| use crate::ops::{ControlFlow, Try}; |
| |
| /// An iterator able to yield elements from both ends. |
| /// |
| /// Something that implements `DoubleEndedIterator` has one extra capability |
| /// over something that implements [`Iterator`]: the ability to also take |
| /// `Item`s from the back, as well as the front. |
| /// |
| /// It is important to note that both back and forth work on the same range, |
| /// and do not cross: iteration is over when they meet in the middle. |
| /// |
| /// In a similar fashion to the [`Iterator`] protocol, once a |
| /// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it |
| /// again may or may not ever return [`Some`] again. [`next()`] and |
| /// [`next_back()`] are interchangeable for this purpose. |
| /// |
| /// [`next_back()`]: DoubleEndedIterator::next_back |
| /// [`next()`]: Iterator::next |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let numbers = vec![1, 2, 3, 4, 5, 6]; |
| /// |
| /// let mut iter = numbers.iter(); |
| /// |
| /// assert_eq!(Some(&1), iter.next()); |
| /// assert_eq!(Some(&6), iter.next_back()); |
| /// assert_eq!(Some(&5), iter.next_back()); |
| /// assert_eq!(Some(&2), iter.next()); |
| /// assert_eq!(Some(&3), iter.next()); |
| /// assert_eq!(Some(&4), iter.next()); |
| /// assert_eq!(None, iter.next()); |
| /// assert_eq!(None, iter.next_back()); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")] |
| pub trait DoubleEndedIterator: Iterator { |
| /// Removes and returns an element from the end of the iterator. |
| /// |
| /// Returns `None` when there are no more elements. |
| /// |
| /// The [trait-level] docs contain more details. |
| /// |
| /// [trait-level]: DoubleEndedIterator |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let numbers = vec![1, 2, 3, 4, 5, 6]; |
| /// |
| /// let mut iter = numbers.iter(); |
| /// |
| /// assert_eq!(Some(&1), iter.next()); |
| /// assert_eq!(Some(&6), iter.next_back()); |
| /// assert_eq!(Some(&5), iter.next_back()); |
| /// assert_eq!(Some(&2), iter.next()); |
| /// assert_eq!(Some(&3), iter.next()); |
| /// assert_eq!(Some(&4), iter.next()); |
| /// assert_eq!(None, iter.next()); |
| /// assert_eq!(None, iter.next_back()); |
| /// ``` |
| /// |
| /// # Remarks |
| /// |
| /// The elements yielded by `DoubleEndedIterator`'s methods may differ from |
| /// the ones yielded by [`Iterator`]'s methods: |
| /// |
| /// ``` |
| /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')]; |
| /// let uniq_by_fst_comp = || { |
| /// let mut seen = std::collections::HashSet::new(); |
| /// vec.iter().copied().filter(move |x| seen.insert(x.0)) |
| /// }; |
| /// |
| /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a'))); |
| /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b'))); |
| /// |
| /// assert_eq!( |
| /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}), |
| /// vec![(1, 'a'), (2, 'a')] |
| /// ); |
| /// assert_eq!( |
| /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}), |
| /// vec![(2, 'b'), (1, 'c')] |
| /// ); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn next_back(&mut self) -> Option<Self::Item>; |
| |
| /// Advances the iterator from the back by `n` elements. |
| /// |
| /// `advance_back_by` is the reverse version of [`advance_by`]. This method will |
| /// eagerly skip `n` elements starting from the back by calling [`next_back`] up |
| /// to `n` times until [`None`] is encountered. |
| /// |
| /// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by |
| /// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of |
| /// elements the iterator is advanced by before running out of elements (i.e. the length |
| /// of the iterator). Note that `k` is always less than `n`. |
| /// |
| /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its |
| /// outer iterator until it finds an inner iterator that is not empty, which then often |
| /// allows it to return a more accurate `size_hint()` than in its initial state. |
| /// |
| /// [`advance_by`]: Iterator::advance_by |
| /// [`Flatten`]: crate::iter::Flatten |
| /// [`next_back`]: DoubleEndedIterator::next_back |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// #![feature(iter_advance_by)] |
| /// |
| /// let a = [3, 4, 5, 6]; |
| /// let mut iter = a.iter(); |
| /// |
| /// assert_eq!(iter.advance_back_by(2), Ok(())); |
| /// assert_eq!(iter.next_back(), Some(&4)); |
| /// assert_eq!(iter.advance_back_by(0), Ok(())); |
| /// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped |
| /// ``` |
| /// |
| /// [`Ok(())`]: Ok |
| /// [`Err(k)`]: Err |
| #[inline] |
| #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")] |
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { |
| for i in 0..n { |
| self.next_back().ok_or(i)?; |
| } |
| Ok(()) |
| } |
| |
| /// Returns the `n`th element from the end of the iterator. |
| /// |
| /// This is essentially the reversed version of [`Iterator::nth()`]. |
| /// Although like most indexing operations, the count starts from zero, so |
| /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the |
| /// second, and so on. |
| /// |
| /// Note that all elements between the end and the returned element will be |
| /// consumed, including the returned element. This also means that calling |
| /// `nth_back(0)` multiple times on the same iterator will return different |
| /// elements. |
| /// |
| /// `nth_back()` will return [`None`] if `n` is greater than or equal to the |
| /// length of the iterator. |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// assert_eq!(a.iter().nth_back(2), Some(&1)); |
| /// ``` |
| /// |
| /// Calling `nth_back()` multiple times doesn't rewind the iterator: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// |
| /// let mut iter = a.iter(); |
| /// |
| /// assert_eq!(iter.nth_back(1), Some(&2)); |
| /// assert_eq!(iter.nth_back(1), None); |
| /// ``` |
| /// |
| /// Returning `None` if there are less than `n + 1` elements: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// assert_eq!(a.iter().nth_back(10), None); |
| /// ``` |
| #[inline] |
| #[stable(feature = "iter_nth_back", since = "1.37.0")] |
| fn nth_back(&mut self, n: usize) -> Option<Self::Item> { |
| self.advance_back_by(n).ok()?; |
| self.next_back() |
| } |
| |
| /// This is the reverse version of [`Iterator::try_fold()`]: it takes |
| /// elements starting from the back of the iterator. |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let a = ["1", "2", "3"]; |
| /// let sum = a.iter() |
| /// .map(|&s| s.parse::<i32>()) |
| /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y))); |
| /// assert_eq!(sum, Ok(6)); |
| /// ``` |
| /// |
| /// Short-circuiting: |
| /// |
| /// ``` |
| /// let a = ["1", "rust", "3"]; |
| /// let mut it = a.iter(); |
| /// let sum = it |
| /// .by_ref() |
| /// .map(|&s| s.parse::<i32>()) |
| /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y))); |
| /// assert!(sum.is_err()); |
| /// |
| /// // Because it short-circuited, the remaining elements are still |
| /// // available through the iterator. |
| /// assert_eq!(it.next_back(), Some(&"1")); |
| /// ``` |
| #[inline] |
| #[stable(feature = "iterator_try_fold", since = "1.27.0")] |
| fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R |
| where |
| Self: Sized, |
| F: FnMut(B, Self::Item) -> R, |
| R: Try<Output = B>, |
| { |
| let mut accum = init; |
| while let Some(x) = self.next_back() { |
| accum = f(accum, x)?; |
| } |
| try { accum } |
| } |
| |
| /// An iterator method that reduces the iterator's elements to a single, |
| /// final value, starting from the back. |
| /// |
| /// This is the reverse version of [`Iterator::fold()`]: it takes elements |
| /// starting from the back of the iterator. |
| /// |
| /// `rfold()` takes two arguments: an initial value, and a closure with two |
| /// arguments: an 'accumulator', and an element. The closure returns the value that |
| /// the accumulator should have for the next iteration. |
| /// |
| /// The initial value is the value the accumulator will have on the first |
| /// call. |
| /// |
| /// After applying this closure to every element of the iterator, `rfold()` |
| /// returns the accumulator. |
| /// |
| /// This operation is sometimes called 'reduce' or 'inject'. |
| /// |
| /// Folding is useful whenever you have a collection of something, and want |
| /// to produce a single value from it. |
| /// |
| /// Note: `rfold()` combines elements in a *right-associative* fashion. For associative |
| /// operators like `+`, the order the elements are combined in is not important, but for non-associative |
| /// operators like `-` the order will affect the final result. |
| /// For a *left-associative* version of `rfold()`, see [`Iterator::fold()`]. |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// |
| /// // the sum of all of the elements of a |
| /// let sum = a.iter() |
| /// .rfold(0, |acc, &x| acc + x); |
| /// |
| /// assert_eq!(sum, 6); |
| /// ``` |
| /// |
| /// This example demonstrates the right-associative nature of `rfold()`: |
| /// it builds a string, starting with an initial value |
| /// and continuing with each element from the back until the front: |
| /// |
| /// ``` |
| /// let numbers = [1, 2, 3, 4, 5]; |
| /// |
| /// let zero = "0".to_string(); |
| /// |
| /// let result = numbers.iter().rfold(zero, |acc, &x| { |
| /// format!("({x} + {acc})") |
| /// }); |
| /// |
| /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))"); |
| /// ``` |
| #[doc(alias = "foldr")] |
| #[inline] |
| #[stable(feature = "iter_rfold", since = "1.27.0")] |
| fn rfold<B, F>(mut self, init: B, mut f: F) -> B |
| where |
| Self: Sized, |
| F: FnMut(B, Self::Item) -> B, |
| { |
| let mut accum = init; |
| while let Some(x) = self.next_back() { |
| accum = f(accum, x); |
| } |
| accum |
| } |
| |
| /// Searches for an element of an iterator from the back that satisfies a predicate. |
| /// |
| /// `rfind()` takes a closure that returns `true` or `false`. It applies |
| /// this closure to each element of the iterator, starting at the end, and if any |
| /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return |
| /// `false`, it returns [`None`]. |
| /// |
| /// `rfind()` is short-circuiting; in other words, it will stop processing |
| /// as soon as the closure returns `true`. |
| /// |
| /// Because `rfind()` takes a reference, and many iterators iterate over |
| /// references, this leads to a possibly confusing situation where the |
| /// argument is a double reference. You can see this effect in the |
| /// examples below, with `&&x`. |
| /// |
| /// [`Some(element)`]: Some |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// |
| /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2)); |
| /// |
| /// assert_eq!(a.iter().rfind(|&&x| x == 5), None); |
| /// ``` |
| /// |
| /// Stopping at the first `true`: |
| /// |
| /// ``` |
| /// let a = [1, 2, 3]; |
| /// |
| /// let mut iter = a.iter(); |
| /// |
| /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2)); |
| /// |
| /// // we can still use `iter`, as there are more elements. |
| /// assert_eq!(iter.next_back(), Some(&1)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "iter_rfind", since = "1.27.0")] |
| fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> |
| where |
| Self: Sized, |
| P: FnMut(&Self::Item) -> bool, |
| { |
| #[inline] |
| fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> { |
| move |(), x| { |
| if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE } |
| } |
| } |
| |
| self.try_rfold((), check(predicate)).break_value() |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I { |
| fn next_back(&mut self) -> Option<I::Item> { |
| (**self).next_back() |
| } |
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { |
| (**self).advance_back_by(n) |
| } |
| fn nth_back(&mut self, n: usize) -> Option<I::Item> { |
| (**self).nth_back(n) |
| } |
| } |