one
1264 /// level of indirection.
1271 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1272 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1273 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1276 /// Mapping and then flattening:
1279 /// let words = ["alpha", "beta", "gamma"];
1281 /// // chars() returns an iterator
1282 /// let merged: String = words.iter()
1283 /// .map(|s| s.chars())
1286 /// assert_eq!(merged, "alphabetagamma");
1289 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1290 /// in this case since it conveys intent more clearly:
1293 /// let words = ["alpha", "beta", "gamma"];
1295 /// // chars() returns an iterator
1296 /// let merged: String = words.iter()
1297 /// .flat_map(|s| s.chars())
1299 /// assert_eq!(merged, "alphabetagamma");
1302 /// Flattening once only removes one level of nesting:
1305 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1307 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1308 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1310 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1311 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1314 /// Here we see that `flatten()` does not perform a "deep" flatten.
1315 /// Instead, only one level of nesting is removed. That is, if you
1316 /// `flatten()` a three-dimensional array the result will be
1317 /// two-dimensional and not one-dimensional. To get a one-dimensional
1318 /// structure, you have to `flatten()` again.
1320 /// [`flat_map()`]: Iterator::flat_map
1322 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1323 fn flatten(self) -> Flatten<Self>
1326 Self::Item: IntoIterator,
1331 /// Creates an iterator which ends after the first [`None`].
1333 /// After an iterator returns [`None`], future calls may or may not yield
1334 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1335 /// [`None`] is given, it will always return [`None`] forever.
1337 /// [`Some(T)`]: Some
1344 /// // an iterator which alternates between Some and None
1345 /// struct Alternate {
1349 /// impl Iterator for Alternate {
1350 /// type Item = i32;
1352 /// fn next(&mut self) -> Option<i32> {
1353 /// let val = self.state;
1354 /// self.state = self.state + 1;
1356 /// // if it's even, Some(i32), else None
1357 /// if val % 2 == 0 {
1365 /// let mut iter = Alternate { state: 0 };
1367 /// // we can see our iterator going back and forth
1368 /// assert_eq!(iter.next(), Some(0));
1369 /// assert_eq!(iter.next(), None);
1370 /// assert_eq!(iter.next(), Some(2));
1371 /// assert_eq!(iter.next(), None);
1373 /// // however, once we fuse it...
1374 /// let mut iter = iter.fuse();
1376 /// assert_eq!(iter.next(), Some(4));
1377 /// assert_eq!(iter.next(), None);
1379 /// // it will always return `None` after the first time.
1380 /// assert_eq!(iter.next(), None);
1381 /// assert_eq!(iter.next(), None);
1382 /// assert_eq!(iter.next(), None);
1385 #[stable(feature = "rust1", since = "1.0.0")]
1386 fn fuse(self) -> Fuse<Self>
1393 /// Does something with each element of an iterator, passing the value on.
1395 /// When using iterators, you'll often chain several of them together.
1396 /// While working on such code, you might want to check out what's
1397 /// happening at various parts in the pipeline. To do that, insert
1398 /// a call to `inspect()`.
1400 /// It's more common for `inspect()` to be used as a debugging tool than to
1401 /// exist in your final code, but applications may find it useful in certain
1402 /// situations when errors need to be logged before being discarded.
1409 /// let a = [1, 4, 2, 3];
1411 /// // this iterator sequence is complex.
1412 /// let sum = a.iter()
1414 /// .filter(|x| x % 2 == 0)
1415 /// .fold(0, |sum, i| sum + i);
1417 /// println!("{}", sum);
1419 /// // let's add some inspect() calls to investigate what's happening
1420 /// let sum = a.iter()
1422 /// .inspect(|x| println!("about to filter: {}", x))
1423 /// .filter(|x| x % 2 == 0)
1424 /// .inspect(|x| println!("made it through filter: {}", x))
1425 /// .fold(0, |sum, i| sum + i);
1427 /// println!("{}", sum);
1430 /// This will print:
1434 /// about to filter: 1
1435 /// about to filter: 4
1436 /// made it through filter: 4
1437 /// about to filter: 2
1438 /// made it through filter: 2
1439 /// about to filter: 3
1443 /// Logging errors before discarding them:
1446 /// let lines = ["1", "2", "a"];
1448 /// let sum: i32 = lines
1450 /// .map(|line| line.parse::<i32>())
1451 /// .inspect(|num| {
1452 /// if let Err(ref e) = *num {
1453 /// println!("Parsing error: {}", e);
1456 /// .filter_map(Result::ok)
1459 /// println!("Sum: {}", sum);
1462 /// This will print:
1465 /// Parsing error: invalid digit found in string
1469 #[stable(feature = "rust1", since = "1.0.0")]
1470 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1473 F: FnMut(&Self::Item),
1475 Inspect::new(self, f)
1478 /// Borrows an iterator, rather than consuming it.
1480 /// This is useful to allow applying iterator adaptors while still
1481 /// retaining ownership of the original iterator.
1488 /// let a = [1, 2, 3];
1490 /// let iter = a.iter();
1492 /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
1494 /// assert_eq!(sum, 6);
1496 /// // if we try to use iter again, it won't work. The following line
1497 /// // gives "error: use of moved value: `iter`
1498 /// // assert_eq!(iter.next(), None);
1500 /// // let's try that again
1501 /// let a = [1, 2, 3];
1503 /// let mut iter = a.iter();
1505 /// // instead, we add in a .by_ref()
1506 /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
1508 /// assert_eq!(sum, 3);
1510 /// // now this is just fine:
1511 /// assert_eq!(iter.next(), Some(&3));
1512 /// assert_eq!(iter.next(), None);
1514 #[stable(feature = "rust1", since = "1.0.0")]
1515 fn by_ref(&mut self) -> &mut Self
1522 /// Transforms an iterator into a collection.
1524 /// `collect()` can take anything iterable, and turn it into a relevant
1525 /// collection. This is one of the more powerful methods in the standard
1526 /// library, used in a variety of contexts.
1528 /// The most basic pattern in which `collect()` is used is to turn one
1529 /// collection into another. You take a collection, call [`iter`] on it,
1530 /// do a bunch of transformations, and then `collect()` at the end.
1532 /// `collect()` can also create instances of types that are not typical
1533 /// collections. For example, a [`String`] can be built from [`char`]s,
1534 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1535 /// into `Result<Collection<T>, E>`. See the examples below for more.
1537 /// Because `collect()` is so general, it can cause problems with type
1538 /// inference. As such, `collect()` is one of the few times you'll see
1539 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1540 /// helps the inference algorithm understand specifically which collection
1541 /// you're trying to collect into.
1548 /// let a = [1, 2, 3];
1550 /// let doubled: Vec<i32> = a.iter()
1551 /// .map(|&x| x * 2)
1554 /// assert_eq!(vec![2, 4, 6], doubled);
1557 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1558 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1560 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1563 /// use std::collections::VecDeque;
1565 /// let a = [1, 2, 3];
1567 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1569 /// assert_eq!(2, doubled[0]);
1570 /// assert_eq!(4, doubled[1]);
1571 /// assert_eq!(6, doubled[2]);
1574 /// Using the 'turbofish' instead of annotating `doubled`:
1577 /// let a = [1, 2, 3];
1579 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1581 /// assert_eq!(vec![2, 4, 6], doubled);
1584 /// Because `collect()` only cares about what you're collecting into, you can
1585 /// still use a partial type hint, `_`, with the turbofish:
1588 /// let a = [1, 2, 3];
1590 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1592 /// assert_eq!(vec![2, 4, 6], doubled);
1595 /// Using `collect()` to make a [`String`]:
1598 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1600 /// let hello: String = chars.iter()
1601 /// .map(|&x| x as u8)
1602 /// .map(|x| (x + 1) as char)
1605 /// assert_eq!("hello", hello);
1608 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1609 /// see if any of them failed:
1612 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1614 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1616 /// // gives us the first error
1617 /// assert_eq!(Err("nope"), result);
1619 /// let results = [Ok(1), Ok(3)];
1621 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1623 /// // gives us the list of answers
1624 /// assert_eq!(Ok(vec![1, 3]), result);
1627 /// [`iter`]: Iterator::next
1628 /// [`String`]: ../../std/string/struct.String.html
1629 /// [`char`]: type@char
1631 #[stable(feature = "rust1", since = "1.0.0")]
1632 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1633 fn collect<B: FromIterator<Self::Item>>(self) -> B
1637 FromIterator::from_iter(self)
1640 /// Consumes an iterator, creating two collections from it.
1642 /// The predicate passed to `partition()` can return `true`, or `false`.
1643 /// `partition()` returns a pair, all of the elements for which it returned
1644 /// `true`, and all of the elements for which it returned `false`.
1646 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1648 /// [`is_partitioned()`]: Iterator::is_partitioned
1649 /// [`partition_in_place()`]: Iterator::partition_in_place
1656 /// let a = [1, 2, 3];
1658 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1660 /// .partition(|&n| n % 2 == 0);
1662 /// assert_eq!(even, vec![2]);
1663 /// assert_eq!(odd, vec![1, 3]);
1665 #[stable(feature = "rust1", since = "1.0.0")]
1666 fn partition<B, F>(self, f: F) -> (B, B)
1669 B: Default + Extend<Self::Item>,
1670 F: FnMut(&Self::Item) -> bool,
1673 fn extend<'a, T, B: Extend<T>>(
1674 mut f: impl FnMut(&T) -> bool + 'a,
1677 ) -> impl FnMut((), T) + 'a {
1682 right.extend_one(x);
1687 let mut left: B = Default::default();
1688 let mut right: B = Default::default();
1690 self.fold((), extend(f, &mut left, &mut right));
1695 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1696 /// such that all those that return `true` precede all those that return `false`.
1697 /// Returns the number of `true` elements found.
1699 /// The relative order of partitioned items is not maintained.
1701 /// See also [`is_partitioned()`] and [`partition()`].
1703 /// [`is_partitioned()`]: Iterator::is_partitioned
1704 /// [`partition()`]: Iterator::partition
1709 /// #![feature(iter_partition_in_place)]
1711 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1713 /// // Partition in-place between evens and odds
1714 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1716 /// assert_eq!(i, 3);
1717 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1718 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1720 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1721 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1723 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1724 P: FnMut(&T) -> bool,
1726 // FIXME: should we worry about the count overflowing? The only way to have more than
1727 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1729 // These closure "factory" functions exist to avoid genericity in `Self`.
1733 predicate: &'a mut impl FnMut(&T) -> bool,
1734 true_count: &'a mut usize,
1735 ) -> impl FnMut(&&mut T) -> bool + 'a {
1737 let p = predicate(&**x);
1738 *true_count += p as usize;
1744 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1745 move |x| predicate(&**x)
1748 // Repeatedly find the first `false` and swap it with the last `true`.
1749 let mut true_count = 0;
1750 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1751 if let Some(tail) = self.rfind(is_true(predicate)) {
1752 crate::mem::swap(head, tail);
1761 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1762 /// such that all those that return `true` precede all those that return `false`.
1764 /// See also [`partition()`] and [`partition_in_place()`].
1766 /// [`partition()`]: Iterator::partition
1767 /// [`partition_in_place()`]: Iterator::partition_in_place
1772 /// #![feature(iter_is_partitioned)]
1774 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1775 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1777 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1778 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1781 P: FnMut(Self::Item) -> bool,
1783 // Either all items test `true`, or the first clause stops at `false`
1784 // and we check that there are no more `true` items after that.
1785 self.all(&mut predicate) || !self.any(predicate)
1788 /// An iterator method that applies a function as long as it returns
1789 /// successfully, producing a single, final value.
1791 /// `try_fold()` takes two arguments: an initial value, and a closure with
1792 /// two arguments: an 'accumulator', and an element. The closure either
1793 /// returns successfully, with the value that the accumulator should have
1794 /// for the next iteration, or it returns failure, with an error value that
1795 /// is propagated back to the caller immediately (short-circuiting).
1797 /// The initial value is the value the accumulator will have on the first
1798 /// call. If applying the closure succeeded against every element of the
1799 /// iterator, `try_fold()` returns the final accumulator as success.
1801 /// Folding is useful whenever you have a collection of something, and want
1802 /// to produce a single value from it.
1804 /// # Note to Implementors
1806 /// Several of the other (forward) methods have default implementations in
1807 /// terms of this one, so try to implement this explicitly if it can
1808 /// do something better than the default `for` loop implementation.
1810 /// In particular, try to have this call `try_fold()` on the internal parts
1811 /// from which this iterator is composed. If multiple calls are needed,
1812 /// the `?` operator may be convenient for chaining the accumulator value
1813 /// along, but beware any invariants that need to be upheld before those
1814 /// early returns. This is a `&mut self` method, so iteration needs to be
1815 /// resumable after hitting an error here.
1822 /// let a = [1, 2, 3];
1824 /// // the checked sum of all of the elements of the array
1825 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1827 /// assert_eq!(sum, Some(6));
1830 /// Short-circuiting:
1833 /// let a = [10, 20, 30, 100, 40, 50];
1834 /// let mut it = a.iter();
1836 /// // This sum overflows when adding the 100 element
1837 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1838 /// assert_eq!(sum, None);
1840 /// // Because it short-circuited, the remaining elements are still
1841 /// // available through the iterator.
1842 /// assert_eq!(it.len(), 2);
1843 /// assert_eq!(it.next(), Some(&40));
1846 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1847 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1850 F: FnMut(B, Self::Item) -> R,
1853 let mut accum = init;
1854 while let Some(x) = self.next() {
1855 accum = f(accum, x)?;
1860 /// An iterator method that applies a fallible function to each item in the
1861 /// iterator, stopping at the first error and returning that error.
1863 /// This can also be thought of as the fallible form of [`for_each()`]
1864 /// or as the stateless version of [`try_fold()`].
1866 /// [`for_each()`]: Iterator::for_each
1867 /// [`try_fold()`]: Iterator::try_fold
1872 /// use std::fs::rename;
1873 /// use std::io::{stdout, Write};
1874 /// use std::path::Path;
1876 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
1878 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
1879 /// assert!(res.is_ok());
1881 /// let mut it = data.iter().cloned();
1882 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
1883 /// assert!(res.is_err());
1884 /// // It short-circuited, so the remaining items are still in the iterator:
1885 /// assert_eq!(it.next(), Some("stale_bread.json"));
1888 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1889 fn try_for_each<F, R>(&mut self, f: F) -> R
1892 F: FnMut(Self::Item) -> R,
1896 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
1900 self.try_fold((), call(f))
1903 /// An iterator method that applies a function, producing a single, final value.
1905 /// `fold()` takes two arguments: an initial value, and a closure with two
1906 /// arguments: an 'accumulator', and an element. The closure returns the value that
1907 /// the accumulator should have for the next iteration.
1909 /// The initial value is the value the accumulator will have on the first
1912 /// After applying this closure to every element of the iterator, `fold()`
1913 /// returns the accumulator.
1915 /// This operation is sometimes called 'reduce' or 'inject'.
1917 /// Folding is useful whenever you have a collection of something, and want
1918 /// to produce a single value from it.
1920 /// Note: `fold()`, and similar methods that traverse the entire iterator,
1921 /// may not terminate for infinite iterators, even on traits for which a
1922 /// result is determinable in finite time.
1924 /// # Note to Implementors
1926 /// Several of the other (forward) methods have default implementations in
1927 /// terms of this one, so try to implement this explicitly if it can
1928 /// do something better than the default `for` loop implementation.
1930 /// In particular, try to have this call `fold()` on the internal parts
1931 /// from which this iterator is composed.
1938 /// let a = [1, 2, 3];
1940 /// // the sum of all of the elements of the array
1941 /// let sum = a.iter().fold(0, |acc, x| acc + x);
1943 /// assert_eq!(sum, 6);
1946 /// Let's walk through each step of the iteration here:
1948 /// | element | acc | x | result |
1949 /// |---------|-----|---|--------|
1951 /// | 1 | 0 | 1 | 1 |
1952 /// | 2 | 1 | 2 | 3 |
1953 /// | 3 | 3 | 3 | 6 |
1955 /// And so, our final result, `6`.
1957 /// It's common for people who haven't used iterators a lot to
1958 /// use a `for` loop with a list of things to build up a result. Those
1959 /// can be turned into `fold()`s:
1961 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
1964 /// let numbers = [1, 2, 3, 4, 5];
1966 /// let mut result = 0;
1969 /// for i in &numbers {
1970 /// result = result + i;
1974 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
1976 /// // they're the same
1977 /// assert_eq!(result, result2);
1980 #[stable(feature = "rust1", since = "1.0.0")]
1981 fn fold<B, F>(mut self, init: B, mut f: F) -> B
1984 F: FnMut(B, Self::Item) -> B,
1986 let mut accum = init;
1987 while let Some(x) = self.next() {
1988 accum = f(accum, x);
1993 /// The same as [`fold()`], but uses the first element in the
1994 /// iterator as the initial value, folding every subsequent element into it.
1995 /// If the iterator is empty, return [`None`]; otherwise, return the result
1998 /// [`fold()`]: Iterator::fold
2002 /// Find the maximum value:
2005 /// #![feature(iterator_fold_self)]
2007 /// fn find_max<I>(iter: I) -> Option<I::Item>
2008 /// where I: Iterator,
2011 /// iter.fold_first(|a, b| {
2012 /// if a >= b { a } else { b }
2015 /// let a = [10, 20, 5, -23, 0];
2016 /// let b: [u32; 0] = [];
2018 /// assert_eq!(find_max(a.iter()), Some(&20));
2019 /// assert_eq!(find_max(b.iter()), None);
2022 #[unstable(feature = "iterator_fold_self", issue = "68125")]
2023 fn fold_first<F>(mut self, f: F) -> Option<Self::Item>
2026 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2028 let first = self.next()?;
2029 Some(self.fold(first, f))
2032 /// Tests if every element of the iterator matches a predicate.
2034 /// `all()` takes a closure that returns `true` or `false`. It applies
2035 /// this closure to each element of the iterator, and if they all return
2036 /// `true`, then so does `all()`. If any of them return `false`, it
2037 /// returns `false`.
2039 /// `all()` is short-circuiting; in other words, it will stop processing
2040 /// as soon as it finds a `false`, given that no matter what else happens,
2041 /// the result will also be `false`.
2043 /// An empty iterator returns `true`.
2050 /// let a = [1, 2, 3];
2052 /// assert!(a.iter().all(|&x| x > 0));
2054 /// assert!(!a.iter().all(|&x| x > 2));
2057 /// Stopping at the first `false`:
2060 /// let a = [1, 2, 3];
2062 /// let mut iter = a.iter();
2064 /// assert!(!iter.all(|&x| x != 2));
2066 /// // we can still use `iter`, as there are more elements.
2067 /// assert_eq!(iter.next(), Some(&3));
2070 #[stable(feature = "rust1", since = "1.0.0")]
2071 fn all<F>(&mut self, f: F) -> bool
2074 F: FnMut(Self::Item) -> bool,
2077 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<(), ()> {
2079 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2082 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2085 /// Tests if any element of the iterator matches a predicate.
2087 /// `any()` takes a closure that returns `true` or `false`. It applies
2088 /// this closure to each element of the iterator, and if any of them return
2089 /// `true`, then so does `any()`. If they all return `false`, it
2090 /// returns `false`.
2092 /// `any()` is short-circuiting; in other words, it will stop processing
2093 /// as soon as it finds a `true`, given that no matter what else happens,
2094 /// the result will also be `true`.
2096 /// An empty iterator returns `false`.
2103 /// let a = [1, 2, 3];
2105 /// assert!(a.iter().any(|&x| x > 0));
2107 /// assert!(!a.iter().any(|&x| x > 5));
2110 /// Stopping at the first `true`:
2113 /// let a = [1, 2, 3];
2115 /// let mut iter = a.iter();
2117 /// assert!(iter.any(|&x| x != 2));
2119 /// // we can still use `iter`, as there are more elements.
2120 /// assert_eq!(iter.next(), Some(&2));
2123 #[stable(feature = "rust1", since = "1.0.0")]
2124 fn any<F>(&mut self, f: F) -> bool
2127 F: FnMut(Self::Item) -> bool,
2130 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<(), ()> {
2132 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2136 self.try_fold((), check(f)) == ControlFlow::BREAK
2139 /// Searches for an element of an iterator that satisfies a predicate.
2141 /// `find()` takes a closure that returns `true` or `false`. It applies
2142 /// this closure to each element of the iterator, and if any of them return
2143 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2144 /// `false`, it returns [`None`].
2146 /// `find()` is short-circuiting; in other words, it will stop processing
2147 /// as soon as the closure returns `true`.
2149 /// Because `find()` takes a reference, and many iterators iterate over
2150 /// references, this leads to a possibly confusing situation where the
2151 /// argument is a double reference. You can see this effect in the
2152 /// examples below, with `&&x`.
2154 /// [`Some(element)`]: Some
2161 /// let a = [1, 2, 3];
2163 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2165 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2168 /// Stopping at the first `true`:
2171 /// let a = [1, 2, 3];
2173 /// let mut iter = a.iter();
2175 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2177 /// // we can still use `iter`, as there are more elements.
2178 /// assert_eq!(iter.next(), Some(&3));
2181 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2183 #[stable(feature = "rust1", since = "1.0.0")]
2184 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2187 P: FnMut(&Self::Item) -> bool,
2191 mut predicate: impl FnMut(&T) -> bool,
2192 ) -> impl FnMut((), T) -> ControlFlow<(), T> {
2194 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2198 self.try_fold((), check(predicate)).break_value()
2201 /// Applies function to the elements of iterator and returns
2202 /// the first non-none result.
2204 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2210 /// let a = ["lol", "NaN", "2", "5"];
2212 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2214 /// assert_eq!(first_number, Some(2));
2217 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2218 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2221 F: FnMut(Self::Item) -> Option<B>,
2225 mut f: impl FnMut(T) -> Option<B>,
2226 ) -> impl FnMut((), T) -> ControlFlow<(), B> {
2227 move |(), x| match f(x) {
2228 Some(x) => ControlFlow::Break(x),
2229 None => ControlFlow::CONTINUE,
2233 self.try_fold((), check(f)).break_value()
2236 /// Applies function to the elements of iterator and returns
2237 /// the first true result or the first error.
2242 /// #![feature(try_find)]
2244 /// let a = ["1", "2", "lol", "NaN", "5"];
2246 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2247 /// Ok(s.parse::<i32>()? == search)
2250 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2251 /// assert_eq!(result, Ok(Some(&"2")));
2253 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2254 /// assert!(result.is_err());
2257 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2258 fn try_find<F, R>(&mut self, f: F) -> Result<Option<Self::Item>, R::Error>
2261 F: FnMut(&Self::Item) -> R,
2265 fn check<F, T, R>(mut f: F) -> impl FnMut((), T) -> ControlFlow<(), Result<T, R::Error>>
2270 move |(), x| match f(&x).into_result() {
2271 Ok(false) => ControlFlow::CONTINUE,
2272 Ok(true) => ControlFlow::Break(Ok(x)),
2273 Err(x) => ControlFlow::Break(Err(x)),
2277 self.try_fold((), check(f)).break_value().transpose()
2280 /// Searches for an element in an iterator, returning its index.
2282 /// `position()` takes a closure that returns `true` or `false`. It applies
2283 /// this closure to each element of the iterator, and if one of them
2284 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2285 /// them return `false`, it returns [`None`].
2287 /// `position()` is short-circuiting; in other words, it will stop
2288 /// processing as soon as it finds a `true`.
2290 /// # Overflow Behavior
2292 /// The method does no guarding against overflows, so if there are more
2293 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2294 /// result or panics. If debug assertions are enabled, a panic is
2299 /// This function might panic if the iterator has more than `usize::MAX`
2300 /// non-matching elements.
2302 /// [`Some(index)`]: Some
2303 /// [`usize::MAX`]: crate::usize::MAX
2310 /// let a = [1, 2, 3];
2312 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2314 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2317 /// Stopping at the first `true`:
2320 /// let a = [1, 2, 3, 4];
2322 /// let mut iter = a.iter();
2324 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2326 /// // we can still use `iter`, as there are more elements.
2327 /// assert_eq!(iter.next(), Some(&3));
2329 /// // The returned index depends on iterator state
2330 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2334 #[stable(feature = "rust1", since = "1.0.0")]
2335 fn position<P>(&mut self, predicate: P) -> Option<usize>
2338 P: FnMut(Self::Item) -> bool,
2342 mut predicate: impl FnMut(T) -> bool,
2343 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2344 // The addition might panic on overflow
2347 ControlFlow::Break(i)
2349 ControlFlow::Continue(Add::add(i, 1))
2354 self.try_fold(0, check(predicate)).break_value()
2357 /// Searches for an element in an iterator from the right, returning its
2360 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2361 /// this closure to each element of the iterator, starting from the end,
2362 /// and if one of them returns `true`, then `rposition()` returns
2363 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2365 /// `rposition()` is short-circuiting; in other words, it will stop
2366 /// processing as soon as it finds a `true`.
2368 /// [`Some(index)`]: Some
2375 /// let a = [1, 2, 3];
2377 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2379 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2382 /// Stopping at the first `true`:
2385 /// let a = [1, 2, 3];
2387 /// let mut iter = a.iter();
2389 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2391 /// // we can still use `iter`, as there are more elements.
2392 /// assert_eq!(iter.next(), Some(&1));
2395 #[stable(feature = "rust1", since = "1.0.0")]
2396 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2398 P: FnMut(Self::Item) -> bool,
2399 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2401 // No need for an overflow check here, because `ExactSizeIterator`
2402 // implies that the number of elements fits into a `usize`.
2405 mut predicate: impl FnMut(T) -> bool,
2406 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2409 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2414 self.try_rfold(n, check(predicate)).break_value()
2417 /// Returns the maximum element of an iterator.
2419 /// If several elements are equally maximum, the last element is
2420 /// returned. If the iterator is empty, [`None`] is returned.
2427 /// let a = [1, 2, 3];
2428 /// let b: Vec<u32> = Vec::new();
2430 /// assert_eq!(a.iter().max(), Some(&3));
2431 /// assert_eq!(b.iter().max(), None);
2434 #[stable(feature = "rust1", since = "1.0.0")]
2435 fn max(self) -> Option<Self::Item>
2440 self.max_by(Ord::cmp)
2443 /// Returns the minimum element of an iterator.
2445 /// If several elements are equally minimum, the first element is
2446 /// returned. If the iterator is empty, [`None`] is returned.
2453 /// let a = [1, 2, 3];
2454 /// let b: Vec<u32> = Vec::new();
2456 /// assert_eq!(a.iter().min(), Some(&1));
2457 /// assert_eq!(b.iter().min(), None);
2460 #[stable(feature = "rust1", since = "1.0.0")]
2461 fn min(self) -> Option<Self::Item>
2466 self.min_by(Ord::cmp)
2469 /// Returns the element that gives the maximum value from the
2470 /// specified function.
2472 /// If several elements are equally maximum, the last element is
2473 /// returned. If the iterator is empty, [`None`] is returned.
2478 /// let a = [-3_i32, 0, 1, 5, -10];
2479 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2482 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2483 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2486 F: FnMut(&Self::Item) -> B,
2489 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2494 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2498 let (_, x) = self.map(key(f)).max_by(compare)?;
2502 /// Returns the element that gives the maximum value with respect to the
2503 /// specified comparison function.
2505 /// If several elements are equally maximum, the last element is
2506 /// returned. If the iterator is empty, [`None`] is returned.
2511 /// let a = [-3_i32, 0, 1, 5, -10];
2512 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2515 #[stable(feature = "iter_max_by", since = "1.15.0")]
2516 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2519 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2522 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2523 move |x, y| cmp::max_by(x, y, &mut compare)
2526 self.fold_first(fold(compare))
2529 /// Returns the element that gives the minimum value from the
2530 /// specified function.
2532 /// If several elements are equally minimum, the first element is
2533 /// returned. If the iterator is empty, [`None`] is returned.
2538 /// let a = [-3_i32, 0, 1, 5, -10];
2539 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2542 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2543 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2546 F: FnMut(&Self::Item) -> B,
2549 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2554 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2558 let (_, x) = self.map(key(f)).min_by(compare)?;
2562 /// Returns the element that gives the minimum value with respect to the
2563 /// specified comparison function.
2565 /// If several elements are equally minimum, the first element is
2566 /// returned. If the iterator is empty, [`None`] is returned.
2571 /// let a = [-3_i32, 0, 1, 5, -10];
2572 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2575 #[stable(feature = "iter_min_by", since = "1.15.0")]
2576 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2579 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2582 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2583 move |x, y| cmp::min_by(x, y, &mut compare)
2586 self.fold_first(fold(compare))
2589 /// Reverses an iterator's direction.
2591 /// Usually, iterators iterate from left to right. After using `rev()`,
2592 /// an iterator will instead iterate from right to left.
2594 /// This is only possible if the iterator has an end, so `rev()` only
2595 /// works on [`DoubleEndedIterator`]s.
2600 /// let a = [1, 2, 3];
2602 /// let mut iter = a.iter().rev();
2604 /// assert_eq!(iter.next(), Some(&3));
2605 /// assert_eq!(iter.next(), Some(&2));
2606 /// assert_eq!(iter.next(), Some(&1));
2608 /// assert_eq!(iter.next(), None);
2611 #[stable(feature = "rust1", since = "1.0.0")]
2612 fn rev(self) -> Rev<Self>
2614 Self: Sized + DoubleEndedIterator,
2619 /// Converts an iterator of pairs into a pair of containers.
2621 /// `unzip()` consumes an entire iterator of pairs, producing two
2622 /// collections: one from the left elements of the pairs, and one
2623 /// from the right elements.
2625 /// This function is, in some sense, the opposite of [`zip`].
2627 /// [`zip`]: Iterator::zip
2634 /// let a = [(1, 2), (3, 4)];
2636 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2638 /// assert_eq!(left, [1, 3]);
2639 /// assert_eq!(right, [2, 4]);
2641 #[stable(feature = "rust1", since = "1.0.0")]
2642 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2644 FromA: Default + Extend<A>,
2645 FromB: Default + Extend<B>,
2646 Self: Sized + Iterator<Item = (A, B)>,
2648 fn extend<'a, A, B>(
2649 ts: &'a mut impl Extend<A>,
2650 us: &'a mut impl Extend<B>,
2651 ) -> impl FnMut((), (A, B)) + 'a {
2658 let mut ts: FromA = Default::default();
2659 let mut us: FromB = Default::default();
2661 let (lower_bound, _) = self.size_hint();
2662 if lower_bound > 0 {
2663 ts.extend_reserve(lower_bound);
2664 us.extend_reserve(lower_bound);
2667 self.fold((), extend(&mut ts, &mut us));
2672 /// Creates an iterator which copies all of its elements.
2674 /// This is useful when you have an iterator over `&T`, but you need an
2675 /// iterator over `T`.
2682 /// let a = [1, 2, 3];
2684 /// let v_copied: Vec<_> = a.iter().copied().collect();
2686 /// // copied is the same as .map(|&x| x)
2687 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2689 /// assert_eq!(v_copied, vec![1, 2, 3]);
2690 /// assert_eq!(v_map, vec![1, 2, 3]);
2692 #[stable(feature = "iter_copied", since = "1.36.0")]
2693 fn copied<'a, T: 'a>(self) -> Copied<Self>
2695 Self: Sized + Iterator<Item = &'a T>,
2701 /// Creates an iterator which [`clone`]s all of its elements.
2703 /// This is useful when you have an iterator over `&T`, but you need an
2704 /// iterator over `T`.
2706 /// [`clone`]: Clone::clone
2713 /// let a = [1, 2, 3];
2715 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2717 /// // cloned is the same as .map(|&x| x), for integers
2718 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2720 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2721 /// assert_eq!(v_map, vec![1, 2, 3]);
2723 #[stable(feature = "rust1", since = "1.0.0")]
2724 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2726 Self: Sized + Iterator<Item = &'a T>,
2732 /// Repeats an iterator endlessly.
2734 /// Instead of stopping at [`None`], the iterator will instead start again,
2735 /// from the beginning. After iterating again, it will start at the
2736 /// beginning again. And again. And again. Forever.
2743 /// let a = [1, 2, 3];
2745 /// let mut it = a.iter().cycle();
2747 /// assert_eq!(it.next(), Some(&1));
2748 /// assert_eq!(it.next(), Some(&2));
2749 /// assert_eq!(it.next(), Some(&3));
2750 /// assert_eq!(it.next(), Some(&1));
2751 /// assert_eq!(it.next(), Some(&2));
2752 /// assert_eq!(it.next(), Some(&3));
2753 /// assert_eq!(it.next(), Some(&1));
2755 #[stable(feature = "rust1", since = "1.0.0")]
2757 fn cycle(self) -> Cycle<Self>
2759 Self: Sized + Clone,
2764 /// Sums the elements of an iterator.
2766 /// Takes each element, adds them together, and returns the result.
2768 /// An empty iterator returns the zero value of the type.
2772 /// When calling `sum()` and a primitive integer type is being returned, this
2773 /// method will panic if the computation overflows and debug assertions are
2781 /// let a = [1, 2, 3];
2782 /// let sum: i32 = a.iter().sum();
2784 /// assert_eq!(sum, 6);
2786 #[stable(feature = "iter_arith", since = "1.11.0")]
2787 fn sum<S>(self) -> S
2795 /// Iterates over the entire iterator, multiplying all the elements
2797 /// An empty iterator returns the one value of the type.
2801 /// When calling `product()` and a primitive integer type is being returned,
2802 /// method will panic if the computation overflows and debug assertions are
2808 /// fn factorial(n: u32) -> u32 {
2809 /// (1..=n).product()
2811 /// assert_eq!(factorial(0), 1);
2812 /// assert_eq!(factorial(1), 1);
2813 /// assert_eq!(factorial(5), 120);
2815 #[stable(feature = "iter_arith", since = "1.11.0")]
2816 fn product<P>(self) -> P
2819 P: Product<Self::Item>,
2821 Product::product(self)
2824 /// Lexicographically compares the elements of this [`Iterator`] with those
2830 /// use std::cmp::Ordering;
2832 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
2833 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
2834 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
2836 #[stable(feature = "iter_order", since = "1.5.0")]
2837 fn cmp<I>(self, other: I) -> Ordering
2839 I: IntoIterator<Item = Self::Item>,
2843 self.cmp_by(other, |x, y| x.cmp(&y))
2846 /// Lexicographically compares the elements of this [`Iterator`] with those
2847 /// of another with respect to the specified comparison function.
2854 /// #![feature(iter_order_by)]
2856 /// use std::cmp::Ordering;
2858 /// let xs = [1, 2, 3, 4];
2859 /// let ys = [1, 4, 9, 16];
2861 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
2862 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
2863 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
2865 #[unstable(feature = "iter_order_by", issue = "64295")]
2866 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
2870 F: FnMut(Self::Item, I::Item) -> Ordering,
2872 let mut other = other.into_iter();
2875 let x = match self.next() {
2877 if other.next().is_none() {
2878 return Ordering::Equal;
2880 return Ordering::Less;
2886 let y = match other.next() {
2887 None => return Ordering::Greater,
2892 Ordering::Equal => (),
2893 non_eq => return non_eq,
2898 /// Lexicographically compares the elements of this [`Iterator`] with those
2904 /// use std::cmp::Ordering;
2906 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
2907 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
2908 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
2910 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
2912 #[stable(feature = "iter_order", since = "1.5.0")]
2913 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
2916 Self::Item: PartialOrd<I::Item>,
2919 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
2922 /// Lexicographically compares the elements of this [`Iterator`] with those
2923 /// of another with respect to the specified comparison function.
2930 /// #![feature(iter_order_by)]
2932 /// use std::cmp::Ordering;
2934 /// let xs = [1.0, 2.0, 3.0, 4.0];
2935 /// let ys = [1.0, 4.0, 9.0, 16.0];
2938 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
2939 /// Some(Ordering::Less)
2942 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
2943 /// Some(Ordering::Equal)
2946 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
2947 /// Some(Ordering::Greater)
2950 #[unstable(feature = "iter_order_by", issue = "64295")]
2951 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
2955 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
2957 let mut other = other.into_iter();
2960 let x = match self.next() {
2962 if other.next().is_none() {
2963 return Some(Ordering::Equal);
2965 return Some(Ordering::Less);
2971 let y = match other.next() {
2972 None => return Some(Ordering::Greater),
2976 match partial_cmp(x, y) {
2977 Some(Ordering::Equal) => (),
2978 non_eq => return non_eq,
2983 /// Determines if the elements of this [`Iterator`] are equal to those of
2989 /// assert_eq!([1].iter().eq([1].iter()), true);
2990 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
2992 #[stable(feature = "iter_order", since = "1.5.0")]
2993 fn eq<I>(self, other: I) -> bool
2996 Self::Item: PartialEq<I::Item>,
2999 self.eq_by(other, |x, y| x == y)
3002 /// Determines if the elements of this [`Iterator`] are equal to those of
3003 /// another with respect to the specified equality function.
3010 /// #![feature(iter_order_by)]
3012 /// let xs = [1, 2, 3, 4];
3013 /// let ys = [1, 4, 9, 16];
3015 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3017 #[unstable(feature = "iter_order_by", issue = "64295")]
3018 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3022 F: FnMut(Self::Item, I::Item) -> bool,
3024 let mut other = other.into_iter();
3027 let x = match self.next() {
3028 None => return other.next().is_none(),
3032 let y = match other.next() {
3033 None => return false,
3043 /// Determines if the elements of this [`Iterator`] are unequal to those of
3049 /// assert_eq!([1].iter().ne([1].iter()), false);
3050 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3052 #[stable(feature = "iter_order", since = "1.5.0")]
3053 fn ne<I>(self, other: I) -> bool
3056 Self::Item: PartialEq<I::Item>,
3062 /// Determines if the elements of this [`Iterator`] are lexicographically
3063 /// less than those of another.
3068 /// assert_eq!([1].iter().lt([1].iter()), false);
3069 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3070 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3071 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3073 #[stable(feature = "iter_order", since = "1.5.0")]
3074 fn lt<I>(self, other: I) -> bool
3077 Self::Item: PartialOrd<I::Item>,
3080 self.partial_cmp(other) == Some(Ordering::Less)
3083 /// Determines if the elements of this [`Iterator`] are lexicographically
3084 /// less or equal to those of another.
3089 /// assert_eq!([1].iter().le([1].iter()), true);
3090 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3091 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3092 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3094 #[stable(feature = "iter_order", since = "1.5.0")]
3095 fn le<I>(self, other: I) -> bool
3098 Self::Item: PartialOrd<I::Item>,
3101 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3104 /// Determines if the elements of this [`Iterator`] are lexicographically
3105 /// greater than those of another.
3110 /// assert_eq!([1].iter().gt([1].iter()), false);
3111 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3112 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3113 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3115 #[stable(feature = "iter_order", since = "1.5.0")]
3116 fn gt<I>(self, other: I) -> bool
3119 Self::Item: PartialOrd<I::Item>,
3122 self.partial_cmp(other) == Some(Ordering::Greater)
3125 /// Determines if the elements of this [`Iterator`] are lexicographically
3126 /// greater than or equal to those of another.
3131 /// assert_eq!([1].iter().ge([1].iter()), true);
3132 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3133 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3134 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3136 #[stable(feature = "iter_order", since = "1.5.0")]
3137 fn ge<I>(self, other: I) -> bool
3140 Self::Item: PartialOrd<I::Item>,
3143 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3146 /// Checks if the elements of this iterator are sorted.
3148 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3149 /// iterator yields exactly zero or one element, `true` is returned.
3151 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3152 /// implies that this function returns `false` if any two consecutive items are not
3158 /// #![feature(is_sorted)]
3160 /// assert!([1, 2, 2, 9].iter().is_sorted());
3161 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3162 /// assert!([0].iter().is_sorted());
3163 /// assert!(std::iter::empty::<i32>().is_sorted());
3164 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3167 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3168 fn is_sorted(self) -> bool
3171 Self::Item: PartialOrd,
3173 self.is_sorted_by(PartialOrd::partial_cmp)
3176 /// Checks if the elements of this iterator are sorted using the given comparator function.
3178 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3179 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3180 /// [`is_sorted`]; see its documentation for more information.
3185 /// #![feature(is_sorted)]
3187 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3188 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3189 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3190 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3191 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3194 /// [`is_sorted`]: Iterator::is_sorted
3195 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3196 fn is_sorted_by<F>(mut self, mut compare: F) -> bool
3199 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3201 let mut last = match self.next() {
3203 None => return true,
3206 while let Some(curr) = self.next() {
3207 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3216 /// Checks if the elements of this iterator are sorted using the given key extraction
3219 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3220 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3221 /// its documentation for more information.
3223 /// [`is_sorted`]: Iterator::is_sorted
3228 /// #![feature(is_sorted)]
3230 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3231 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3234 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3235 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3238 F: FnMut(Self::Item) -> K,
3241 self.map(f).is_sorted()
3244 /// See [TrustedRandomAccess]
3247 #[unstable(feature = "trusted_random_access", issue = "none")]
3248 unsafe fn get_unchecked(&mut self, _idx: usize) -> Self::Item
3250 Self: TrustedRandomAccess,
3252 unreachable!("Always specialized");
3256 #[stable(feature = "rust1", since = "1.0.0")]
3257 impl<I: Iterator + ?Sized> Iterator for &mut I {
3258 type Item = I::Item;
3259 fn next(&mut self) -> Option<I::Item> {
3262 fn size_hint(&self) -> (usize, Option<usize>) {
3263 (**self).size_hint()
3265 fn nth(&mut self, n: usize) -> Option<Self::Item> {