1 use crate::ops::{ControlFlow, Try};
3 /// An iterator able to yield elements from both ends.
5 /// Something that implements `DoubleEndedIterator` has one extra capability
6 /// over something that implements [`Iterator`]: the ability to also take
7 /// `Item`s from the back, as well as the front.
9 /// It is important to note that both back and forth work on the same range,
10 /// and do not cross: iteration is over when they meet in the middle.
12 /// In a similar fashion to the [`Iterator`] protocol, once a
13 /// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
14 /// again may or may not ever return [`Some`] again. [`next()`] and
15 /// [`next_back()`] are interchangeable for this purpose.
17 /// [`next_back()`]: DoubleEndedIterator::next_back
18 /// [`next()`]: Iterator::next
25 /// let numbers = vec![1, 2, 3, 4, 5, 6];
27 /// let mut iter = numbers.iter();
29 /// assert_eq!(Some(&1), iter.next());
30 /// assert_eq!(Some(&6), iter.next_back());
31 /// assert_eq!(Some(&5), iter.next_back());
32 /// assert_eq!(Some(&2), iter.next());
33 /// assert_eq!(Some(&3), iter.next());
34 /// assert_eq!(Some(&4), iter.next());
35 /// assert_eq!(None, iter.next());
36 /// assert_eq!(None, iter.next_back());
38 #[stable(feature = "rust1", since = "1.0.0")]
39 pub trait DoubleEndedIterator: Iterator {
40 /// Removes and returns an element from the end of the iterator.
42 /// Returns `None` when there are no more elements.
44 /// The [trait-level] docs contain more details.
46 /// [trait-level]: DoubleEndedIterator
53 /// let numbers = vec![1, 2, 3, 4, 5, 6];
55 /// let mut iter = numbers.iter();
57 /// assert_eq!(Some(&1), iter.next());
58 /// assert_eq!(Some(&6), iter.next_back());
59 /// assert_eq!(Some(&5), iter.next_back());
60 /// assert_eq!(Some(&2), iter.next());
61 /// assert_eq!(Some(&3), iter.next());
62 /// assert_eq!(Some(&4), iter.next());
63 /// assert_eq!(None, iter.next());
64 /// assert_eq!(None, iter.next_back());
69 /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
70 /// the ones yielded by [`Iterator`]'s methods:
73 /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
74 /// let uniq_by_fst_comp = || {
75 /// let mut seen = std::collections::HashSet::new();
76 /// vec.iter().copied().filter(move |x| seen.insert(x.0))
79 /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
80 /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
83 /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
84 /// vec![(1, 'a'), (2, 'a')]
87 /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
88 /// vec![(2, 'b'), (1, 'c')]
91 #[stable(feature = "rust1", since = "1.0.0")]
92 fn next_back(&mut self) -> Option<Self::Item>;
94 /// Returns the `n`th element from the end of the iterator.
96 /// This is essentially the reversed version of [`Iterator::nth()`].
97 /// Although like most indexing operations, the count starts from zero, so
98 /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
99 /// second, and so on.
101 /// Note that all elements between the end and the returned element will be
102 /// consumed, including the returned element. This also means that calling
103 /// `nth_back(0)` multiple times on the same iterator will return different
106 /// `nth_back()` will return [`None`] if `n` is greater than or equal to the
107 /// length of the iterator.
114 /// let a = [1, 2, 3];
115 /// assert_eq!(a.iter().nth_back(2), Some(&1));
118 /// Calling `nth_back()` multiple times doesn't rewind the iterator:
121 /// let a = [1, 2, 3];
123 /// let mut iter = a.iter();
125 /// assert_eq!(iter.nth_back(1), Some(&2));
126 /// assert_eq!(iter.nth_back(1), None);
129 /// Returning `None` if there are less than `n + 1` elements:
132 /// let a = [1, 2, 3];
133 /// assert_eq!(a.iter().nth_back(10), None);
136 #[stable(feature = "iter_nth_back", since = "1.37.0")]
137 fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
138 for x in self.rev() {
147 /// This is the reverse version of [`Iterator::try_fold()`]: it takes
148 /// elements starting from the back of the iterator.
155 /// let a = ["1", "2", "3"];
156 /// let sum = a.iter()
157 /// .map(|&s| s.parse::<i32>())
158 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
159 /// assert_eq!(sum, Ok(6));
162 /// Short-circuiting:
165 /// let a = ["1", "rust", "3"];
166 /// let mut it = a.iter();
169 /// .map(|&s| s.parse::<i32>())
170 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
171 /// assert!(sum.is_err());
173 /// // Because it short-circuited, the remaining elements are still
174 /// // available through the iterator.
175 /// assert_eq!(it.next_back(), Some(&"1"));
178 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
179 fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
182 F: FnMut(B, Self::Item) -> R,
185 let mut accum = init;
186 while let Some(x) = self.next_back() {
187 accum = f(accum, x)?;
192 /// An iterator method that reduces the iterator's elements to a single,
193 /// final value, starting from the back.
195 /// This is the reverse version of [`Iterator::fold()`]: it takes elements
196 /// starting from the back of the iterator.
198 /// `rfold()` takes two arguments: an initial value, and a closure with two
199 /// arguments: an 'accumulator', and an element. The closure returns the value that
200 /// the accumulator should have for the next iteration.
202 /// The initial value is the value the accumulator will have on the first
205 /// After applying this closure to every element of the iterator, `rfold()`
206 /// returns the accumulator.
208 /// This operation is sometimes called 'reduce' or 'inject'.
210 /// Folding is useful whenever you have a collection of something, and want
211 /// to produce a single value from it.
218 /// let a = [1, 2, 3];
220 /// // the sum of all of the elements of a
221 /// let sum = a.iter()
222 /// .rfold(0, |acc, &x| acc + x);
224 /// assert_eq!(sum, 6);
227 /// This example builds a string, starting with an initial value
228 /// and continuing with each element from the back until the front:
231 /// let numbers = [1, 2, 3, 4, 5];
233 /// let zero = "0".to_string();
235 /// let result = numbers.iter().rfold(zero, |acc, &x| {
236 /// format!("({} + {})", x, acc)
239 /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
242 #[stable(feature = "iter_rfold", since = "1.27.0")]
243 fn rfold<B, F>(mut self, init: B, mut f: F) -> B
246 F: FnMut(B, Self::Item) -> B,
248 let mut accum = init;
249 while let Some(x) = self.next_back() {
255 /// Searches for an element of an iterator from the back that satisfies a predicate.
257 /// `rfind()` takes a closure that returns `true` or `false`. It applies
258 /// this closure to each element of the iterator, starting at the end, and if any
259 /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
260 /// `false`, it returns [`None`].
262 /// `rfind()` is short-circuiting; in other words, it will stop processing
263 /// as soon as the closure returns `true`.
265 /// Because `rfind()` takes a reference, and many iterators iterate over
266 /// references, this leads to a possibly confusing situation where the
267 /// argument is a double reference. You can see this effect in the
268 /// examples below, with `&&x`.
270 /// [`Some(element)`]: Some
277 /// let a = [1, 2, 3];
279 /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
281 /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
284 /// Stopping at the first `true`:
287 /// let a = [1, 2, 3];
289 /// let mut iter = a.iter();
291 /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
293 /// // we can still use `iter`, as there are more elements.
294 /// assert_eq!(iter.next_back(), Some(&1));
297 #[stable(feature = "iter_rfind", since = "1.27.0")]
298 fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
301 P: FnMut(&Self::Item) -> bool,
305 mut predicate: impl FnMut(&T) -> bool,
306 ) -> impl FnMut((), T) -> ControlFlow<(), T> {
308 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
312 self.try_rfold((), check(predicate)).break_value()
316 #[stable(feature = "rust1", since = "1.0.0")]
317 impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
318 fn next_back(&mut self) -> Option<I::Item> {
321 fn nth_back(&mut self, n: usize) -> Option<I::Item> {