# class Deque(T)

- Deque(T)
- Reference
- Object

##
Overview

A Deque ("double-ended queue") is a collection of objects of type T that behaves much like an Array.

Deque has a subset of Array's API. It performs better than an `Array` when there are frequent insertions or deletions of items near the beginning or the end.

The most typical use case of a Deque is a queue: use `#push` to add items to the end of the queue and `#shift` to get and remove the item at the beginning of the queue.

This Deque is implemented with a dynamic array used as a circular buffer.

##
Included Modules

- Indexable(T)

##
Defined in:

##
Constructors

- .new(size : Int, value : T)

  Creates a new `Deque` of the given size filled with the same value in each position.

- .new(array : Array(T))

  Creates a new `Deque` that copies its items from an Array.

- .new(initial_capacity : Int)

  Creates a new empty `Deque` backed by a buffer that is initially `initial_capacity` big.

- .new

  Creates a new empty Deque

- .new(size : Int, &block : Int32 -> T)

  Creates a new `Deque` of the given size and invokes the block once for each index of the deque, assigning the block's value in that index.

##
Instance Method Summary

- #+(other : Deque(U)) forall U

  Concatenation.

- #<<(value : T)

  Alias for `#push`.

- #==(other : Deque)

  Returns `true` if it is passed a `Deque` and `equals?` returns `true` for both deques, the caller and the argument.

- #[]=(index : Int, value : T)

  Sets the given value at the given index.

- #clear

  Removes all elements from `self`.

- #clone

  Returns a new `Deque` that has this deque's elements cloned.

- #concat(other : Enumerable(T))

  Appends the elements of other to `self`, and returns `self`.

- #delete(obj)

  Removes all items from `self` that are equal to obj.

- #delete_at(index : Int)

  Deletes the item that is present at the index.

- #delete_if(&)
- #dup

  Returns a new `Deque` that has exactly this deque's elements.

- #each(&) : Nil

  Yields each item in this deque, from first to last.

- #insert(index : Int, value : T)

  Insert a new item before the item at index.

- #inspect(io : IO) : Nil

  Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

- #pop(&)

  Removes and returns the last item, if not empty, otherwise executes the given block and returns its value.

- #pop

  Removes and returns the last item.

- #pop(n : Int)

  Removes the last n (at most) items in the deque.

- #pop?

  Removes and returns the last item, if not empty, otherwise `nil`.

- #pretty_print(pp)
- #push(value : T)

  Adds an item to the end of the deque.

- #rotate!(n : Int = 1)

  Rotates this deque in place so that the element at n becomes first.

- #shift(n : Int)

  Removes the first n (at most) items in the deque.

- #shift

  Removes and returns the first item.

- #shift(&)

  Removes and returns the first item, if not empty, otherwise executes the given block and returns its value.

- #shift?

  Removes and returns the first item, if not empty, otherwise `nil`.

- #size

  Returns the number of elements in the deque.

- #swap(i, j)

  Swaps the items at the indices i and j.

- #to_s(io : IO) : Nil

  Appends a short String representation of this object which includes its class name and its object address.

- #unsafe_fetch(index : Int)

  Returns the element at the given index, without doing any bounds check.

- #unshift(value : T)

  Adds an item to the beginning of the deque.

### Instance methods inherited from module `Indexable(T)`

[](index : Int) [], []?(index : Int) []?, bsearch(&block : T -> Bool) bsearch, bsearch_index(&block : T, Int32 -> Bool) bsearch_index, dig(index : Int, *subindexes) dig, dig?(index : Int, *subindexes) dig?, each(*, start : Int, count : Int, &)
each(*, within range : Range, &)
each(&)
each each, each_index(*, start : Int, count : Int, &)
each_index(&) : Nil
each_index each_index, empty? empty?, equals?(other : Indexable, &)
equals?(other, &) equals?, fetch(index, default)
fetch(index : Int, &) fetch, first
first(&) first, first? first?, hash(hasher) hash, index(object, offset : Int = 0)
index(offset : Int = 0, &) index, join(separator = "") join, last
last(&) last, last? last?, reverse_each(&) : Nil
reverse_each reverse_each, rindex(offset = size - 1, &)
rindex(value, offset = size - 1) rindex, sample(random = Random::DEFAULT) sample, size size, to_a to_a, unsafe_fetch(index : Int) unsafe_fetch, values_at(*indexes : Int) values_at

### Instance methods inherited from module `Enumerable(T)`

all?(&)
all?(pattern)
all? all?, any?(&)
any?(pattern)
any? any?, chunks(&block : T -> U) forall U chunks, compact_map(&) compact_map, count(&)
count(item) count, cycle(n, &)
cycle(&) cycle, each(&block : T -> _) each, each_cons(count : Int, reuse = false, &) each_cons, each_cons_pair(& : T, T -> _) : Nil each_cons_pair, each_slice(count : Int, reuse = false, &) each_slice, each_with_index(offset = 0, &) each_with_index, each_with_object(obj, &) each_with_object, find(if_none = nil, &) find, first(count : Int)
first first, first? first?, flat_map(&block : T -> Array(U) | Iterator(U) | U) forall U flat_map, grep(pattern) grep, group_by(&block : T -> U) forall U group_by, in_groups_of(size : Int, filled_up_with : U = nil) forall U
in_groups_of(size : Int, filled_up_with : U = nil, reuse = false, &) forall U in_groups_of, includes?(obj) includes?, index(&)
index(obj) index, index_by(&block : T -> U) forall U index_by, join(separator = "", &)
join(separator, io, &)
join(separator, io)
join(separator = "") join, map(&block : T -> U) forall U map, map_with_index(offset = 0, &block : T, Int32 -> U) forall U map_with_index, max max, max? max?, max_by(&block : T -> U) forall U max_by, max_by?(&block : T -> U) forall U max_by?, max_of(&block : T -> U) forall U max_of, max_of?(&block : T -> U) forall U max_of?, min min, min? min?, min_by(&block : T -> U) forall U min_by, min_by?(&block : T -> U) forall U min_by?, min_of(&block : T -> U) forall U min_of, min_of?(&block : T -> U) forall U min_of?, minmax minmax, minmax? minmax?, minmax_by(&block : T -> U) forall U minmax_by, minmax_by?(&block : T -> U) forall U minmax_by?, minmax_of(&block : T -> U) forall U minmax_of, minmax_of?(&block : T -> U) forall U minmax_of?, none?
none?(pattern)
none?(&) none?, one?(&)
one?(pattern)
one? one?, partition(&) partition, product(&)
product(initial : Number, &)
product
product(initial : Number) product, reduce(memo, &)
reduce(&) reduce, reduce?(&) reduce?, reject(&block : T -> )
reject(type : U.class) forall U
reject(pattern) reject, select(pattern)
select(type : U.class) forall U
select(&block : T -> ) select, size size, skip(count : Int) skip, skip_while(&) skip_while, sum(initial)
sum
sum(initial, &)
sum(&) sum, take_while(&) take_while, tally : Hash(T, Int32) tally, to_a to_a, to_h
to_h(&block : T -> Tuple(K, V)) forall K, V to_h, to_set to_set, zip(*others : Indexable | Iterable | Iterator, &)
zip(*others : Indexable | Iterable | Iterator) zip, zip?(*others : Indexable | Iterable | Iterator, &)
zip?(*others : Indexable | Iterable | Iterator) zip?

### Instance methods inherited from module `Iterable(T)`

chunk(reuse = false, &block : T -> U) forall U chunk, chunk_while(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B chunk_while, cycle(n)
cycle cycle, each each, each_cons(count : Int, reuse = false) each_cons, each_slice(count : Int, reuse = false) each_slice, each_with_index(offset = 0) each_with_index, each_with_object(obj) each_with_object, slice_after(reuse : Bool | Array(T) = false, &block : T -> B) forall B
slice_after(pattern, reuse : Bool | Array(T) = false) slice_after, slice_before(reuse : Bool | Array(T) = false, &block : T -> B) forall B
slice_before(pattern, reuse : Bool | Array(T) = false) slice_before, slice_when(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B slice_when

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference)
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new

### Instance methods inherited from class `Object`

!=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, __crystal_pseudo_! : Bool __crystal_pseudo_!, __crystal_pseudo_as(type : Class) __crystal_pseudo_as, __crystal_pseudo_as?(type : Class) __crystal_pseudo_as?, __crystal_pseudo_is_a?(type : Class) : Bool __crystal_pseudo_is_a?, __crystal_pseudo_nil? : Bool __crystal_pseudo_nil?, __crystal_pseudo_responds_to?(name : Symbol) : Bool __crystal_pseudo_responds_to?, class class, dup dup, hash(hasher)
hash hash, inspect : String
inspect(io : IO) : Nil inspect, itself itself, not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, tap(&) tap, to_json(io : IO)
to_json to_json, to_pretty_json(indent : String = " ")
to_pretty_json(io : IO, indent : String = " ") to_pretty_json, to_s : String
to_s(io : IO) : Nil to_s, to_yaml(io : IO)
to_yaml to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io, root : String)
from_json(string_or_io) from_json, from_yaml(string_or_io : String | IO) from_yaml

##
Constructor Detail

def self.new(size : Int, value : T) #

Creates a new `Deque` of the given size filled with the same value in each position.

    Deque.new(3, 'a') # => Deque{'a', 'a', 'a'}

def self.new(array : Array(T)) #

Creates a new `Deque` that copies its items from an Array.

    Deque.new([1, 2, 3]) # => Deque{1, 2, 3}

def self.new(initial_capacity : Int) #

Creates a new empty `Deque` backed by a buffer that is initially `initial_capacity` big.

The `initial_capacity` is useful to avoid unnecessary reallocations of the internal buffer in case of growth. If you have an estimate of the maximum number of elements a deque will hold, you should initialize it with that capacity for improved execution performance.

    deq = Deque(Int32).new(5)
    deq.size # => 0

def self.new #

Creates a new empty Deque

def self.new(size : Int, &block : Int32 -> T) #

Creates a new `Deque` of the given size and invokes the block once for each index of the deque, assigning the block's value in that index.

    Deque.new(3) { |i| (i + 1) ** 2 } # => Deque{1, 4, 9}

##
Instance Method Detail

def +(other : Deque(U)) forall U #

Concatenation. Returns a new `Deque` built by concatenating two deques together to create a third. The type of the new deque is the union of the types of both the other deques.

def <<(value : T) #

Alias for `#push`.

def ==(other : Deque) #

Returns `true` if it is passed a `Deque` and `equals?` returns `true` for both deques, the caller and the argument.

    deq = Deque{2, 3}
    deq.unshift 1
    deq == Deque{1, 2, 3} # => true
    deq == Deque{2, 3}    # => false

def []=(index : Int, value : T) #

Sets the given value at the given index.

Raises `IndexError` if the deque had no previous value at the given index.

def clear #

Removes all elements from `self`.

def clone #

Returns a new `Deque` that has this deque's elements cloned. That is, it returns a deep copy of this deque.

Use `#dup` if you want a shallow copy.

def concat(other : Enumerable(T)) #

Appends the elements of other to `self`, and returns `self`.

def delete(obj) #

Removes all items from `self` that are equal to obj.

    a = Deque{"a", "b", "b", "b", "c"}
    a.delete("b") # => true
    a             # => Deque{"a", "c"}

def delete_at(index : Int) #

Deletes the item that is present at the index. Items to the right of this one will have their indices decremented. Raises `IndexError` if trying to delete an element outside the deque's range.

    a = Deque{1, 2, 3}
    a.delete_at(1) # => 2
    a              # => Deque{1, 3}

def delete_if(&) #
def dup #

Returns a new `Deque` that has exactly this deque's elements. That is, it returns a shallow copy of this deque.

def each(&) : Nil #

Yields each item in this deque, from first to last.

Do not modify the deque while using this variant of `#each`!

def insert(index : Int, value : T) #

Insert a new item before the item at index. Items to the right of this one will have their indices incremented.

    a = Deque{0, 1, 2}
    a.insert(1, 7) # => Deque{0, 7, 1, 2}

def inspect(io : IO) : Nil #
Description copied from class Reference

Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

    class Person
      def initialize(@name : String, @age : Int32)
      end
    end

    Person.new("John", 32).inspect # => #<Person:0x10fd31f20 @name="John", @age=32>

def pop(&) #

Removes and returns the last item, if not empty, otherwise executes the given block and returns its value.

def pop #

Removes and returns the last item. Raises `IndexError` if empty.

    a = Deque{1, 2, 3}
    a.pop # => 3
    a     # => Deque{1, 2}

def pop(n : Int) #

Removes the last n (at most) items in the deque.

def pop? #

Removes and returns the last item, if not empty, otherwise `nil`.

def pretty_print(pp) #
def push(value : T) #

Adds an item to the end of the deque.

    a = Deque{1, 2}
    a.push 3 # => Deque{1, 2, 3}

def rotate!(n : Int = 1) #

Rotates this deque in place so that the element at n becomes first.

- For positive n, equivalent to `n.times { push(shift) }`.
- For negative n, equivalent to `(-n).times { unshift(pop) }`.

def shift(n : Int) #

Removes the first n (at most) items in the deque.

def shift #

Removes and returns the first item. Raises `IndexError` if empty.

    a = Deque{1, 2, 3}
    a.shift # => 1
    a       # => Deque{2, 3}

def shift(&) #

Removes and returns the first item, if not empty, otherwise executes the given block and returns its value.

def shift? #

Removes and returns the first item, if not empty, otherwise `nil`.

def size #

Returns the number of elements in the deque.

    Deque{:foo, :bar}.size # => 2

def swap(i, j) #

Swaps the items at the indices i and j.

def to_s(io : IO) : Nil #
Description copied from class Reference

Appends a short String representation of this object which includes its class name and its object address.

    class Person
      def initialize(@name : String, @age : Int32)
      end
    end

    Person.new("John", 32).to_s # => #<Person:0x10a199f20>

def unsafe_fetch(index : Int) #
Description copied from module Indexable(T)

Returns the element at the given index, without doing any bounds check.

`Indexable` makes sure to invoke this method with index in `0...size`, so converting negative indices to positive ones is not needed here.

Clients never invoke this method directly. Instead, they access elements with `#[](index)` and `#[]?(index)`.

This method should only be directly invoked if you are absolutely sure the index is in bounds, to avoid a bounds check for a small boost of performance.

def unshift(value : T) #

Adds an item to the beginning of the deque.

    a = Deque{1, 2}
    a.unshift 0 # => Deque{0, 1, 2}