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 /// Advances the iterator by `n` elements.
289 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
290 /// times until [`None`] is encountered.
292 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
293 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
294 /// of elements the iterator is advanced by before running out of elements (i.e. the
295 /// length of the iterator). Note that `k` is always less than `n`.
297 /// Calling `advance_by(0)` does not consume any elements and always returns [`Ok(())`][Ok].
299 /// [`next`]: Iterator::next
306 /// #![feature(iter_advance_by)]
308 /// let a = [1, 2, 3, 4];
309 /// let mut iter = a.iter();
311 /// assert_eq!(iter.advance_by(2), Ok(()));
312 /// assert_eq!(iter.next(), Some(&3));
313 /// assert_eq!(iter.advance_by(0), Ok(()));
314 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
317 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
318 fn advance_by(&mut self, n
: usize) -> Result
<(), usize> {
320 self.next().ok_or(i
)?
;
325 /// Returns the `n`th element of the iterator.
327 /// Like most indexing operations, the count starts from zero, so `nth(0)`
328 /// returns the first value, `nth(1)` the second, and so on.
330 /// Note that all preceding elements, as well as the returned element, will be
331 /// consumed from the iterator. That means that the preceding elements will be
332 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
333 /// will return different elements.
335 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
343 /// let a = [1, 2, 3];
344 /// assert_eq!(a.iter().nth(1), Some(&2));
347 /// Calling `nth()` multiple times doesn't rewind the iterator:
350 /// let a = [1, 2, 3];
352 /// let mut iter = a.iter();
354 /// assert_eq!(iter.nth(1), Some(&2));
355 /// assert_eq!(iter.nth(1), None);
358 /// Returning `None` if there are less than `n + 1` elements:
361 /// let a = [1, 2, 3];
362 /// assert_eq!(a.iter().nth(10), None);
365 #[stable(feature = "rust1", since = "1.0.0")]
366 fn nth(&mut self, n
: usize) -> Option
<Self::Item
> {
367 self.advance_by(n
).ok()?
;
371 /// Creates an iterator starting at the same point, but stepping by
372 /// the given amount at each iteration.
374 /// Note 1: The first element of the iterator will always be returned,
375 /// regardless of the step given.
377 /// Note 2: The time at which ignored elements are pulled is not fixed.
378 /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
379 /// but is also free to behave like the sequence
380 /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
381 /// Which way is used may change for some iterators for performance reasons.
382 /// The second way will advance the iterator earlier and may consume more items.
384 /// `advance_n_and_return_first` is the equivalent of:
386 /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
390 /// let next = iter.next();
391 /// if total_step > 1 {
392 /// iter.nth(total_step-2);
400 /// The method will panic if the given step is `0`.
407 /// let a = [0, 1, 2, 3, 4, 5];
408 /// let mut iter = a.iter().step_by(2);
410 /// assert_eq!(iter.next(), Some(&0));
411 /// assert_eq!(iter.next(), Some(&2));
412 /// assert_eq!(iter.next(), Some(&4));
413 /// assert_eq!(iter.next(), None);
416 #[stable(feature = "iterator_step_by", since = "1.28.0")]
417 fn step_by(self, step
: usize) -> StepBy
<Self>
421 StepBy
::new(self, step
)
424 /// Takes two iterators and creates a new iterator over both in sequence.
426 /// `chain()` will return a new iterator which will first iterate over
427 /// values from the first iterator and then over values from the second
430 /// In other words, it links two iterators together, in a chain. 🔗
432 /// [`once`] is commonly used to adapt a single value into a chain of
433 /// other kinds of iteration.
440 /// let a1 = [1, 2, 3];
441 /// let a2 = [4, 5, 6];
443 /// let mut iter = a1.iter().chain(a2.iter());
445 /// assert_eq!(iter.next(), Some(&1));
446 /// assert_eq!(iter.next(), Some(&2));
447 /// assert_eq!(iter.next(), Some(&3));
448 /// assert_eq!(iter.next(), Some(&4));
449 /// assert_eq!(iter.next(), Some(&5));
450 /// assert_eq!(iter.next(), Some(&6));
451 /// assert_eq!(iter.next(), None);
454 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
455 /// anything that can be converted into an [`Iterator`], not just an
456 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
457 /// [`IntoIterator`], and so can be passed to `chain()` directly:
460 /// let s1 = &[1, 2, 3];
461 /// let s2 = &[4, 5, 6];
463 /// let mut iter = s1.iter().chain(s2);
465 /// assert_eq!(iter.next(), Some(&1));
466 /// assert_eq!(iter.next(), Some(&2));
467 /// assert_eq!(iter.next(), Some(&3));
468 /// assert_eq!(iter.next(), Some(&4));
469 /// assert_eq!(iter.next(), Some(&5));
470 /// assert_eq!(iter.next(), Some(&6));
471 /// assert_eq!(iter.next(), None);
474 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
478 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
479 /// use std::os::windows::ffi::OsStrExt;
480 /// s.encode_wide().chain(std::iter::once(0)).collect()
484 /// [`once`]: crate::iter::once
485 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
487 #[stable(feature = "rust1", since = "1.0.0")]
488 fn chain
<U
>(self, other
: U
) -> Chain
<Self, U
::IntoIter
>
491 U
: IntoIterator
<Item
= Self::Item
>,
493 Chain
::new(self, other
.into_iter())
496 /// 'Zips up' two iterators into a single iterator of pairs.
498 /// `zip()` returns a new iterator that will iterate over two other
499 /// iterators, returning a tuple where the first element comes from the
500 /// first iterator, and the second element comes from the second iterator.
502 /// In other words, it zips two iterators together, into a single one.
504 /// If either iterator returns [`None`], [`next`] from the zipped iterator
505 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
506 /// short-circuit and `next` will not be called on the second iterator.
513 /// let a1 = [1, 2, 3];
514 /// let a2 = [4, 5, 6];
516 /// let mut iter = a1.iter().zip(a2.iter());
518 /// assert_eq!(iter.next(), Some((&1, &4)));
519 /// assert_eq!(iter.next(), Some((&2, &5)));
520 /// assert_eq!(iter.next(), Some((&3, &6)));
521 /// assert_eq!(iter.next(), None);
524 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
525 /// anything that can be converted into an [`Iterator`], not just an
526 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
527 /// [`IntoIterator`], and so can be passed to `zip()` directly:
530 /// let s1 = &[1, 2, 3];
531 /// let s2 = &[4, 5, 6];
533 /// let mut iter = s1.iter().zip(s2);
535 /// assert_eq!(iter.next(), Some((&1, &4)));
536 /// assert_eq!(iter.next(), Some((&2, &5)));
537 /// assert_eq!(iter.next(), Some((&3, &6)));
538 /// assert_eq!(iter.next(), None);
541 /// `zip()` is often used to zip an infinite iterator to a finite one.
542 /// This works because the finite iterator will eventually return [`None`],
543 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
546 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
548 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
550 /// assert_eq!((0, 'f'), enumerate[0]);
551 /// assert_eq!((0, 'f'), zipper[0]);
553 /// assert_eq!((1, 'o'), enumerate[1]);
554 /// assert_eq!((1, 'o'), zipper[1]);
556 /// assert_eq!((2, 'o'), enumerate[2]);
557 /// assert_eq!((2, 'o'), zipper[2]);
560 /// [`enumerate`]: Iterator::enumerate
561 /// [`next`]: Iterator::next
563 #[stable(feature = "rust1", since = "1.0.0")]
564 fn zip
<U
>(self, other
: U
) -> Zip
<Self, U
::IntoIter
>
569 Zip
::new(self, other
.into_iter())
572 /// Takes a closure and creates an iterator which calls that closure on each
575 /// `map()` transforms one iterator into another, by means of its argument:
576 /// something that implements [`FnMut`]. It produces a new iterator which
577 /// calls this closure on each element of the original iterator.
579 /// If you are good at thinking in types, you can think of `map()` like this:
580 /// If you have an iterator that gives you elements of some type `A`, and
581 /// you want an iterator of some other type `B`, you can use `map()`,
582 /// passing a closure that takes an `A` and returns a `B`.
584 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
585 /// lazy, it is best used when you're already working with other iterators.
586 /// If you're doing some sort of looping for a side effect, it's considered
587 /// more idiomatic to use [`for`] than `map()`.
589 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
590 /// [`FnMut`]: crate::ops::FnMut
597 /// let a = [1, 2, 3];
599 /// let mut iter = a.iter().map(|x| 2 * x);
601 /// assert_eq!(iter.next(), Some(2));
602 /// assert_eq!(iter.next(), Some(4));
603 /// assert_eq!(iter.next(), Some(6));
604 /// assert_eq!(iter.next(), None);
607 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
610 /// # #![allow(unused_must_use)]
611 /// // don't do this:
612 /// (0..5).map(|x| println!("{}", x));
614 /// // it won't even execute, as it is lazy. Rust will warn you about this.
616 /// // Instead, use for:
618 /// println!("{}", x);
622 #[stable(feature = "rust1", since = "1.0.0")]
623 fn map
<B
, F
>(self, f
: F
) -> Map
<Self, F
>
626 F
: FnMut(Self::Item
) -> B
,
631 /// Calls a closure on each element of an iterator.
633 /// This is equivalent to using a [`for`] loop on the iterator, although
634 /// `break` and `continue` are not possible from a closure. It's generally
635 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
636 /// when processing items at the end of longer iterator chains. In some
637 /// cases `for_each` may also be faster than a loop, because it will use
638 /// internal iteration on adaptors like `Chain`.
640 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
647 /// use std::sync::mpsc::channel;
649 /// let (tx, rx) = channel();
650 /// (0..5).map(|x| x * 2 + 1)
651 /// .for_each(move |x| tx.send(x).unwrap());
653 /// let v: Vec<_> = rx.iter().collect();
654 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
657 /// For such a small example, a `for` loop may be cleaner, but `for_each`
658 /// might be preferable to keep a functional style with longer iterators:
661 /// (0..5).flat_map(|x| x * 100 .. x * 110)
663 /// .filter(|&(i, x)| (i + x) % 3 == 0)
664 /// .for_each(|(i, x)| println!("{}:{}", i, x));
667 #[stable(feature = "iterator_for_each", since = "1.21.0")]
668 fn for_each
<F
>(self, f
: F
)
671 F
: FnMut(Self::Item
),
674 fn call
<T
>(mut f
: impl FnMut(T
)) -> impl FnMut((), T
) {
675 move |(), item
| f(item
)
678 self.fold((), call(f
));
681 /// Creates an iterator which uses a closure to determine if an element
682 /// should be yielded.
684 /// Given an element the closure must return `true` or `false`. The returned
685 /// iterator will yield only the elements for which the closure returns
693 /// let a = [0i32, 1, 2];
695 /// let mut iter = a.iter().filter(|x| x.is_positive());
697 /// assert_eq!(iter.next(), Some(&1));
698 /// assert_eq!(iter.next(), Some(&2));
699 /// assert_eq!(iter.next(), None);
702 /// Because the closure passed to `filter()` takes a reference, and many
703 /// iterators iterate over references, this leads to a possibly confusing
704 /// situation, where the type of the closure is a double reference:
707 /// let a = [0, 1, 2];
709 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
711 /// assert_eq!(iter.next(), Some(&2));
712 /// assert_eq!(iter.next(), None);
715 /// It's common to instead use destructuring on the argument to strip away
719 /// let a = [0, 1, 2];
721 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
723 /// assert_eq!(iter.next(), Some(&2));
724 /// assert_eq!(iter.next(), None);
730 /// let a = [0, 1, 2];
732 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
734 /// assert_eq!(iter.next(), Some(&2));
735 /// assert_eq!(iter.next(), None);
740 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
742 #[stable(feature = "rust1", since = "1.0.0")]
743 fn filter
<P
>(self, predicate
: P
) -> Filter
<Self, P
>
746 P
: FnMut(&Self::Item
) -> bool
,
748 Filter
::new(self, predicate
)
751 /// Creates an iterator that both filters and maps.
753 /// The returned iterator yields only the `value`s for which the supplied
754 /// closure returns `Some(value)`.
756 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
757 /// concise. The example below shows how a `map().filter().map()` can be
758 /// shortened to a single call to `filter_map`.
760 /// [`filter`]: Iterator::filter
761 /// [`map`]: Iterator::map
768 /// let a = ["1", "two", "NaN", "four", "5"];
770 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
772 /// assert_eq!(iter.next(), Some(1));
773 /// assert_eq!(iter.next(), Some(5));
774 /// assert_eq!(iter.next(), None);
777 /// Here's the same example, but with [`filter`] and [`map`]:
780 /// let a = ["1", "two", "NaN", "four", "5"];
781 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
782 /// assert_eq!(iter.next(), Some(1));
783 /// assert_eq!(iter.next(), Some(5));
784 /// assert_eq!(iter.next(), None);
787 /// [`Option<T>`]: Option
789 #[stable(feature = "rust1", since = "1.0.0")]
790 fn filter_map
<B
, F
>(self, f
: F
) -> FilterMap
<Self, F
>
793 F
: FnMut(Self::Item
) -> Option
<B
>,
795 FilterMap
::new(self, f
)
798 /// Creates an iterator which gives the current iteration count as well as
801 /// The iterator returned yields pairs `(i, val)`, where `i` is the
802 /// current index of iteration and `val` is the value returned by the
805 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
806 /// different sized integer, the [`zip`] function provides similar
809 /// # Overflow Behavior
811 /// The method does no guarding against overflows, so enumerating more than
812 /// [`usize::MAX`] elements either produces the wrong result or panics. If
813 /// debug assertions are enabled, a panic is guaranteed.
817 /// The returned iterator might panic if the to-be-returned index would
818 /// overflow a [`usize`].
820 /// [`usize`]: type@usize
821 /// [`usize::MAX`]: crate::usize::MAX
822 /// [`zip`]: Iterator::zip
827 /// let a = ['a', 'b', 'c'];
829 /// let mut iter = a.iter().enumerate();
831 /// assert_eq!(iter.next(), Some((0, &'a')));
832 /// assert_eq!(iter.next(), Some((1, &'b')));
833 /// assert_eq!(iter.next(), Some((2, &'c')));
834 /// assert_eq!(iter.next(), None);
837 #[stable(feature = "rust1", since = "1.0.0")]
838 fn enumerate(self) -> Enumerate
<Self>
845 /// Creates an iterator which can use [`peek`] to look at the next element of
846 /// the iterator without consuming it.
848 /// Adds a [`peek`] method to an iterator. See its documentation for
849 /// more information.
851 /// Note that the underlying iterator is still advanced when [`peek`] is
852 /// called for the first time: In order to retrieve the next element,
853 /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
854 /// anything other than fetching the next value) of the [`next`] method
857 /// [`peek`]: Peekable::peek
858 /// [`next`]: Iterator::next
865 /// let xs = [1, 2, 3];
867 /// let mut iter = xs.iter().peekable();
869 /// // peek() lets us see into the future
870 /// assert_eq!(iter.peek(), Some(&&1));
871 /// assert_eq!(iter.next(), Some(&1));
873 /// assert_eq!(iter.next(), Some(&2));
875 /// // we can peek() multiple times, the iterator won't advance
876 /// assert_eq!(iter.peek(), Some(&&3));
877 /// assert_eq!(iter.peek(), Some(&&3));
879 /// assert_eq!(iter.next(), Some(&3));
881 /// // after the iterator is finished, so is peek()
882 /// assert_eq!(iter.peek(), None);
883 /// assert_eq!(iter.next(), None);
886 #[stable(feature = "rust1", since = "1.0.0")]
887 fn peekable(self) -> Peekable
<Self>
894 /// Creates an iterator that [`skip`]s elements based on a predicate.
896 /// [`skip`]: Iterator::skip
898 /// `skip_while()` takes a closure as an argument. It will call this
899 /// closure on each element of the iterator, and ignore elements
900 /// until it returns `false`.
902 /// After `false` is returned, `skip_while()`'s job is over, and the
903 /// rest of the elements are yielded.
910 /// let a = [-1i32, 0, 1];
912 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
914 /// assert_eq!(iter.next(), Some(&0));
915 /// assert_eq!(iter.next(), Some(&1));
916 /// assert_eq!(iter.next(), None);
919 /// Because the closure passed to `skip_while()` takes a reference, and many
920 /// iterators iterate over references, this leads to a possibly confusing
921 /// situation, where the type of the closure is a double reference:
924 /// let a = [-1, 0, 1];
926 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
928 /// assert_eq!(iter.next(), Some(&0));
929 /// assert_eq!(iter.next(), Some(&1));
930 /// assert_eq!(iter.next(), None);
933 /// Stopping after an initial `false`:
936 /// let a = [-1, 0, 1, -2];
938 /// let mut iter = a.iter().skip_while(|x| **x < 0);
940 /// assert_eq!(iter.next(), Some(&0));
941 /// assert_eq!(iter.next(), Some(&1));
943 /// // while this would have been false, since we already got a false,
944 /// // skip_while() isn't used any more
945 /// assert_eq!(iter.next(), Some(&-2));
947 /// assert_eq!(iter.next(), None);
950 #[stable(feature = "rust1", since = "1.0.0")]
951 fn skip_while
<P
>(self, predicate
: P
) -> SkipWhile
<Self, P
>
954 P
: FnMut(&Self::Item
) -> bool
,
956 SkipWhile
::new(self, predicate
)
959 /// Creates an iterator that yields elements based on a predicate.
961 /// `take_while()` takes a closure as an argument. It will call this
962 /// closure on each element of the iterator, and yield elements
963 /// while it returns `true`.
965 /// After `false` is returned, `take_while()`'s job is over, and the
966 /// rest of the elements are ignored.
973 /// let a = [-1i32, 0, 1];
975 /// let mut iter = a.iter().take_while(|x| x.is_negative());
977 /// assert_eq!(iter.next(), Some(&-1));
978 /// assert_eq!(iter.next(), None);
981 /// Because the closure passed to `take_while()` takes a reference, and many
982 /// iterators iterate over references, this leads to a possibly confusing
983 /// situation, where the type of the closure is a double reference:
986 /// let a = [-1, 0, 1];
988 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
990 /// assert_eq!(iter.next(), Some(&-1));
991 /// assert_eq!(iter.next(), None);
994 /// Stopping after an initial `false`:
997 /// let a = [-1, 0, 1, -2];
999 /// let mut iter = a.iter().take_while(|x| **x < 0);
1001 /// assert_eq!(iter.next(), Some(&-1));
1003 /// // We have more elements that are less than zero, but since we already
1004 /// // got a false, take_while() isn't used any more
1005 /// assert_eq!(iter.next(), None);
1008 /// Because `take_while()` needs to look at the value in order to see if it
1009 /// should be included or not, consuming iterators will see that it is
1013 /// let a = [1, 2, 3, 4];
1014 /// let mut iter = a.iter();
1016 /// let result: Vec<i32> = iter.by_ref()
1017 /// .take_while(|n| **n != 3)
1021 /// assert_eq!(result, &[1, 2]);
1023 /// let result: Vec<i32> = iter.cloned().collect();
1025 /// assert_eq!(result, &[4]);
1028 /// The `3` is no longer there, because it was consumed in order to see if
1029 /// the iteration should stop, but wasn't placed back into the iterator.
1031 #[stable(feature = "rust1", since = "1.0.0")]
1032 fn take_while
<P
>(self, predicate
: P
) -> TakeWhile
<Self, P
>
1035 P
: FnMut(&Self::Item
) -> bool
,
1037 TakeWhile
::new(self, predicate
)
1040 /// Creates an iterator that both yields elements based on a predicate and maps.
1042 /// `map_while()` takes a closure as an argument. It will call this
1043 /// closure on each element of the iterator, and yield elements
1044 /// while it returns [`Some(_)`][`Some`].
1051 /// #![feature(iter_map_while)]
1052 /// let a = [-1i32, 4, 0, 1];
1054 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1056 /// assert_eq!(iter.next(), Some(-16));
1057 /// assert_eq!(iter.next(), Some(4));
1058 /// assert_eq!(iter.next(), None);
1061 /// Here's the same example, but with [`take_while`] and [`map`]:
1063 /// [`take_while`]: Iterator::take_while
1064 /// [`map`]: Iterator::map
1067 /// let a = [-1i32, 4, 0, 1];
1069 /// let mut iter = a.iter()
1070 /// .map(|x| 16i32.checked_div(*x))
1071 /// .take_while(|x| x.is_some())
1072 /// .map(|x| x.unwrap());
1074 /// assert_eq!(iter.next(), Some(-16));
1075 /// assert_eq!(iter.next(), Some(4));
1076 /// assert_eq!(iter.next(), None);
1079 /// Stopping after an initial [`None`]:
1082 /// #![feature(iter_map_while)]
1083 /// use std::convert::TryFrom;
1085 /// let a = [0, 1, 2, -3, 4, 5, -6];
1087 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1088 /// let vec = iter.collect::<Vec<_>>();
1090 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1091 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1092 /// assert_eq!(vec, vec![0, 1, 2]);
1095 /// Because `map_while()` needs to look at the value in order to see if it
1096 /// should be included or not, consuming iterators will see that it is
1100 /// #![feature(iter_map_while)]
1101 /// use std::convert::TryFrom;
1103 /// let a = [1, 2, -3, 4];
1104 /// let mut iter = a.iter();
1106 /// let result: Vec<u32> = iter.by_ref()
1107 /// .map_while(|n| u32::try_from(*n).ok())
1110 /// assert_eq!(result, &[1, 2]);
1112 /// let result: Vec<i32> = iter.cloned().collect();
1114 /// assert_eq!(result, &[4]);
1117 /// The `-3` is no longer there, because it was consumed in order to see if
1118 /// the iteration should stop, but wasn't placed back into the iterator.
1120 /// Note that unlike [`take_while`] this iterator is **not** fused.
1121 /// It is also not specified what this iterator returns after the first` None` is returned.
1122 /// If you need fused iterator, use [`fuse`].
1124 /// [`fuse`]: Iterator::fuse
1126 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1127 fn map_while
<B
, P
>(self, predicate
: P
) -> MapWhile
<Self, P
>
1130 P
: FnMut(Self::Item
) -> Option
<B
>,
1132 MapWhile
::new(self, predicate
)
1135 /// Creates an iterator that skips the first `n` elements.
1137 /// After they have been consumed, the rest of the elements are yielded.
1138 /// Rather than overriding this method directly, instead override the `nth` method.
1145 /// let a = [1, 2, 3];
1147 /// let mut iter = a.iter().skip(2);
1149 /// assert_eq!(iter.next(), Some(&3));
1150 /// assert_eq!(iter.next(), None);
1153 #[stable(feature = "rust1", since = "1.0.0")]
1154 fn skip(self, n
: usize) -> Skip
<Self>
1161 /// Creates an iterator that yields its first `n` elements.
1168 /// let a = [1, 2, 3];
1170 /// let mut iter = a.iter().take(2);
1172 /// assert_eq!(iter.next(), Some(&1));
1173 /// assert_eq!(iter.next(), Some(&2));
1174 /// assert_eq!(iter.next(), None);
1177 /// `take()` is often used with an infinite iterator, to make it finite:
1180 /// let mut iter = (0..).take(3);
1182 /// assert_eq!(iter.next(), Some(0));
1183 /// assert_eq!(iter.next(), Some(1));
1184 /// assert_eq!(iter.next(), Some(2));
1185 /// assert_eq!(iter.next(), None);
1188 /// If less than `n` elements are available,
1189 /// `take` will limit itself to the size of the underlying iterator:
1192 /// let v = vec![1, 2];
1193 /// let mut iter = v.into_iter().take(5);
1194 /// assert_eq!(iter.next(), Some(1));
1195 /// assert_eq!(iter.next(), Some(2));
1196 /// assert_eq!(iter.next(), None);
1199 #[stable(feature = "rust1", since = "1.0.0")]
1200 fn take(self, n
: usize) -> Take
<Self>
1207 /// An iterator adaptor similar to [`fold`] that holds internal state and
1208 /// produces a new iterator.
1210 /// [`fold`]: Iterator::fold
1212 /// `scan()` takes two arguments: an initial value which seeds the internal
1213 /// state, and a closure with two arguments, the first being a mutable
1214 /// reference to the internal state and the second an iterator element.
1215 /// The closure can assign to the internal state to share state between
1218 /// On iteration, the closure will be applied to each element of the
1219 /// iterator and the return value from the closure, an [`Option`], is
1220 /// yielded by the iterator.
1227 /// let a = [1, 2, 3];
1229 /// let mut iter = a.iter().scan(1, |state, &x| {
1230 /// // each iteration, we'll multiply the state by the element
1231 /// *state = *state * x;
1233 /// // then, we'll yield the negation of the state
1237 /// assert_eq!(iter.next(), Some(-1));
1238 /// assert_eq!(iter.next(), Some(-2));
1239 /// assert_eq!(iter.next(), Some(-6));
1240 /// assert_eq!(iter.next(), None);
1243 #[stable(feature = "rust1", since = "1.0.0")]
1244 fn scan
<St
, B
, F
>(self, initial_state
: St
, f
: F
) -> Scan
<Self, St
, F
>
1247 F
: FnMut(&mut St
, Self::Item
) -> Option
<B
>,
1249 Scan
::new(self, initial_state
, f
)
1252 /// Creates an iterator that works like map, but flattens nested structure.
1254 /// The [`map`] adapter is very useful, but only when the closure
1255 /// argument produces values. If it produces an iterator instead, there's
1256 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1259 /// You can think of `flat_map(f)` as the semantic equivalent
1260 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1262 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1263 /// one item for each element, and `flat_map()`'s closure returns an
1264 /// iterator for each element.
1266 /// [`map`]: Iterator::map
1267 /// [`flatten`]: Iterator::flatten
1274 /// let words = ["alpha", "beta", "gamma"];
1276 /// // chars() returns an iterator
1277 /// let merged: String = words.iter()
1278 /// .flat_map(|s| s.chars())
1280 /// assert_eq!(merged, "alphabetagamma");
1283 #[stable(feature = "rust1", since = "1.0.0")]
1284 fn flat_map
<U
, F
>(self, f
: F
) -> FlatMap
<Self, U
, F
>
1288 F
: FnMut(Self::Item
) -> U
,
1290 FlatMap
::new(self, f
)
1293 /// Creates an iterator that flattens nested structure.
1295 /// This is useful when you have an iterator of iterators or an iterator of
1296 /// things that can be turned into iterators and you want to remove one
1297 /// level of indirection.
1304 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1305 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1306 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1309 /// Mapping and then flattening:
1312 /// let words = ["alpha", "beta", "gamma"];
1314 /// // chars() returns an iterator
1315 /// let merged: String = words.iter()
1316 /// .map(|s| s.chars())
1319 /// assert_eq!(merged, "alphabetagamma");
1322 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1323 /// in this case since it conveys intent more clearly:
1326 /// let words = ["alpha", "beta", "gamma"];
1328 /// // chars() returns an iterator
1329 /// let merged: String = words.iter()
1330 /// .flat_map(|s| s.chars())
1332 /// assert_eq!(merged, "alphabetagamma");
1335 /// Flattening only removes one level of nesting at a time:
1338 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1340 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1341 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1343 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1344 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1347 /// Here we see that `flatten()` does not perform a "deep" flatten.
1348 /// Instead, only one level of nesting is removed. That is, if you
1349 /// `flatten()` a three-dimensional array, the result will be
1350 /// two-dimensional and not one-dimensional. To get a one-dimensional
1351 /// structure, you have to `flatten()` again.
1353 /// [`flat_map()`]: Iterator::flat_map
1355 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1356 fn flatten(self) -> Flatten
<Self>
1359 Self::Item
: IntoIterator
,
1364 /// Creates an iterator which ends after the first [`None`].
1366 /// After an iterator returns [`None`], future calls may or may not yield
1367 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1368 /// [`None`] is given, it will always return [`None`] forever.
1370 /// [`Some(T)`]: Some
1377 /// // an iterator which alternates between Some and None
1378 /// struct Alternate {
1382 /// impl Iterator for Alternate {
1383 /// type Item = i32;
1385 /// fn next(&mut self) -> Option<i32> {
1386 /// let val = self.state;
1387 /// self.state = self.state + 1;
1389 /// // if it's even, Some(i32), else None
1390 /// if val % 2 == 0 {
1398 /// let mut iter = Alternate { state: 0 };
1400 /// // we can see our iterator going back and forth
1401 /// assert_eq!(iter.next(), Some(0));
1402 /// assert_eq!(iter.next(), None);
1403 /// assert_eq!(iter.next(), Some(2));
1404 /// assert_eq!(iter.next(), None);
1406 /// // however, once we fuse it...
1407 /// let mut iter = iter.fuse();
1409 /// assert_eq!(iter.next(), Some(4));
1410 /// assert_eq!(iter.next(), None);
1412 /// // it will always return `None` after the first time.
1413 /// assert_eq!(iter.next(), None);
1414 /// assert_eq!(iter.next(), None);
1415 /// assert_eq!(iter.next(), None);
1418 #[stable(feature = "rust1", since = "1.0.0")]
1419 fn fuse(self) -> Fuse
<Self>
1426 /// Does something with each element of an iterator, passing the value on.
1428 /// When using iterators, you'll often chain several of them together.
1429 /// While working on such code, you might want to check out what's
1430 /// happening at various parts in the pipeline. To do that, insert
1431 /// a call to `inspect()`.
1433 /// It's more common for `inspect()` to be used as a debugging tool than to
1434 /// exist in your final code, but applications may find it useful in certain
1435 /// situations when errors need to be logged before being discarded.
1442 /// let a = [1, 4, 2, 3];
1444 /// // this iterator sequence is complex.
1445 /// let sum = a.iter()
1447 /// .filter(|x| x % 2 == 0)
1448 /// .fold(0, |sum, i| sum + i);
1450 /// println!("{}", sum);
1452 /// // let's add some inspect() calls to investigate what's happening
1453 /// let sum = a.iter()
1455 /// .inspect(|x| println!("about to filter: {}", x))
1456 /// .filter(|x| x % 2 == 0)
1457 /// .inspect(|x| println!("made it through filter: {}", x))
1458 /// .fold(0, |sum, i| sum + i);
1460 /// println!("{}", sum);
1463 /// This will print:
1467 /// about to filter: 1
1468 /// about to filter: 4
1469 /// made it through filter: 4
1470 /// about to filter: 2
1471 /// made it through filter: 2
1472 /// about to filter: 3
1476 /// Logging errors before discarding them:
1479 /// let lines = ["1", "2", "a"];
1481 /// let sum: i32 = lines
1483 /// .map(|line| line.parse::<i32>())
1484 /// .inspect(|num| {
1485 /// if let Err(ref e) = *num {
1486 /// println!("Parsing error: {}", e);
1489 /// .filter_map(Result::ok)
1492 /// println!("Sum: {}", sum);
1495 /// This will print:
1498 /// Parsing error: invalid digit found in string
1502 #[stable(feature = "rust1", since = "1.0.0")]
1503 fn inspect
<F
>(self, f
: F
) -> Inspect
<Self, F
>
1506 F
: FnMut(&Self::Item
),
1508 Inspect
::new(self, f
)
1511 /// Borrows an iterator, rather than consuming it.
1513 /// This is useful to allow applying iterator adaptors while still
1514 /// retaining ownership of the original iterator.
1521 /// let a = [1, 2, 3];
1523 /// let iter = a.iter();
1525 /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
1527 /// assert_eq!(sum, 6);
1529 /// // if we try to use iter again, it won't work. The following line
1530 /// // gives "error: use of moved value: `iter`
1531 /// // assert_eq!(iter.next(), None);
1533 /// // let's try that again
1534 /// let a = [1, 2, 3];
1536 /// let mut iter = a.iter();
1538 /// // instead, we add in a .by_ref()
1539 /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
1541 /// assert_eq!(sum, 3);
1543 /// // now this is just fine:
1544 /// assert_eq!(iter.next(), Some(&3));
1545 /// assert_eq!(iter.next(), None);
1547 #[stable(feature = "rust1", since = "1.0.0")]
1548 fn by_ref(&mut self) -> &mut Self
1555 /// Transforms an iterator into a collection.
1557 /// `collect()` can take anything iterable, and turn it into a relevant
1558 /// collection. This is one of the more powerful methods in the standard
1559 /// library, used in a variety of contexts.
1561 /// The most basic pattern in which `collect()` is used is to turn one
1562 /// collection into another. You take a collection, call [`iter`] on it,
1563 /// do a bunch of transformations, and then `collect()` at the end.
1565 /// `collect()` can also create instances of types that are not typical
1566 /// collections. For example, a [`String`] can be built from [`char`]s,
1567 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1568 /// into `Result<Collection<T>, E>`. See the examples below for more.
1570 /// Because `collect()` is so general, it can cause problems with type
1571 /// inference. As such, `collect()` is one of the few times you'll see
1572 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1573 /// helps the inference algorithm understand specifically which collection
1574 /// you're trying to collect into.
1581 /// let a = [1, 2, 3];
1583 /// let doubled: Vec<i32> = a.iter()
1584 /// .map(|&x| x * 2)
1587 /// assert_eq!(vec![2, 4, 6], doubled);
1590 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1591 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1593 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1596 /// use std::collections::VecDeque;
1598 /// let a = [1, 2, 3];
1600 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1602 /// assert_eq!(2, doubled[0]);
1603 /// assert_eq!(4, doubled[1]);
1604 /// assert_eq!(6, doubled[2]);
1607 /// Using the 'turbofish' instead of annotating `doubled`:
1610 /// let a = [1, 2, 3];
1612 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1614 /// assert_eq!(vec![2, 4, 6], doubled);
1617 /// Because `collect()` only cares about what you're collecting into, you can
1618 /// still use a partial type hint, `_`, with the turbofish:
1621 /// let a = [1, 2, 3];
1623 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1625 /// assert_eq!(vec![2, 4, 6], doubled);
1628 /// Using `collect()` to make a [`String`]:
1631 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1633 /// let hello: String = chars.iter()
1634 /// .map(|&x| x as u8)
1635 /// .map(|x| (x + 1) as char)
1638 /// assert_eq!("hello", hello);
1641 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1642 /// see if any of them failed:
1645 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1647 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1649 /// // gives us the first error
1650 /// assert_eq!(Err("nope"), result);
1652 /// let results = [Ok(1), Ok(3)];
1654 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1656 /// // gives us the list of answers
1657 /// assert_eq!(Ok(vec![1, 3]), result);
1660 /// [`iter`]: Iterator::next
1661 /// [`String`]: ../../std/string/struct.String.html
1662 /// [`char`]: type@char
1664 #[stable(feature = "rust1", since = "1.0.0")]
1665 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1666 fn collect
<B
: FromIterator
<Self::Item
>>(self) -> B
1670 FromIterator
::from_iter(self)
1673 /// Consumes an iterator, creating two collections from it.
1675 /// The predicate passed to `partition()` can return `true`, or `false`.
1676 /// `partition()` returns a pair, all of the elements for which it returned
1677 /// `true`, and all of the elements for which it returned `false`.
1679 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1681 /// [`is_partitioned()`]: Iterator::is_partitioned
1682 /// [`partition_in_place()`]: Iterator::partition_in_place
1689 /// let a = [1, 2, 3];
1691 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1693 /// .partition(|&n| n % 2 == 0);
1695 /// assert_eq!(even, vec![2]);
1696 /// assert_eq!(odd, vec![1, 3]);
1698 #[stable(feature = "rust1", since = "1.0.0")]
1699 fn partition
<B
, F
>(self, f
: F
) -> (B
, B
)
1702 B
: Default
+ Extend
<Self::Item
>,
1703 F
: FnMut(&Self::Item
) -> bool
,
1706 fn extend
<'a
, T
, B
: Extend
<T
>>(
1707 mut f
: impl FnMut(&T
) -> bool
+ 'a
,
1710 ) -> impl FnMut((), T
) + 'a
{
1715 right
.extend_one(x
);
1720 let mut left
: B
= Default
::default();
1721 let mut right
: B
= Default
::default();
1723 self.fold((), extend(f
, &mut left
, &mut right
));
1728 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1729 /// such that all those that return `true` precede all those that return `false`.
1730 /// Returns the number of `true` elements found.
1732 /// The relative order of partitioned items is not maintained.
1734 /// See also [`is_partitioned()`] and [`partition()`].
1736 /// [`is_partitioned()`]: Iterator::is_partitioned
1737 /// [`partition()`]: Iterator::partition
1742 /// #![feature(iter_partition_in_place)]
1744 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1746 /// // Partition in-place between evens and odds
1747 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1749 /// assert_eq!(i, 3);
1750 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1751 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1753 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1754 fn partition_in_place
<'a
, T
: 'a
, P
>(mut self, ref mut predicate
: P
) -> usize
1756 Self: Sized
+ DoubleEndedIterator
<Item
= &'a
mut T
>,
1757 P
: FnMut(&T
) -> bool
,
1759 // FIXME: should we worry about the count overflowing? The only way to have more than
1760 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1762 // These closure "factory" functions exist to avoid genericity in `Self`.
1766 predicate
: &'a
mut impl FnMut(&T
) -> bool
,
1767 true_count
: &'a
mut usize,
1768 ) -> impl FnMut(&&mut T
) -> bool
+ 'a
{
1770 let p
= predicate(&**x
);
1771 *true_count
+= p
as usize;
1777 fn is_true
<T
>(predicate
: &mut impl FnMut(&T
) -> bool
) -> impl FnMut(&&mut T
) -> bool
+ '_
{
1778 move |x
| predicate(&**x
)
1781 // Repeatedly find the first `false` and swap it with the last `true`.
1782 let mut true_count
= 0;
1783 while let Some(head
) = self.find(is_false(predicate
, &mut true_count
)) {
1784 if let Some(tail
) = self.rfind(is_true(predicate
)) {
1785 crate::mem
::swap(head
, tail
);
1794 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1795 /// such that all those that return `true` precede all those that return `false`.
1797 /// See also [`partition()`] and [`partition_in_place()`].
1799 /// [`partition()`]: Iterator::partition
1800 /// [`partition_in_place()`]: Iterator::partition_in_place
1805 /// #![feature(iter_is_partitioned)]
1807 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1808 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1810 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1811 fn is_partitioned
<P
>(mut self, mut predicate
: P
) -> bool
1814 P
: FnMut(Self::Item
) -> bool
,
1816 // Either all items test `true`, or the first clause stops at `false`
1817 // and we check that there are no more `true` items after that.
1818 self.all(&mut predicate
) || !self.any(predicate
)
1821 /// An iterator method that applies a function as long as it returns
1822 /// successfully, producing a single, final value.
1824 /// `try_fold()` takes two arguments: an initial value, and a closure with
1825 /// two arguments: an 'accumulator', and an element. The closure either
1826 /// returns successfully, with the value that the accumulator should have
1827 /// for the next iteration, or it returns failure, with an error value that
1828 /// is propagated back to the caller immediately (short-circuiting).
1830 /// The initial value is the value the accumulator will have on the first
1831 /// call. If applying the closure succeeded against every element of the
1832 /// iterator, `try_fold()` returns the final accumulator as success.
1834 /// Folding is useful whenever you have a collection of something, and want
1835 /// to produce a single value from it.
1837 /// # Note to Implementors
1839 /// Several of the other (forward) methods have default implementations in
1840 /// terms of this one, so try to implement this explicitly if it can
1841 /// do something better than the default `for` loop implementation.
1843 /// In particular, try to have this call `try_fold()` on the internal parts
1844 /// from which this iterator is composed. If multiple calls are needed,
1845 /// the `?` operator may be convenient for chaining the accumulator value
1846 /// along, but beware any invariants that need to be upheld before those
1847 /// early returns. This is a `&mut self` method, so iteration needs to be
1848 /// resumable after hitting an error here.
1855 /// let a = [1, 2, 3];
1857 /// // the checked sum of all of the elements of the array
1858 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1860 /// assert_eq!(sum, Some(6));
1863 /// Short-circuiting:
1866 /// let a = [10, 20, 30, 100, 40, 50];
1867 /// let mut it = a.iter();
1869 /// // This sum overflows when adding the 100 element
1870 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1871 /// assert_eq!(sum, None);
1873 /// // Because it short-circuited, the remaining elements are still
1874 /// // available through the iterator.
1875 /// assert_eq!(it.len(), 2);
1876 /// assert_eq!(it.next(), Some(&40));
1879 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1880 fn try_fold
<B
, F
, R
>(&mut self, init
: B
, mut f
: F
) -> R
1883 F
: FnMut(B
, Self::Item
) -> R
,
1886 let mut accum
= init
;
1887 while let Some(x
) = self.next() {
1888 accum
= f(accum
, x
)?
;
1893 /// An iterator method that applies a fallible function to each item in the
1894 /// iterator, stopping at the first error and returning that error.
1896 /// This can also be thought of as the fallible form of [`for_each()`]
1897 /// or as the stateless version of [`try_fold()`].
1899 /// [`for_each()`]: Iterator::for_each
1900 /// [`try_fold()`]: Iterator::try_fold
1905 /// use std::fs::rename;
1906 /// use std::io::{stdout, Write};
1907 /// use std::path::Path;
1909 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
1911 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
1912 /// assert!(res.is_ok());
1914 /// let mut it = data.iter().cloned();
1915 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
1916 /// assert!(res.is_err());
1917 /// // It short-circuited, so the remaining items are still in the iterator:
1918 /// assert_eq!(it.next(), Some("stale_bread.json"));
1921 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1922 fn try_for_each
<F
, R
>(&mut self, f
: F
) -> R
1925 F
: FnMut(Self::Item
) -> R
,
1929 fn call
<T
, R
>(mut f
: impl FnMut(T
) -> R
) -> impl FnMut((), T
) -> R
{
1933 self.try_fold((), call(f
))
1936 /// An iterator method that applies a function, producing a single, final value.
1938 /// `fold()` takes two arguments: an initial value, and a closure with two
1939 /// arguments: an 'accumulator', and an element. The closure returns the value that
1940 /// the accumulator should have for the next iteration.
1942 /// The initial value is the value the accumulator will have on the first
1945 /// After applying this closure to every element of the iterator, `fold()`
1946 /// returns the accumulator.
1948 /// This operation is sometimes called 'reduce' or 'inject'.
1950 /// Folding is useful whenever you have a collection of something, and want
1951 /// to produce a single value from it.
1953 /// Note: `fold()`, and similar methods that traverse the entire iterator,
1954 /// may not terminate for infinite iterators, even on traits for which a
1955 /// result is determinable in finite time.
1957 /// # Note to Implementors
1959 /// Several of the other (forward) methods have default implementations in
1960 /// terms of this one, so try to implement this explicitly if it can
1961 /// do something better than the default `for` loop implementation.
1963 /// In particular, try to have this call `fold()` on the internal parts
1964 /// from which this iterator is composed.
1971 /// let a = [1, 2, 3];
1973 /// // the sum of all of the elements of the array
1974 /// let sum = a.iter().fold(0, |acc, x| acc + x);
1976 /// assert_eq!(sum, 6);
1979 /// Let's walk through each step of the iteration here:
1981 /// | element | acc | x | result |
1982 /// |---------|-----|---|--------|
1984 /// | 1 | 0 | 1 | 1 |
1985 /// | 2 | 1 | 2 | 3 |
1986 /// | 3 | 3 | 3 | 6 |
1988 /// And so, our final result, `6`.
1990 /// It's common for people who haven't used iterators a lot to
1991 /// use a `for` loop with a list of things to build up a result. Those
1992 /// can be turned into `fold()`s:
1994 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
1997 /// let numbers = [1, 2, 3, 4, 5];
1999 /// let mut result = 0;
2002 /// for i in &numbers {
2003 /// result = result + i;
2007 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2009 /// // they're the same
2010 /// assert_eq!(result, result2);
2012 #[doc(alias = "reduce")]
2013 #[doc(alias = "inject")]
2015 #[stable(feature = "rust1", since = "1.0.0")]
2016 fn fold
<B
, F
>(mut self, init
: B
, mut f
: F
) -> B
2019 F
: FnMut(B
, Self::Item
) -> B
,
2021 let mut accum
= init
;
2022 while let Some(x
) = self.next() {
2023 accum
= f(accum
, x
);
2028 /// The same as [`fold()`], but uses the first element in the
2029 /// iterator as the initial value, folding every subsequent element into it.
2030 /// If the iterator is empty, return [`None`]; otherwise, return the result
2033 /// [`fold()`]: Iterator::fold
2037 /// Find the maximum value:
2040 /// #![feature(iterator_fold_self)]
2042 /// fn find_max<I>(iter: I) -> Option<I::Item>
2043 /// where I: Iterator,
2046 /// iter.fold_first(|a, b| {
2047 /// if a >= b { a } else { b }
2050 /// let a = [10, 20, 5, -23, 0];
2051 /// let b: [u32; 0] = [];
2053 /// assert_eq!(find_max(a.iter()), Some(&20));
2054 /// assert_eq!(find_max(b.iter()), None);
2057 #[unstable(feature = "iterator_fold_self", issue = "68125")]
2058 fn fold_first
<F
>(mut self, f
: F
) -> Option
<Self::Item
>
2061 F
: FnMut(Self::Item
, Self::Item
) -> Self::Item
,
2063 let first
= self.next()?
;
2064 Some(self.fold(first
, f
))
2067 /// Tests if every element of the iterator matches a predicate.
2069 /// `all()` takes a closure that returns `true` or `false`. It applies
2070 /// this closure to each element of the iterator, and if they all return
2071 /// `true`, then so does `all()`. If any of them return `false`, it
2072 /// returns `false`.
2074 /// `all()` is short-circuiting; in other words, it will stop processing
2075 /// as soon as it finds a `false`, given that no matter what else happens,
2076 /// the result will also be `false`.
2078 /// An empty iterator returns `true`.
2085 /// let a = [1, 2, 3];
2087 /// assert!(a.iter().all(|&x| x > 0));
2089 /// assert!(!a.iter().all(|&x| x > 2));
2092 /// Stopping at the first `false`:
2095 /// let a = [1, 2, 3];
2097 /// let mut iter = a.iter();
2099 /// assert!(!iter.all(|&x| x != 2));
2101 /// // we can still use `iter`, as there are more elements.
2102 /// assert_eq!(iter.next(), Some(&3));
2105 #[stable(feature = "rust1", since = "1.0.0")]
2106 fn all
<F
>(&mut self, f
: F
) -> bool
2109 F
: FnMut(Self::Item
) -> bool
,
2112 fn check
<T
>(mut f
: impl FnMut(T
) -> bool
) -> impl FnMut((), T
) -> ControlFlow
<()> {
2114 if f(x
) { ControlFlow::CONTINUE }
else { ControlFlow::BREAK }
2117 self.try_fold((), check(f
)) == ControlFlow
::CONTINUE
2120 /// Tests if any element of the iterator matches a predicate.
2122 /// `any()` takes a closure that returns `true` or `false`. It applies
2123 /// this closure to each element of the iterator, and if any of them return
2124 /// `true`, then so does `any()`. If they all return `false`, it
2125 /// returns `false`.
2127 /// `any()` is short-circuiting; in other words, it will stop processing
2128 /// as soon as it finds a `true`, given that no matter what else happens,
2129 /// the result will also be `true`.
2131 /// An empty iterator returns `false`.
2138 /// let a = [1, 2, 3];
2140 /// assert!(a.iter().any(|&x| x > 0));
2142 /// assert!(!a.iter().any(|&x| x > 5));
2145 /// Stopping at the first `true`:
2148 /// let a = [1, 2, 3];
2150 /// let mut iter = a.iter();
2152 /// assert!(iter.any(|&x| x != 2));
2154 /// // we can still use `iter`, as there are more elements.
2155 /// assert_eq!(iter.next(), Some(&2));
2158 #[stable(feature = "rust1", since = "1.0.0")]
2159 fn any
<F
>(&mut self, f
: F
) -> bool
2162 F
: FnMut(Self::Item
) -> bool
,
2165 fn check
<T
>(mut f
: impl FnMut(T
) -> bool
) -> impl FnMut((), T
) -> ControlFlow
<()> {
2167 if f(x
) { ControlFlow::BREAK }
else { ControlFlow::CONTINUE }
2171 self.try_fold((), check(f
)) == ControlFlow
::BREAK
2174 /// Searches for an element of an iterator that satisfies a predicate.
2176 /// `find()` takes a closure that returns `true` or `false`. It applies
2177 /// this closure to each element of the iterator, and if any of them return
2178 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2179 /// `false`, it returns [`None`].
2181 /// `find()` is short-circuiting; in other words, it will stop processing
2182 /// as soon as the closure returns `true`.
2184 /// Because `find()` takes a reference, and many iterators iterate over
2185 /// references, this leads to a possibly confusing situation where the
2186 /// argument is a double reference. You can see this effect in the
2187 /// examples below, with `&&x`.
2189 /// [`Some(element)`]: Some
2196 /// let a = [1, 2, 3];
2198 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2200 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2203 /// Stopping at the first `true`:
2206 /// let a = [1, 2, 3];
2208 /// let mut iter = a.iter();
2210 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2212 /// // we can still use `iter`, as there are more elements.
2213 /// assert_eq!(iter.next(), Some(&3));
2216 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2218 #[stable(feature = "rust1", since = "1.0.0")]
2219 fn find
<P
>(&mut self, predicate
: P
) -> Option
<Self::Item
>
2222 P
: FnMut(&Self::Item
) -> bool
,
2225 fn check
<T
>(mut predicate
: impl FnMut(&T
) -> bool
) -> impl FnMut((), T
) -> ControlFlow
<T
> {
2227 if predicate(&x
) { ControlFlow::Break(x) }
else { ControlFlow::CONTINUE }
2231 self.try_fold((), check(predicate
)).break_value()
2234 /// Applies function to the elements of iterator and returns
2235 /// the first non-none result.
2237 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2242 /// let a = ["lol", "NaN", "2", "5"];
2244 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2246 /// assert_eq!(first_number, Some(2));
2249 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2250 fn find_map
<B
, F
>(&mut self, f
: F
) -> Option
<B
>
2253 F
: FnMut(Self::Item
) -> Option
<B
>,
2256 fn check
<T
, B
>(mut f
: impl FnMut(T
) -> Option
<B
>) -> impl FnMut((), T
) -> ControlFlow
<B
> {
2257 move |(), x
| match f(x
) {
2258 Some(x
) => ControlFlow
::Break(x
),
2259 None
=> ControlFlow
::CONTINUE
,
2263 self.try_fold((), check(f
)).break_value()
2266 /// Applies function to the elements of iterator and returns
2267 /// the first true result or the first error.
2272 /// #![feature(try_find)]
2274 /// let a = ["1", "2", "lol", "NaN", "5"];
2276 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2277 /// Ok(s.parse::<i32>()? == search)
2280 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2281 /// assert_eq!(result, Ok(Some(&"2")));
2283 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2284 /// assert!(result.is_err());
2287 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2288 fn try_find
<F
, R
>(&mut self, f
: F
) -> Result
<Option
<Self::Item
>, R
::Error
>
2291 F
: FnMut(&Self::Item
) -> R
,
2295 fn check
<F
, T
, R
>(mut f
: F
) -> impl FnMut((), T
) -> ControlFlow
<Result
<T
, R
::Error
>>
2300 move |(), x
| match f(&x
).into_result() {
2301 Ok(false) => ControlFlow
::CONTINUE
,
2302 Ok(true) => ControlFlow
::Break(Ok(x
)),
2303 Err(x
) => ControlFlow
::Break(Err(x
)),
2307 self.try_fold((), check(f
)).break_value().transpose()
2310 /// Searches for an element in an iterator, returning its index.
2312 /// `position()` takes a closure that returns `true` or `false`. It applies
2313 /// this closure to each element of the iterator, and if one of them
2314 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2315 /// them return `false`, it returns [`None`].
2317 /// `position()` is short-circuiting; in other words, it will stop
2318 /// processing as soon as it finds a `true`.
2320 /// # Overflow Behavior
2322 /// The method does no guarding against overflows, so if there are more
2323 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2324 /// result or panics. If debug assertions are enabled, a panic is
2329 /// This function might panic if the iterator has more than `usize::MAX`
2330 /// non-matching elements.
2332 /// [`Some(index)`]: Some
2333 /// [`usize::MAX`]: crate::usize::MAX
2340 /// let a = [1, 2, 3];
2342 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2344 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2347 /// Stopping at the first `true`:
2350 /// let a = [1, 2, 3, 4];
2352 /// let mut iter = a.iter();
2354 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2356 /// // we can still use `iter`, as there are more elements.
2357 /// assert_eq!(iter.next(), Some(&3));
2359 /// // The returned index depends on iterator state
2360 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2364 #[stable(feature = "rust1", since = "1.0.0")]
2365 fn position
<P
>(&mut self, predicate
: P
) -> Option
<usize>
2368 P
: FnMut(Self::Item
) -> bool
,
2372 mut predicate
: impl FnMut(T
) -> bool
,
2373 ) -> impl FnMut(usize, T
) -> ControlFlow
<usize, usize> {
2374 // The addition might panic on overflow
2377 ControlFlow
::Break(i
)
2379 ControlFlow
::Continue(Add
::add(i
, 1))
2384 self.try_fold(0, check(predicate
)).break_value()
2387 /// Searches for an element in an iterator from the right, returning its
2390 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2391 /// this closure to each element of the iterator, starting from the end,
2392 /// and if one of them returns `true`, then `rposition()` returns
2393 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2395 /// `rposition()` is short-circuiting; in other words, it will stop
2396 /// processing as soon as it finds a `true`.
2398 /// [`Some(index)`]: Some
2405 /// let a = [1, 2, 3];
2407 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2409 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2412 /// Stopping at the first `true`:
2415 /// let a = [1, 2, 3];
2417 /// let mut iter = a.iter();
2419 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2421 /// // we can still use `iter`, as there are more elements.
2422 /// assert_eq!(iter.next(), Some(&1));
2425 #[stable(feature = "rust1", since = "1.0.0")]
2426 fn rposition
<P
>(&mut self, predicate
: P
) -> Option
<usize>
2428 P
: FnMut(Self::Item
) -> bool
,
2429 Self: Sized
+ ExactSizeIterator
+ DoubleEndedIterator
,
2431 // No need for an overflow check here, because `ExactSizeIterator`
2432 // implies that the number of elements fits into a `usize`.
2435 mut predicate
: impl FnMut(T
) -> bool
,
2436 ) -> impl FnMut(usize, T
) -> ControlFlow
<usize, usize> {
2439 if predicate(x
) { ControlFlow::Break(i) }
else { ControlFlow::Continue(i) }
2444 self.try_rfold(n
, check(predicate
)).break_value()
2447 /// Returns the maximum element of an iterator.
2449 /// If several elements are equally maximum, the last element is
2450 /// returned. If the iterator is empty, [`None`] is returned.
2457 /// let a = [1, 2, 3];
2458 /// let b: Vec<u32> = Vec::new();
2460 /// assert_eq!(a.iter().max(), Some(&3));
2461 /// assert_eq!(b.iter().max(), None);
2464 #[stable(feature = "rust1", since = "1.0.0")]
2465 fn max(self) -> Option
<Self::Item
>
2470 self.max_by(Ord
::cmp
)
2473 /// Returns the minimum element of an iterator.
2475 /// If several elements are equally minimum, the first element is
2476 /// returned. If the iterator is empty, [`None`] is returned.
2483 /// let a = [1, 2, 3];
2484 /// let b: Vec<u32> = Vec::new();
2486 /// assert_eq!(a.iter().min(), Some(&1));
2487 /// assert_eq!(b.iter().min(), None);
2490 #[stable(feature = "rust1", since = "1.0.0")]
2491 fn min(self) -> Option
<Self::Item
>
2496 self.min_by(Ord
::cmp
)
2499 /// Returns the element that gives the maximum value from the
2500 /// specified function.
2502 /// If several elements are equally maximum, the last element is
2503 /// returned. If the iterator is empty, [`None`] is returned.
2508 /// let a = [-3_i32, 0, 1, 5, -10];
2509 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2512 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2513 fn max_by_key
<B
: Ord
, F
>(self, f
: F
) -> Option
<Self::Item
>
2516 F
: FnMut(&Self::Item
) -> B
,
2519 fn key
<T
, B
>(mut f
: impl FnMut(&T
) -> B
) -> impl FnMut(T
) -> (B
, T
) {
2524 fn compare
<T
, B
: Ord
>((x_p
, _
): &(B
, T
), (y_p
, _
): &(B
, T
)) -> Ordering
{
2528 let (_
, x
) = self.map(key(f
)).max_by(compare
)?
;
2532 /// Returns the element that gives the maximum value with respect to the
2533 /// specified comparison function.
2535 /// If several elements are equally maximum, the last element is
2536 /// returned. If the iterator is empty, [`None`] is returned.
2541 /// let a = [-3_i32, 0, 1, 5, -10];
2542 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2545 #[stable(feature = "iter_max_by", since = "1.15.0")]
2546 fn max_by
<F
>(self, compare
: F
) -> Option
<Self::Item
>
2549 F
: FnMut(&Self::Item
, &Self::Item
) -> Ordering
,
2552 fn fold
<T
>(mut compare
: impl FnMut(&T
, &T
) -> Ordering
) -> impl FnMut(T
, T
) -> T
{
2553 move |x
, y
| cmp
::max_by(x
, y
, &mut compare
)
2556 self.fold_first(fold(compare
))
2559 /// Returns the element that gives the minimum value from the
2560 /// specified function.
2562 /// If several elements are equally minimum, the first element is
2563 /// returned. If the iterator is empty, [`None`] is returned.
2568 /// let a = [-3_i32, 0, 1, 5, -10];
2569 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2572 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2573 fn min_by_key
<B
: Ord
, F
>(self, f
: F
) -> Option
<Self::Item
>
2576 F
: FnMut(&Self::Item
) -> B
,
2579 fn key
<T
, B
>(mut f
: impl FnMut(&T
) -> B
) -> impl FnMut(T
) -> (B
, T
) {
2584 fn compare
<T
, B
: Ord
>((x_p
, _
): &(B
, T
), (y_p
, _
): &(B
, T
)) -> Ordering
{
2588 let (_
, x
) = self.map(key(f
)).min_by(compare
)?
;
2592 /// Returns the element that gives the minimum value with respect to the
2593 /// specified comparison function.
2595 /// If several elements are equally minimum, the first element is
2596 /// returned. If the iterator is empty, [`None`] is returned.
2601 /// let a = [-3_i32, 0, 1, 5, -10];
2602 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2605 #[stable(feature = "iter_min_by", since = "1.15.0")]
2606 fn min_by
<F
>(self, compare
: F
) -> Option
<Self::Item
>
2609 F
: FnMut(&Self::Item
, &Self::Item
) -> Ordering
,
2612 fn fold
<T
>(mut compare
: impl FnMut(&T
, &T
) -> Ordering
) -> impl FnMut(T
, T
) -> T
{
2613 move |x
, y
| cmp
::min_by(x
, y
, &mut compare
)
2616 self.fold_first(fold(compare
))
2619 /// Reverses an iterator's direction.
2621 /// Usually, iterators iterate from left to right. After using `rev()`,
2622 /// an iterator will instead iterate from right to left.
2624 /// This is only possible if the iterator has an end, so `rev()` only
2625 /// works on [`DoubleEndedIterator`]s.
2630 /// let a = [1, 2, 3];
2632 /// let mut iter = a.iter().rev();
2634 /// assert_eq!(iter.next(), Some(&3));
2635 /// assert_eq!(iter.next(), Some(&2));
2636 /// assert_eq!(iter.next(), Some(&1));
2638 /// assert_eq!(iter.next(), None);
2641 #[stable(feature = "rust1", since = "1.0.0")]
2642 fn rev(self) -> Rev
<Self>
2644 Self: Sized
+ DoubleEndedIterator
,
2649 /// Converts an iterator of pairs into a pair of containers.
2651 /// `unzip()` consumes an entire iterator of pairs, producing two
2652 /// collections: one from the left elements of the pairs, and one
2653 /// from the right elements.
2655 /// This function is, in some sense, the opposite of [`zip`].
2657 /// [`zip`]: Iterator::zip
2664 /// let a = [(1, 2), (3, 4)];
2666 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2668 /// assert_eq!(left, [1, 3]);
2669 /// assert_eq!(right, [2, 4]);
2671 #[stable(feature = "rust1", since = "1.0.0")]
2672 fn unzip
<A
, B
, FromA
, FromB
>(self) -> (FromA
, FromB
)
2674 FromA
: Default
+ Extend
<A
>,
2675 FromB
: Default
+ Extend
<B
>,
2676 Self: Sized
+ Iterator
<Item
= (A
, B
)>,
2678 fn extend
<'a
, A
, B
>(
2679 ts
: &'a
mut impl Extend
<A
>,
2680 us
: &'a
mut impl Extend
<B
>,
2681 ) -> impl FnMut((), (A
, B
)) + 'a
{
2688 let mut ts
: FromA
= Default
::default();
2689 let mut us
: FromB
= Default
::default();
2691 let (lower_bound
, _
) = self.size_hint();
2692 if lower_bound
> 0 {
2693 ts
.extend_reserve(lower_bound
);
2694 us
.extend_reserve(lower_bound
);
2697 self.fold((), extend(&mut ts
, &mut us
));
2702 /// Creates an iterator which copies all of its elements.
2704 /// This is useful when you have an iterator over `&T`, but you need an
2705 /// iterator over `T`.
2712 /// let a = [1, 2, 3];
2714 /// let v_copied: Vec<_> = a.iter().copied().collect();
2716 /// // copied is the same as .map(|&x| x)
2717 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2719 /// assert_eq!(v_copied, vec![1, 2, 3]);
2720 /// assert_eq!(v_map, vec![1, 2, 3]);
2722 #[stable(feature = "iter_copied", since = "1.36.0")]
2723 fn copied
<'a
, T
: 'a
>(self) -> Copied
<Self>
2725 Self: Sized
+ Iterator
<Item
= &'a T
>,
2731 /// Creates an iterator which [`clone`]s all of its elements.
2733 /// This is useful when you have an iterator over `&T`, but you need an
2734 /// iterator over `T`.
2736 /// [`clone`]: Clone::clone
2743 /// let a = [1, 2, 3];
2745 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2747 /// // cloned is the same as .map(|&x| x), for integers
2748 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2750 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2751 /// assert_eq!(v_map, vec![1, 2, 3]);
2753 #[stable(feature = "rust1", since = "1.0.0")]
2754 fn cloned
<'a
, T
: 'a
>(self) -> Cloned
<Self>
2756 Self: Sized
+ Iterator
<Item
= &'a T
>,
2762 /// Repeats an iterator endlessly.
2764 /// Instead of stopping at [`None`], the iterator will instead start again,
2765 /// from the beginning. After iterating again, it will start at the
2766 /// beginning again. And again. And again. Forever.
2773 /// let a = [1, 2, 3];
2775 /// let mut it = a.iter().cycle();
2777 /// assert_eq!(it.next(), Some(&1));
2778 /// assert_eq!(it.next(), Some(&2));
2779 /// assert_eq!(it.next(), Some(&3));
2780 /// assert_eq!(it.next(), Some(&1));
2781 /// assert_eq!(it.next(), Some(&2));
2782 /// assert_eq!(it.next(), Some(&3));
2783 /// assert_eq!(it.next(), Some(&1));
2785 #[stable(feature = "rust1", since = "1.0.0")]
2787 fn cycle(self) -> Cycle
<Self>
2789 Self: Sized
+ Clone
,
2794 /// Sums the elements of an iterator.
2796 /// Takes each element, adds them together, and returns the result.
2798 /// An empty iterator returns the zero value of the type.
2802 /// When calling `sum()` and a primitive integer type is being returned, this
2803 /// method will panic if the computation overflows and debug assertions are
2811 /// let a = [1, 2, 3];
2812 /// let sum: i32 = a.iter().sum();
2814 /// assert_eq!(sum, 6);
2816 #[stable(feature = "iter_arith", since = "1.11.0")]
2817 fn sum
<S
>(self) -> S
2825 /// Iterates over the entire iterator, multiplying all the elements
2827 /// An empty iterator returns the one value of the type.
2831 /// When calling `product()` and a primitive integer type is being returned,
2832 /// method will panic if the computation overflows and debug assertions are
2838 /// fn factorial(n: u32) -> u32 {
2839 /// (1..=n).product()
2841 /// assert_eq!(factorial(0), 1);
2842 /// assert_eq!(factorial(1), 1);
2843 /// assert_eq!(factorial(5), 120);
2845 #[stable(feature = "iter_arith", since = "1.11.0")]
2846 fn product
<P
>(self) -> P
2849 P
: Product
<Self::Item
>,
2851 Product
::product(self)
2854 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2860 /// use std::cmp::Ordering;
2862 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
2863 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
2864 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
2866 #[stable(feature = "iter_order", since = "1.5.0")]
2867 fn cmp
<I
>(self, other
: I
) -> Ordering
2869 I
: IntoIterator
<Item
= Self::Item
>,
2873 self.cmp_by(other
, |x
, y
| x
.cmp(&y
))
2876 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2877 /// of another with respect to the specified comparison function.
2884 /// #![feature(iter_order_by)]
2886 /// use std::cmp::Ordering;
2888 /// let xs = [1, 2, 3, 4];
2889 /// let ys = [1, 4, 9, 16];
2891 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
2892 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
2893 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
2895 #[unstable(feature = "iter_order_by", issue = "64295")]
2896 fn cmp_by
<I
, F
>(mut self, other
: I
, mut cmp
: F
) -> Ordering
2900 F
: FnMut(Self::Item
, I
::Item
) -> Ordering
,
2902 let mut other
= other
.into_iter();
2905 let x
= match self.next() {
2907 if other
.next().is_none() {
2908 return Ordering
::Equal
;
2910 return Ordering
::Less
;
2916 let y
= match other
.next() {
2917 None
=> return Ordering
::Greater
,
2922 Ordering
::Equal
=> (),
2923 non_eq
=> return non_eq
,
2928 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2934 /// use std::cmp::Ordering;
2936 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
2937 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
2938 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
2940 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
2942 #[stable(feature = "iter_order", since = "1.5.0")]
2943 fn partial_cmp
<I
>(self, other
: I
) -> Option
<Ordering
>
2946 Self::Item
: PartialOrd
<I
::Item
>,
2949 self.partial_cmp_by(other
, |x
, y
| x
.partial_cmp(&y
))
2952 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2953 /// of another with respect to the specified comparison function.
2960 /// #![feature(iter_order_by)]
2962 /// use std::cmp::Ordering;
2964 /// let xs = [1.0, 2.0, 3.0, 4.0];
2965 /// let ys = [1.0, 4.0, 9.0, 16.0];
2968 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
2969 /// Some(Ordering::Less)
2972 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
2973 /// Some(Ordering::Equal)
2976 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
2977 /// Some(Ordering::Greater)
2980 #[unstable(feature = "iter_order_by", issue = "64295")]
2981 fn partial_cmp_by
<I
, F
>(mut self, other
: I
, mut partial_cmp
: F
) -> Option
<Ordering
>
2985 F
: FnMut(Self::Item
, I
::Item
) -> Option
<Ordering
>,
2987 let mut other
= other
.into_iter();
2990 let x
= match self.next() {
2992 if other
.next().is_none() {
2993 return Some(Ordering
::Equal
);
2995 return Some(Ordering
::Less
);
3001 let y
= match other
.next() {
3002 None
=> return Some(Ordering
::Greater
),
3006 match partial_cmp(x
, y
) {
3007 Some(Ordering
::Equal
) => (),
3008 non_eq
=> return non_eq
,
3013 /// Determines if the elements of this [`Iterator`] are equal to those of
3019 /// assert_eq!([1].iter().eq([1].iter()), true);
3020 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3022 #[stable(feature = "iter_order", since = "1.5.0")]
3023 fn eq
<I
>(self, other
: I
) -> bool
3026 Self::Item
: PartialEq
<I
::Item
>,
3029 self.eq_by(other
, |x
, y
| x
== y
)
3032 /// Determines if the elements of this [`Iterator`] are equal to those of
3033 /// another with respect to the specified equality function.
3040 /// #![feature(iter_order_by)]
3042 /// let xs = [1, 2, 3, 4];
3043 /// let ys = [1, 4, 9, 16];
3045 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3047 #[unstable(feature = "iter_order_by", issue = "64295")]
3048 fn eq_by
<I
, F
>(mut self, other
: I
, mut eq
: F
) -> bool
3052 F
: FnMut(Self::Item
, I
::Item
) -> bool
,
3054 let mut other
= other
.into_iter();
3057 let x
= match self.next() {
3058 None
=> return other
.next().is_none(),
3062 let y
= match other
.next() {
3063 None
=> return false,
3073 /// Determines if the elements of this [`Iterator`] are unequal to those of
3079 /// assert_eq!([1].iter().ne([1].iter()), false);
3080 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3082 #[stable(feature = "iter_order", since = "1.5.0")]
3083 fn ne
<I
>(self, other
: I
) -> bool
3086 Self::Item
: PartialEq
<I
::Item
>,
3092 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3093 /// less than those of another.
3098 /// assert_eq!([1].iter().lt([1].iter()), false);
3099 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3100 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3101 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3103 #[stable(feature = "iter_order", since = "1.5.0")]
3104 fn lt
<I
>(self, other
: I
) -> bool
3107 Self::Item
: PartialOrd
<I
::Item
>,
3110 self.partial_cmp(other
) == Some(Ordering
::Less
)
3113 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3114 /// less or equal to those of another.
3119 /// assert_eq!([1].iter().le([1].iter()), true);
3120 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3121 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3122 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3124 #[stable(feature = "iter_order", since = "1.5.0")]
3125 fn le
<I
>(self, other
: I
) -> bool
3128 Self::Item
: PartialOrd
<I
::Item
>,
3131 matches
!(self.partial_cmp(other
), Some(Ordering
::Less
| Ordering
::Equal
))
3134 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3135 /// greater than those of another.
3140 /// assert_eq!([1].iter().gt([1].iter()), false);
3141 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3142 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3143 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3145 #[stable(feature = "iter_order", since = "1.5.0")]
3146 fn gt
<I
>(self, other
: I
) -> bool
3149 Self::Item
: PartialOrd
<I
::Item
>,
3152 self.partial_cmp(other
) == Some(Ordering
::Greater
)
3155 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3156 /// greater than or equal to those of another.
3161 /// assert_eq!([1].iter().ge([1].iter()), true);
3162 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3163 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3164 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3166 #[stable(feature = "iter_order", since = "1.5.0")]
3167 fn ge
<I
>(self, other
: I
) -> bool
3170 Self::Item
: PartialOrd
<I
::Item
>,
3173 matches
!(self.partial_cmp(other
), Some(Ordering
::Greater
| Ordering
::Equal
))
3176 /// Checks if the elements of this iterator are sorted.
3178 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3179 /// iterator yields exactly zero or one element, `true` is returned.
3181 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3182 /// implies that this function returns `false` if any two consecutive items are not
3188 /// #![feature(is_sorted)]
3190 /// assert!([1, 2, 2, 9].iter().is_sorted());
3191 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3192 /// assert!([0].iter().is_sorted());
3193 /// assert!(std::iter::empty::<i32>().is_sorted());
3194 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3197 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3198 fn is_sorted(self) -> bool
3201 Self::Item
: PartialOrd
,
3203 self.is_sorted_by(PartialOrd
::partial_cmp
)
3206 /// Checks if the elements of this iterator are sorted using the given comparator function.
3208 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3209 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3210 /// [`is_sorted`]; see its documentation for more information.
3215 /// #![feature(is_sorted)]
3217 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3218 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3219 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3220 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3221 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3224 /// [`is_sorted`]: Iterator::is_sorted
3225 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3226 fn is_sorted_by
<F
>(mut self, mut compare
: F
) -> bool
3229 F
: FnMut(&Self::Item
, &Self::Item
) -> Option
<Ordering
>,
3231 let mut last
= match self.next() {
3233 None
=> return true,
3236 while let Some(curr
) = self.next() {
3237 if let Some(Ordering
::Greater
) | None
= compare(&last
, &curr
) {
3246 /// Checks if the elements of this iterator are sorted using the given key extraction
3249 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3250 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3251 /// its documentation for more information.
3253 /// [`is_sorted`]: Iterator::is_sorted
3258 /// #![feature(is_sorted)]
3260 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3261 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3264 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3265 fn is_sorted_by_key
<F
, K
>(self, f
: F
) -> bool
3268 F
: FnMut(Self::Item
) -> K
,
3271 self.map(f
).is_sorted()
3274 /// See [TrustedRandomAccess]
3275 // The unusual name is to avoid name collisions in method resolution
3279 #[unstable(feature = "trusted_random_access", issue = "none")]
3280 unsafe fn __iterator_get_unchecked(&mut self, _idx
: usize) -> Self::Item
3282 Self: TrustedRandomAccess
,
3284 unreachable
!("Always specialized");
3288 #[stable(feature = "rust1", since = "1.0.0")]
3289 impl<I
: Iterator
+ ?Sized
> Iterator
for &mut I
{
3290 type Item
= I
::Item
;
3291 fn next(&mut self) -> Option
<I
::Item
> {
3294 fn size_hint(&self) -> (usize, Option
<usize>) {
3295 (**self).size_hint()
3297 fn advance_by(&mut self, n
: usize) -> Result
<(), usize> {
3298 (**self).advance_by(n
)
3300 fn nth(&mut self, n
: usize) -> Option
<Self::Item
> {