Trait core::iter::DoubleEndedIterator
1.0.0 · source · [−]pub trait DoubleEndedIterator: Iterator {
fn next_back(&mut self) -> Option<Self::Item>;
fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { ... }
fn nth_back(&mut self, n: usize) -> Option<Self::Item> { ... }
fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
where
Self: Sized,
F: FnMut(B, Self::Item) -> R,
R: Try<Output = B>,
{ ... }
fn rfold<B, F>(self, init: B, f: F) -> B
where
Self: Sized,
F: FnMut(B, Self::Item) -> B,
{ ... }
fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{ ... }
}Expand description
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
Items 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.
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());Required Methods
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.
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')]
);Provided Methods
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.
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 skippedReturns the nth 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);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"));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)))))");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.
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));