1 use crate::iter::LoopState;
4 /// An iterator able to yield elements from both ends.
6 /// Something that implements `DoubleEndedIterator` has one extra capability
7 /// over something that implements [`Iterator`]: the ability to also take
8 /// `Item`s from the back, as well as the front.
10 /// It is important to note that both back and forth work on the same range,
11 /// and do not cross: iteration is over when they meet in the middle.
13 /// In a similar fashion to the [`Iterator`] protocol, once a
14 /// `DoubleEndedIterator` returns `None` from a `next_back()`, calling it again
15 /// may or may not ever return `Some` again. `next()` and `next_back()` are
16 /// interchangeable for this purpose.
18 /// [`Iterator`]: trait.Iterator.html
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]: trait.DoubleEndedIterator.html
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());
66 #[stable(feature = "rust1", since = "1.0.0")]
67 fn next_back(&mut self) -> Option<Self::Item>;
69 /// Returns the `n`th element from the end of the iterator.
71 /// This is essentially the reversed version of [`nth`]. Although like most indexing
72 /// operations, the count starts from zero, so `nth_back(0)` returns the first value from
73 /// the end, `nth_back(1)` the second, and so on.
75 /// Note that all elements between the end and the returned element will be
76 /// consumed, including the returned element. This also means that calling
77 /// `nth_back(0)` multiple times on the same iterator will return different
80 /// `nth_back()` will return [`None`] if `n` is greater than or equal to the length of the
83 /// [`None`]: ../../std/option/enum.Option.html#variant.None
84 /// [`nth`]: ../../std/iter/trait.Iterator.html#method.nth
91 /// let a = [1, 2, 3];
92 /// assert_eq!(a.iter().nth_back(2), Some(&1));
95 /// Calling `nth_back()` multiple times doesn't rewind the iterator:
98 /// let a = [1, 2, 3];
100 /// let mut iter = a.iter();
102 /// assert_eq!(iter.nth_back(1), Some(&2));
103 /// assert_eq!(iter.nth_back(1), None);
106 /// Returning `None` if there are less than `n + 1` elements:
109 /// let a = [1, 2, 3];
110 /// assert_eq!(a.iter().nth_back(10), None);
113 #[stable(feature = "iter_nth_back", since = "1.37.0")]
114 fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
115 for x in self.rev() {
124 /// This is the reverse version of [`try_fold()`]: it takes elements
125 /// starting from the back of the iterator.
127 /// [`try_fold()`]: trait.Iterator.html#method.try_fold
134 /// let a = ["1", "2", "3"];
135 /// let sum = a.iter()
136 /// .map(|&s| s.parse::<i32>())
137 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
138 /// assert_eq!(sum, Ok(6));
141 /// Short-circuiting:
144 /// let a = ["1", "rust", "3"];
145 /// let mut it = a.iter();
148 /// .map(|&s| s.parse::<i32>())
149 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
150 /// assert!(sum.is_err());
152 /// // Because it short-circuited, the remaining elements are still
153 /// // available through the iterator.
154 /// assert_eq!(it.next_back(), Some(&"1"));
157 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
158 fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
161 F: FnMut(B, Self::Item) -> R,
164 let mut accum = init;
165 while let Some(x) = self.next_back() {
166 accum = f(accum, x)?;
171 /// An iterator method that reduces the iterator's elements to a single,
172 /// final value, starting from the back.
174 /// This is the reverse version of [`fold()`]: it takes elements starting from
175 /// the back of the iterator.
177 /// `rfold()` takes two arguments: an initial value, and a closure with two
178 /// arguments: an 'accumulator', and an element. The closure returns the value that
179 /// the accumulator should have for the next iteration.
181 /// The initial value is the value the accumulator will have on the first
184 /// After applying this closure to every element of the iterator, `rfold()`
185 /// returns the accumulator.
187 /// This operation is sometimes called 'reduce' or 'inject'.
189 /// Folding is useful whenever you have a collection of something, and want
190 /// to produce a single value from it.
192 /// [`fold()`]: trait.Iterator.html#method.fold
199 /// let a = [1, 2, 3];
201 /// // the sum of all of the elements of a
202 /// let sum = a.iter()
203 /// .rfold(0, |acc, &x| acc + x);
205 /// assert_eq!(sum, 6);
208 /// This example builds a string, starting with an initial value
209 /// and continuing with each element from the back until the front:
212 /// let numbers = [1, 2, 3, 4, 5];
214 /// let zero = "0".to_string();
216 /// let result = numbers.iter().rfold(zero, |acc, &x| {
217 /// format!("({} + {})", x, acc)
220 /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
223 #[stable(feature = "iter_rfold", since = "1.27.0")]
224 fn rfold<B, F>(mut self, init: B, mut f: F) -> B
227 F: FnMut(B, Self::Item) -> B,
229 let mut accum = init;
230 while let Some(x) = self.next_back() {
236 /// Searches for an element of an iterator from the back that satisfies a predicate.
238 /// `rfind()` takes a closure that returns `true` or `false`. It applies
239 /// this closure to each element of the iterator, starting at the end, and if any
240 /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
241 /// `false`, it returns [`None`].
243 /// `rfind()` is short-circuiting; in other words, it will stop processing
244 /// as soon as the closure returns `true`.
246 /// Because `rfind()` takes a reference, and many iterators iterate over
247 /// references, this leads to a possibly confusing situation where the
248 /// argument is a double reference. You can see this effect in the
249 /// examples below, with `&&x`.
251 /// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some
252 /// [`None`]: ../../std/option/enum.Option.html#variant.None
259 /// let a = [1, 2, 3];
261 /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
263 /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
266 /// Stopping at the first `true`:
269 /// let a = [1, 2, 3];
271 /// let mut iter = a.iter();
273 /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
275 /// // we can still use `iter`, as there are more elements.
276 /// assert_eq!(iter.next_back(), Some(&1));
279 #[stable(feature = "iter_rfind", since = "1.27.0")]
280 fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
283 P: FnMut(&Self::Item) -> bool,
287 mut predicate: impl FnMut(&T) -> bool,
288 ) -> impl FnMut((), T) -> LoopState<(), T> {
290 if predicate(&x) { LoopState::Break(x) } else { LoopState::Continue(()) }
294 self.try_rfold((), check(predicate)).break_value()
298 #[stable(feature = "rust1", since = "1.0.0")]
299 impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
300 fn next_back(&mut self) -> Option<I::Item> {
303 fn nth_back(&mut self, n: usize) -> Option<I::Item> {