Struct VecDeque

pub struct VecDeque<T, A = Global>
where
    A: Allocator,
{ /* private fields */ }

A double-ended queue implemented with a growable ring buffer.

The “default” usage of this type as a queue is to use push_back to add to the queue, and pop_front to remove from the queue. extend and append push onto the back in this manner, and iterating over VecDeque goes front to back.

A VecDeque with a known list of items can be initialized from an array:

use std::collections::VecDeque;

let deq = VecDeque::from([-1, 0, 1]);

Since VecDeque is a ring buffer, its elements are not necessarily contiguous in memory. If you want to access the elements as a single slice, such as for efficient sorting, you can use make_contiguous. It rotates the VecDeque so that its elements do not wrap, and returns a mutable slice to the now-contiguous element sequence.

Implementations

impl<T> VecDeque<T>

pub const fn new() -> VecDeque<T>

Creates an empty deque.

Examples

use std::collections::VecDeque;

let deque: VecDeque<u32> = VecDeque::new();

pub fn with_capacity(capacity: usize) -> VecDeque<T>

Creates an empty deque with space for at least capacity elements.

Examples

use std::collections::VecDeque;

let deque: VecDeque<u32> = VecDeque::with_capacity(10);

pub fn try_with_capacity( capacity: usize, ) -> Result<VecDeque<T>, TryReserveError>

🔬This is a nightly-only experimental API. (try_with_capacity #91913)

Creates an empty deque with space for at least capacity elements.

Errors

Returns an error if the capacity exceeds isize::MAX bytes, or if the allocator reports allocation failure.

Examples

use std::collections::VecDeque;

let deque: VecDeque<u32> = VecDeque::try_with_capacity(10)?;

impl<T, A> VecDeque<T, A>

where A: Allocator,

pub const fn new_in(alloc: A) -> VecDeque<T, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty deque.

Examples

use std::collections::VecDeque;

let deque: VecDeque<u32> = VecDeque::new();

pub fn with_capacity_in(capacity: usize, alloc: A) -> VecDeque<T, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty deque with space for at least capacity elements.

Examples

use std::collections::VecDeque;

let deque: VecDeque<u32> = VecDeque::with_capacity(10);

pub fn get(&self, index: usize) -> Option<&T>

Provides a reference to the element at the given index.

Element at index 0 is the front of the queue.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(3);
buf.push_back(4);
buf.push_back(5);
buf.push_back(6);
assert_eq!(buf.get(1), Some(&4));

pub fn get_mut(&mut self, index: usize) -> Option<&mut T>

Provides a mutable reference to the element at the given index.

Element at index 0 is the front of the queue.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(3);
buf.push_back(4);
buf.push_back(5);
buf.push_back(6);
assert_eq!(buf[1], 4);
if let Some(elem) = buf.get_mut(1) {
    *elem = 7;
}
assert_eq!(buf[1], 7);

pub fn swap(&mut self, i: usize, j: usize)

Swaps elements at indices i and j.

i and j may be equal.

Element at index 0 is the front of the queue.

Panics

Panics if either index is out of bounds.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(3);
buf.push_back(4);
buf.push_back(5);
assert_eq!(buf, [3, 4, 5]);
buf.swap(0, 2);
assert_eq!(buf, [5, 4, 3]);

pub fn capacity(&self) -> usize

Returns the number of elements the deque can hold without reallocating.

Examples

use std::collections::VecDeque;

let buf: VecDeque<i32> = VecDeque::with_capacity(10);
assert!(buf.capacity() >= 10);

pub fn reserve_exact(&mut self, additional: usize)

Reserves the minimum capacity for at least additional more elements to be inserted in the given deque. Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it requests. Therefore capacity can not be relied upon to be precisely minimal. Prefer reserve if future insertions are expected.

Panics

Panics if the new capacity overflows usize.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<i32> = [1].into();
buf.reserve_exact(10);
assert!(buf.capacity() >= 11);

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional more elements to be inserted in the given deque. The collection may reserve more space to speculatively avoid frequent reallocations.

Panics

Panics if the new capacity overflows usize.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<i32> = [1].into();
buf.reserve(10);
assert!(buf.capacity() >= 11);

pub fn try_reserve_exact( &mut self, additional: usize, ) -> Result<(), TryReserveError>

Tries to reserve the minimum capacity for at least additional more elements to be inserted in the given deque. After calling try_reserve_exact, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it requests. Therefore, capacity can not be relied upon to be precisely minimal. Prefer try_reserve if future insertions are expected.

Errors

If the capacity overflows usize, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::TryReserveError;
use std::collections::VecDeque;

fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
    let mut output = VecDeque::new();

    // Pre-reserve the memory, exiting if we can't
    output.try_reserve_exact(data.len())?;

    // Now we know this can't OOM(Out-Of-Memory) in the middle of our complex work
    output.extend(data.iter().map(|&val| {
        val * 2 + 5 // very complicated
    }));

    Ok(output)
}

pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least additional more elements to be inserted in the given deque. The collection may reserve more space to speculatively avoid frequent reallocations. After calling try_reserve, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if capacity is already sufficient. This method preserves the contents even if an error occurs.

Errors

If the capacity overflows usize, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::TryReserveError;
use std::collections::VecDeque;

fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
    let mut output = VecDeque::new();

    // Pre-reserve the memory, exiting if we can't
    output.try_reserve(data.len())?;

    // Now we know this can't OOM in the middle of our complex work
    output.extend(data.iter().map(|&val| {
        val * 2 + 5 // very complicated
    }));

    Ok(output)
}

pub fn shrink_to_fit(&mut self)

Shrinks the capacity of the deque as much as possible.

It will drop down as close as possible to the length but the allocator may still inform the deque that there is space for a few more elements.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::with_capacity(15);
buf.extend(0..4);
assert_eq!(buf.capacity(), 15);
buf.shrink_to_fit();
assert!(buf.capacity() >= 4);

pub fn shrink_to(&mut self, min_capacity: usize)

Shrinks the capacity of the deque with a lower bound.

The capacity will remain at least as large as both the length and the supplied value.

If the current capacity is less than the lower limit, this is a no-op.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::with_capacity(15);
buf.extend(0..4);
assert_eq!(buf.capacity(), 15);
buf.shrink_to(6);
assert!(buf.capacity() >= 6);
buf.shrink_to(0);
assert!(buf.capacity() >= 4);

pub fn truncate(&mut self, len: usize)

Shortens the deque, keeping the first len elements and dropping the rest.

If len is greater or equal to the deque’s current length, this has no effect.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(5);
buf.push_back(10);
buf.push_back(15);
assert_eq!(buf, [5, 10, 15]);
buf.truncate(1);
assert_eq!(buf, [5]);

pub fn allocator(&self) -> &A

🔬This is a nightly-only experimental API. (allocator_api #32838)

Returns a reference to the underlying allocator.

pub fn iter(&self) -> Iter<'_, T>

Returns a front-to-back iterator.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(5);
buf.push_back(3);
buf.push_back(4);
let b: &[_] = &[&5, &3, &4];
let c: Vec<&i32> = buf.iter().collect();
assert_eq!(&c[..], b);

pub fn iter_mut(&mut self) -> IterMut<'_, T>

Returns a front-to-back iterator that returns mutable references.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(5);
buf.push_back(3);
buf.push_back(4);
for num in buf.iter_mut() {
    *num = *num - 2;
}
let b: &[_] = &[&mut 3, &mut 1, &mut 2];
assert_eq!(&buf.iter_mut().collect::<Vec<&mut i32>>()[..], b);

pub fn as_slices(&self) -> (&[T], &[T])

Returns a pair of slices which contain, in order, the contents of the deque.

If make_contiguous was previously called, all elements of the deque will be in the first slice and the second slice will be empty.

Examples

use std::collections::VecDeque;

let mut deque = VecDeque::new();

deque.push_back(0);
deque.push_back(1);
deque.push_back(2);

assert_eq!(deque.as_slices(), (&[0, 1, 2][..], &[][..]));

deque.push_front(10);
deque.push_front(9);

assert_eq!(deque.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));

pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T])

Returns a pair of slices which contain, in order, the contents of the deque.

If make_contiguous was previously called, all elements of the deque will be in the first slice and the second slice will be empty.

Examples

use std::collections::VecDeque;

let mut deque = VecDeque::new();

deque.push_back(0);
deque.push_back(1);

deque.push_front(10);
deque.push_front(9);

deque.as_mut_slices().0[0] = 42;
deque.as_mut_slices().1[0] = 24;
assert_eq!(deque.as_slices(), (&[42, 10][..], &[24, 1][..]));

pub fn len(&self) -> usize

Returns the number of elements in the deque.

Examples

use std::collections::VecDeque;

let mut deque = VecDeque::new();
assert_eq!(deque.len(), 0);
deque.push_back(1);
assert_eq!(deque.len(), 1);

pub fn is_empty(&self) -> bool

Returns true if the deque is empty.

Examples

use std::collections::VecDeque;

let mut deque = VecDeque::new();
assert!(deque.is_empty());
deque.push_front(1);
assert!(!deque.is_empty());

pub fn range<R>(&self, range: R) -> Iter<'_, T>

where R: RangeBounds<usize>,

Creates an iterator that covers the specified range in the deque.

Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

Examples

use std::collections::VecDeque;

let deque: VecDeque<_> = [1, 2, 3].into();
let range = deque.range(2..).copied().collect::<VecDeque<_>>();
assert_eq!(range, [3]);

// A full range covers all contents
let all = deque.range(..);
assert_eq!(all.len(), 3);

pub fn range_mut<R>(&mut self, range: R) -> IterMut<'_, T>

where R: RangeBounds<usize>,

Creates an iterator that covers the specified mutable range in the deque.

Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

Examples

use std::collections::VecDeque;

let mut deque: VecDeque<_> = [1, 2, 3].into();
for v in deque.range_mut(2..) {
  *v *= 2;
}
assert_eq!(deque, [1, 2, 6]);

// A full range covers all contents
for v in deque.range_mut(..) {
  *v *= 2;
}
assert_eq!(deque, [2, 4, 12]);

pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>

where R: RangeBounds<usize>,

Removes the specified range from the deque in bulk, returning all removed elements as an iterator. If the iterator is dropped before being fully consumed, it drops the remaining removed elements.

The returned iterator keeps a mutable borrow on the queue to optimize its implementation.

Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

Leaking

If the returned iterator goes out of scope without being dropped (due to mem::forget, for example), the deque may have lost and leaked elements arbitrarily, including elements outside the range.

Examples

use std::collections::VecDeque;

let mut deque: VecDeque<_> = [1, 2, 3].into();
let drained = deque.drain(2..).collect::<VecDeque<_>>();
assert_eq!(drained, [3]);
assert_eq!(deque, [1, 2]);

// A full range clears all contents, like `clear()` does
deque.drain(..);
assert!(deque.is_empty());

pub fn clear(&mut self)

Clears the deque, removing all values.

Examples

use std::collections::VecDeque;

let mut deque = VecDeque::new();
deque.push_back(1);
deque.clear();
assert!(deque.is_empty());

pub fn contains(&self, x: &T) -> bool

where T: PartialEq,

Returns true if the deque contains an element equal to the given value.

This operation is O(n).

Note that if you have a sorted VecDeque, binary_search may be faster.

Examples

use std::collections::VecDeque;

let mut deque: VecDeque<u32> = VecDeque::new();

deque.push_back(0);
deque.push_back(1);

assert_eq!(deque.contains(&1), true);
assert_eq!(deque.contains(&10), false);

pub fn front(&self) -> Option<&T>

Provides a reference to the front element, or None if the deque is empty.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
assert_eq!(d.front(), None);

d.push_back(1);
d.push_back(2);
assert_eq!(d.front(), Some(&1));

pub fn front_mut(&mut self) -> Option<&mut T>

Provides a mutable reference to the front element, or None if the deque is empty.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
assert_eq!(d.front_mut(), None);

d.push_back(1);
d.push_back(2);
match d.front_mut() {
    Some(x) => *x = 9,
    None => (),
}
assert_eq!(d.front(), Some(&9));

pub fn back(&self) -> Option<&T>

Provides a reference to the back element, or None if the deque is empty.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
assert_eq!(d.back(), None);

d.push_back(1);
d.push_back(2);
assert_eq!(d.back(), Some(&2));

pub fn back_mut(&mut self) -> Option<&mut T>

Provides a mutable reference to the back element, or None if the deque is empty.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
assert_eq!(d.back(), None);

d.push_back(1);
d.push_back(2);
match d.back_mut() {
    Some(x) => *x = 9,
    None => (),
}
assert_eq!(d.back(), Some(&9));

pub fn pop_front(&mut self) -> Option<T>

Removes the first element and returns it, or None if the deque is empty.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
d.push_back(1);
d.push_back(2);

assert_eq!(d.pop_front(), Some(1));
assert_eq!(d.pop_front(), Some(2));
assert_eq!(d.pop_front(), None);

pub fn pop_back(&mut self) -> Option<T>

Removes the last element from the deque and returns it, or None if it is empty.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
assert_eq!(buf.pop_back(), None);
buf.push_back(1);
buf.push_back(3);
assert_eq!(buf.pop_back(), Some(3));

pub fn push_front(&mut self, value: T)

Prepends an element to the deque.

Examples

use std::collections::VecDeque;

let mut d = VecDeque::new();
d.push_front(1);
d.push_front(2);
assert_eq!(d.front(), Some(&2));

pub fn push_back(&mut self, value: T)

Appends an element to the back of the deque.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(1);
buf.push_back(3);
assert_eq!(3, *buf.back().unwrap());

pub fn swap_remove_front(&mut self, index: usize) -> Option<T>

Removes an element from anywhere in the deque and returns it, replacing it with the first element.

This does not preserve ordering, but is O(1).

Returns None if index is out of bounds.

Element at index 0 is the front of the queue.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
assert_eq!(buf.swap_remove_front(0), None);
buf.push_back(1);
buf.push_back(2);
buf.push_back(3);
assert_eq!(buf, [1, 2, 3]);

assert_eq!(buf.swap_remove_front(2), Some(3));
assert_eq!(buf, [2, 1]);

pub fn swap_remove_back(&mut self, index: usize) -> Option<T>

Removes an element from anywhere in the deque and returns it, replacing it with the last element.

This does not preserve ordering, but is O(1).

Returns None if index is out of bounds.

Element at index 0 is the front of the queue.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
assert_eq!(buf.swap_remove_back(0), None);
buf.push_back(1);
buf.push_back(2);
buf.push_back(3);
assert_eq!(buf, [1, 2, 3]);

assert_eq!(buf.swap_remove_back(0), Some(1));
assert_eq!(buf, [3, 2]);

pub fn insert(&mut self, index: usize, value: T)

Inserts an element at index within the deque, shifting all elements with indices greater than or equal to index towards the back.

Element at index 0 is the front of the queue.

Panics

Panics if index is greater than deque’s length

Examples

use std::collections::VecDeque;

let mut vec_deque = VecDeque::new();
vec_deque.push_back('a');
vec_deque.push_back('b');
vec_deque.push_back('c');
assert_eq!(vec_deque, &['a', 'b', 'c']);

vec_deque.insert(1, 'd');
assert_eq!(vec_deque, &['a', 'd', 'b', 'c']);

pub fn remove(&mut self, index: usize) -> Option<T>

Removes and returns the element at index from the deque. Whichever end is closer to the removal point will be moved to make room, and all the affected elements will be moved to new positions. Returns None if index is out of bounds.

Element at index 0 is the front of the queue.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(1);
buf.push_back(2);
buf.push_back(3);
assert_eq!(buf, [1, 2, 3]);

assert_eq!(buf.remove(1), Some(2));
assert_eq!(buf, [1, 3]);

pub fn split_off(&mut self, at: usize) -> VecDeque<T, A>

where A: Clone,

Splits the deque into two at the given index.

Returns a newly allocated VecDeque. self contains elements [0, at), and the returned deque contains elements [at, len).

Note that the capacity of self does not change.

Element at index 0 is the front of the queue.

Panics

Panics if at > len.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<_> = [1, 2, 3].into();
let buf2 = buf.split_off(1);
assert_eq!(buf, [1]);
assert_eq!(buf2, [2, 3]);

pub fn append(&mut self, other: &mut VecDeque<T, A>)

Moves all the elements of other into self, leaving other empty.

Panics

Panics if the new number of elements in self overflows a usize.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<_> = [1, 2].into();
let mut buf2: VecDeque<_> = [3, 4].into();
buf.append(&mut buf2);
assert_eq!(buf, [1, 2, 3, 4]);
assert_eq!(buf2, []);

pub fn retain<F>(&mut self, f: F)

where F: FnMut(&T) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all elements e for which f(&e) returns false. This method operates in place, visiting each element exactly once in the original order, and preserves the order of the retained elements.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.extend(1..5);
buf.retain(|&x| x % 2 == 0);
assert_eq!(buf, [2, 4]);

Because the elements are visited exactly once in the original order, external state may be used to decide which elements to keep.

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.extend(1..6);

let keep = [false, true, true, false, true];
let mut iter = keep.iter();
buf.retain(|_| *iter.next().unwrap());
assert_eq!(buf, [2, 3, 5]);

pub fn retain_mut<F>(&mut self, f: F)

where F: FnMut(&mut T) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all elements e for which f(&e) returns false. This method operates in place, visiting each element exactly once in the original order, and preserves the order of the retained elements.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.extend(1..5);
buf.retain_mut(|x| if *x % 2 == 0 {
    *x += 1;
    true
} else {
    false
});
assert_eq!(buf, [3, 5]);

pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut() -> T)

Modifies the deque in-place so that len() is equal to new_len, either by removing excess elements from the back or by appending elements generated by calling generator to the back.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(5);
buf.push_back(10);
buf.push_back(15);
assert_eq!(buf, [5, 10, 15]);

buf.resize_with(5, Default::default);
assert_eq!(buf, [5, 10, 15, 0, 0]);

buf.resize_with(2, || unreachable!());
assert_eq!(buf, [5, 10]);

let mut state = 100;
buf.resize_with(5, || { state += 1; state });
assert_eq!(buf, [5, 10, 101, 102, 103]);

pub fn make_contiguous(&mut self) -> &mut [T]

Rearranges the internal storage of this deque so it is one contiguous slice, which is then returned.

This method does not allocate and does not change the order of the inserted elements. As it returns a mutable slice, this can be used to sort a deque.

Once the internal storage is contiguous, the as_slices and as_mut_slices methods will return the entire contents of the deque in a single slice.

Examples

Sorting the content of a deque.

use std::collections::VecDeque;

let mut buf = VecDeque::with_capacity(15);

buf.push_back(2);
buf.push_back(1);
buf.push_front(3);

// sorting the deque
buf.make_contiguous().sort();
assert_eq!(buf.as_slices(), (&[1, 2, 3] as &[_], &[] as &[_]));

// sorting it in reverse order
buf.make_contiguous().sort_by(|a, b| b.cmp(a));
assert_eq!(buf.as_slices(), (&[3, 2, 1] as &[_], &[] as &[_]));

Getting immutable access to the contiguous slice.

use std::collections::VecDeque;

let mut buf = VecDeque::new();

buf.push_back(2);
buf.push_back(1);
buf.push_front(3);

buf.make_contiguous();
if let (slice, &[]) = buf.as_slices() {
    // we can now be sure that `slice` contains all elements of the deque,
    // while still having immutable access to `buf`.
    assert_eq!(buf.len(), slice.len());
    assert_eq!(slice, &[3, 2, 1] as &[_]);
}

pub fn rotate_left(&mut self, n: usize)

Rotates the double-ended queue n places to the left.

Equivalently,

• Rotates item n into the first position.
• Pops the first n items and pushes them to the end.
• Rotates len() - n places to the right.

Panics

If n is greater than len(). Note that n == len() does not panic and is a no-op rotation.

Complexity

Takes *O*(min(n, len() - n)) time and no extra space.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<_> = (0..10).collect();

buf.rotate_left(3);
assert_eq!(buf, [3, 4, 5, 6, 7, 8, 9, 0, 1, 2]);

for i in 1..10 {
    assert_eq!(i * 3 % 10, buf[0]);
    buf.rotate_left(3);
}
assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

pub fn rotate_right(&mut self, n: usize)

Rotates the double-ended queue n places to the right.

Equivalently,

• Rotates the first item into position n.
• Pops the last n items and pushes them to the front.
• Rotates len() - n places to the left.

Panics

If n is greater than len(). Note that n == len() does not panic and is a no-op rotation.

Complexity

Takes *O*(min(n, len() - n)) time and no extra space.

Examples

use std::collections::VecDeque;

let mut buf: VecDeque<_> = (0..10).collect();

buf.rotate_right(3);
assert_eq!(buf, [7, 8, 9, 0, 1, 2, 3, 4, 5, 6]);

for i in 1..10 {
    assert_eq!(0, buf[i * 3 % 10]);
    buf.rotate_right(3);
}
assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

pub fn binary_search(&self, x: &T) -> Result<usize, usize>

where T: Ord,

Binary searches this VecDeque for a given element. If the VecDeque is not sorted, the returned result is unspecified and meaningless.

If the value is found then Result::Ok is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then Result::Err is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also binary_search_by, binary_search_by_key, and partition_point.

Examples

Looks up a series of four elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in [1, 4].

use std::collections::VecDeque;

let deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();

assert_eq!(deque.binary_search(&13),  Ok(9));
assert_eq!(deque.binary_search(&4),   Err(7));
assert_eq!(deque.binary_search(&100), Err(13));
let r = deque.binary_search(&1);
assert!(matches!(r, Ok(1..=4)));

If you want to insert an item to a sorted deque, while maintaining sort order, consider using partition_point:

use std::collections::VecDeque;

let mut deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
let num = 42;
let idx = deque.partition_point(|&x| x <= num);
// If `num` is unique, `s.partition_point(|&x| x < num)` (with `<`) is equivalent to
// `s.binary_search(&num).unwrap_or_else(|x| x)`, but using `<=` may allow `insert`
// to shift less elements.
deque.insert(idx, num);
assert_eq!(deque, &[0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);

pub fn binary_search_by<'a, F>(&'a self, f: F) -> Result<usize, usize>

where F: FnMut(&'a T) -> Ordering,

Binary searches this VecDeque with a comparator function.

The comparator function should return an order code that indicates whether its argument is Less, Equal or Greater the desired target. If the VecDeque is not sorted or if the comparator function does not implement an order consistent with the sort order of the underlying VecDeque, the returned result is unspecified and meaningless.

If the value is found then Result::Ok is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then Result::Err is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also binary_search, binary_search_by_key, and partition_point.

Examples

Looks up a series of four elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in [1, 4].

use std::collections::VecDeque;

let deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();

assert_eq!(deque.binary_search_by(|x| x.cmp(&13)),  Ok(9));
assert_eq!(deque.binary_search_by(|x| x.cmp(&4)),   Err(7));
assert_eq!(deque.binary_search_by(|x| x.cmp(&100)), Err(13));
let r = deque.binary_search_by(|x| x.cmp(&1));
assert!(matches!(r, Ok(1..=4)));

pub fn binary_search_by_key<'a, B, F>( &'a self, b: &B, f: F, ) -> Result<usize, usize>

where F: FnMut(&'a T) -> B, B: Ord,

Binary searches this VecDeque with a key extraction function.

Assumes that the deque is sorted by the key, for instance with make_contiguous().sort_by_key() using the same key extraction function. If the deque is not sorted by the key, the returned result is unspecified and meaningless.

If the value is found then Result::Ok is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then Result::Err is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also binary_search, binary_search_by, and partition_point.

Examples

Looks up a series of four elements in a slice of pairs sorted by their second elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in [1, 4].

use std::collections::VecDeque;

let deque: VecDeque<_> = [(0, 0), (2, 1), (4, 1), (5, 1),
         (3, 1), (1, 2), (2, 3), (4, 5), (5, 8), (3, 13),
         (1, 21), (2, 34), (4, 55)].into();

assert_eq!(deque.binary_search_by_key(&13, |&(a, b)| b),  Ok(9));
assert_eq!(deque.binary_search_by_key(&4, |&(a, b)| b),   Err(7));
assert_eq!(deque.binary_search_by_key(&100, |&(a, b)| b), Err(13));
let r = deque.binary_search_by_key(&1, |&(a, b)| b);
assert!(matches!(r, Ok(1..=4)));

pub fn partition_point<P>(&self, pred: P) -> usize

where P: FnMut(&T) -> bool,

Returns the index of the partition point according to the given predicate (the index of the first element of the second partition).

The deque is assumed to be partitioned according to the given predicate. This means that all elements for which the predicate returns true are at the start of the deque and all elements for which the predicate returns false are at the end. For example, [7, 15, 3, 5, 4, 12, 6] is partitioned under the predicate x % 2 != 0 (all odd numbers are at the start, all even at the end).

If the deque is not partitioned, the returned result is unspecified and meaningless, as this method performs a kind of binary search.

See also binary_search, binary_search_by, and binary_search_by_key.

Examples

use std::collections::VecDeque;

let deque: VecDeque<_> = [1, 2, 3, 3, 5, 6, 7].into();
let i = deque.partition_point(|&x| x < 5);

assert_eq!(i, 4);
assert!(deque.iter().take(i).all(|&x| x < 5));
assert!(deque.iter().skip(i).all(|&x| !(x < 5)));

If you want to insert an item to a sorted deque, while maintaining sort order:

use std::collections::VecDeque;

let mut deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
let num = 42;
let idx = deque.partition_point(|&x| x < num);
deque.insert(idx, num);
assert_eq!(deque, &[0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);

impl<T, A> VecDeque<T, A>

where T: Clone, A: Allocator,

pub fn resize(&mut self, new_len: usize, value: T)

Modifies the deque in-place so that len() is equal to new_len, either by removing excess elements from the back or by appending clones of value to the back.

Examples

use std::collections::VecDeque;

let mut buf = VecDeque::new();
buf.push_back(5);
buf.push_back(10);
buf.push_back(15);
assert_eq!(buf, [5, 10, 15]);

buf.resize(2, 0);
assert_eq!(buf, [5, 10]);

buf.resize(5, 20);
assert_eq!(buf, [5, 10, 20, 20, 20]);

Trait Implementations

impl<A: Allocator> BufRead for VecDeque<u8, A>

BufRead is implemented for VecDeque<u8> by reading bytes from the front of the VecDeque.

fn fill_buf(&mut self) -> Result<&[u8]>

Returns the contents of the “front” slice as returned by as_slices. If the contained byte slices of the VecDeque are discontiguous, multiple calls to fill_buf will be needed to read the entire content.

fn consume(&mut self, amt: usize)

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read.

fn has_data_left(&mut self) -> Result<bool>

🔬This is a nightly-only experimental API. (buf_read_has_data_left #86423)

Checks if the underlying Read has any data left to be read.

fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes into buf until the delimiter byte or EOF is reached.

fn skip_until(&mut self, byte: u8) -> Result<usize>

Skips all bytes until the delimiter byte or EOF is reached.

fn read_line(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until a newline (the 0xA byte) is reached, and append them to the provided String buffer.

fn split(self, byte: u8) -> Split<Self>

where Self: Sized,

Returns an iterator over the contents of this reader split on the byte byte.

fn lines(self) -> Lines<Self>

where Self: Sized,

Returns an iterator over the lines of this reader.

impl<T, A> Clone for VecDeque<T, A>

where T: Clone, A: Allocator + Clone,

fn clone_from(&mut self, source: &VecDeque<T, A>)

Overwrites the contents of self with a clone of the contents of source.

This method is preferred over simply assigning source.clone() to self, as it avoids reallocation if possible.

fn clone(&self) -> VecDeque<T, A>

Returns a copy of the value.

impl<T, A> Debug for VecDeque<T, A>

where T: Debug, A: Allocator,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for VecDeque<T>

fn default() -> VecDeque<T>

Creates an empty deque.

impl<T, A> Drop for VecDeque<T, A>

where A: Allocator,

fn drop(&mut self)

Executes the destructor for this type.

impl<'a, T, A> Extend<&'a T> for VecDeque<T, A>

where T: 'a + Copy, A: Allocator,

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = &'a T>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, _: &'a T)

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<T, A> Extend<T> for VecDeque<T, A>

where A: Allocator,

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = T>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, elem: T)

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<T, const N: usize> From<[T; N]> for VecDeque<T>

fn from(arr: [T; N]) -> VecDeque<T>

Converts a [T; N] into a VecDeque<T>.

use std::collections::VecDeque;

let deq1 = VecDeque::from([1, 2, 3, 4]);
let deq2: VecDeque<_> = [1, 2, 3, 4].into();
assert_eq!(deq1, deq2);

impl<T, A> From<Vec<T, A>> for VecDeque<T, A>

where A: Allocator,

fn from(other: Vec<T, A>) -> VecDeque<T, A>

Turn a Vec<T> into a VecDeque<T>.

This conversion is guaranteed to run in O(1) time and to not re-allocate the Vec’s buffer or allocate any additional memory.

impl<T, A> From<VecDeque<T, A>> for Vec<T, A>

where A: Allocator,

fn from(other: VecDeque<T, A>) -> Vec<T, A>

Turn a VecDeque<T> into a Vec<T>.

This never needs to re-allocate, but does need to do O(n) data movement if the circular buffer doesn’t happen to be at the beginning of the allocation.

Examples

use std::collections::VecDeque;

// This one is *O*(1).
let deque: VecDeque<_> = (1..5).collect();
let ptr = deque.as_slices().0.as_ptr();
let vec = Vec::from(deque);
assert_eq!(vec, [1, 2, 3, 4]);
assert_eq!(vec.as_ptr(), ptr);

// This one needs data rearranging.
let mut deque: VecDeque<_> = (1..5).collect();
deque.push_front(9);
deque.push_front(8);
let ptr = deque.as_slices().1.as_ptr();
let vec = Vec::from(deque);
assert_eq!(vec, [8, 9, 1, 2, 3, 4]);
assert_eq!(vec.as_ptr(), ptr);

impl<T> FromIterator<T> for VecDeque<T>

fn from_iter<I>(iter: I) -> VecDeque<T>

where I: IntoIterator<Item = T>,

Creates a value from an iterator.

impl<T, A> Hash for VecDeque<T, A>

where T: Hash, A: Allocator,

fn hash<H>(&self, state: &mut H)

where H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T, A> Index<usize> for VecDeque<T, A>

where A: Allocator,

type Output = T

The returned type after indexing.

fn index(&self, index: usize) -> &T

Performs the indexing (container[index]) operation.

impl<T, A> IndexMut<usize> for VecDeque<T, A>

where A: Allocator,

fn index_mut(&mut self, index: usize) -> &mut T

Performs the mutable indexing (container[index]) operation.

impl<'a, T, A> IntoIterator for &'a VecDeque<T, A>

where A: Allocator,

type Item = &'a T

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<'a, T, A> IntoIterator for &'a mut VecDeque<T, A>

where A: Allocator,

type Item = &'a mut T

The type of the elements being iterated over.

type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> IterMut<'a, T>

Creates an iterator from a value.

impl<T, A> IntoIterator for VecDeque<T, A>

where A: Allocator,

fn into_iter(self) -> IntoIter<T, A>

Consumes the deque into a front-to-back iterator yielding elements by value.

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T, A>

Which kind of iterator are we turning this into?

impl<T, A> Ord for VecDeque<T, A>

where T: Ord, A: Allocator,

fn cmp(&self, other: &VecDeque<T, A>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<T, U, A> PartialEq<&[U]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &&[U]) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A, const N: usize> PartialEq<&[U; N]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &&[U; N]) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A> PartialEq<&mut [U]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &&mut [U]) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A, const N: usize> PartialEq<&mut [U; N]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &&mut [U; N]) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A, const N: usize> PartialEq<[U; N]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &[U; N]) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A> PartialEq<Vec<U, A>> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

fn eq(&self, other: &Vec<U, A>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, A> PartialEq for VecDeque<T, A>

where T: PartialEq, A: Allocator,

fn eq(&self, other: &VecDeque<T, A>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, A> PartialOrd for VecDeque<T, A>

where T: PartialOrd, A: Allocator,

fn partial_cmp(&self, other: &VecDeque<T, A>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<A: Allocator> Read for VecDeque<u8, A>

Read is implemented for VecDeque<u8> by consuming bytes from the front of the VecDeque.

fn read(&mut self, buf: &mut [u8]) -> Result<usize>

Fill buf with the contents of the “front” slice as returned by as_slices. If the contained byte slices of the VecDeque are discontiguous, multiple calls to read will be needed to read the entire content.

fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Pull some bytes from this source into the specified buffer.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until EOF in this source, appending them to buf.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like read, except that it reads into a slice of buffers.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if this Reader has an efficient read_vectored implementation.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Reads the exact number of bytes required to fill buf.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R: Read>(self, next: R) -> Chain<Self, R>

where Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl<A: Allocator> Write for VecDeque<u8, A>

Write is implemented for VecDeque<u8> by appending to the VecDeque, growing it as needed.

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes a buffer into this writer, returning how many bytes were written.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if this Writer has an efficient write_vectored implementation.

fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer.

fn flush(&mut self) -> Result<()>

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (write_all_vectored #70436)

Attempts to write multiple buffers into this writer.

fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.

impl<T, A> Eq for VecDeque<T, A>

where T: Eq, A: Allocator,