deque

The deque, short for double-ended queue, is a Python data structure that efficiently adds and removes elements from both ends. It is a component of the collections module and serves as an alternative to the list for scenarios where frequent insertions and deletions occur at both ends. Deques are notably advantageous when a queue is needed to enable fast appends and pops from both ends or when a stack is required to support the same operations efficiently.

Syntax

from collections import deque
d = deque([iterable[, maxlen]])

• iterable: An optional parameter representing an iterable object (like a list, tuple, or string) used to initialize the deque. If no iterable is provided, an empty deque is created.
• maxlen: An optional parameter that specifies the maximum length of the deque.

Example

The following example demonstrates the usage of deque:

from collections import deque

# Create a deque using a tuple of integers
a = deque((8, 7, 9, 6))
print(a)

# Create a deque using a list of integers
b = deque([45, 845, 65])
print(b)

# Create a deque using a range of integers from 5 to 9
c = deque(range(5, 10))
print(c)

# Create a deque using a string, which will be split into individual characters
d = deque("wxyz")
print(d)

# Create a dictionary with some key-value pairs
numbers = {"firstname": "John", "age":25}

# Create a deque containing the keys of the dictionary
e = deque(numbers.keys())
print(e)

# Create a deque containing the values of the dictionary
f = deque(numbers.values())
print(f)

# Create a deque containing the items (key-value pairs) of the dictionary
g = deque(numbers.items())
print(g)

The above code produces the following output:

deque([8, 7, 9, 6])
deque([45, 845, 65])
deque([5, 6, 7, 8, 9])
deque(['w', 'x', 'y', 'z'])
deque(['firstname', 'age'])
deque(['John', 25])
deque([('firstname', 'John'), ('age', 25)])

Equivalent Methods for Stacks and Queues

Deques can be used to implement both stacks and queues efficiently:

• Stacks: A stack operates on the Last In, First Out (LIFO) principle, where pushing (adding an item) is done with .append() to the right end, and popping (removing the most recent item) is performed using .pop() from the right end.
• Queues: A queue, based on the First In, First Out (FIFO) principle, utilizes .append() at the right end for adding (enqueueing) and .popleft() from the left end for removing (dequeuing).