Deque in Python

A deque stands for Double-Ended Queue. It is a data structure that allows adding and removing elements from both ends efficiently. Unlike regular queues, which are typically operated on using FIFO (First In, First Out) principles, a deque supports both FIFO and LIFO (Last In, First Out) operations.

Example:

from collections import deque 
    
# Declaring deque 
de = deque(['name','age','DOB'])  
    
print(de)

from collections import deque 

# Declaring deque 

de = deque(['name','age','DOB'])  

print(de)

deque(['name', 'age', 'DOB'])

Why Do We Need deque

• It supports O(1) time for adding/removing elements from both ends.
• It is more efficient than lists for front-end operations.
• It can function as both a queue (FIFO) and a stack (LIFO).
• Ideal for scheduling, sliding window problems and real-time data processing.
• It offers powerful built-in methods like appendleft(), popleft() and rotate().

Types of Restricted Deque Input

• Input Restricted Deque:  Input is limited at one end while deletion is permitted at both ends.
• Output Restricted Deque: output is limited at one end but insertion is permitted at both ends.

Operations on deque 

Here’s a table listing built-in operations of a deque in Python with descriptions and their corresponding time complexities:

Appending and Deleting Dequeue Items

• append(x): Adds x to the right end of the deque.
• appendleft(x): Adds x to the left end of the deque.
• extend(iterable): Adds all elements from the iterable to the right end.
• extendleft(iterable): Adds all elements from the iterable to the left end (in reverse order).
• remove(value): Removes the first occurrence of the specified value from the deque. If value is not found, it raises a ValueError.
• pop(): Removes and returns an element from the right end.
• popleft(): Removes and returns an element from the left end.
• clear(): Removes all elements from the deque.

from collections import deque

dq = deque([10, 20, 30])

# Add elements to the right
dq.append(40)  

# Add elements to the left
dq.appendleft(5)  

# extend(iterable)
dq.extend([50, 60, 70]) 
print("After extend([50, 60, 70]):", dq)

# extendleft(iterable)
dq.extendleft([0, 5])  
print("After extendleft([0, 5]):", dq)

# remove method
dq.remove(20)
print("After remove(20):", dq)

# Remove elements from the right
dq.pop()

# Remove elements from the left
dq.popleft()  

print("After pop and popleft:", dq)

# clear() - Removes all elements from the deque
dq.clear()  # deque: []
print("After clear():", dq)

from collections import deque

​

dq = deque([10, 20, 30])

​

# Add elements to the right

dq.append(40)  

​

# Add elements to the left

dq.appendleft(5)  

​

# extend(iterable)

dq.extend([50, 60, 70]) 

print("After extend([50, 60, 70]):", dq)

​

# extendleft(iterable)

dq.extendleft([0, 5])  

print("After extendleft([0, 5]):", dq)

​

# remove method

dq.remove(20)

print("After remove(20):", dq)

​

# Remove elements from the right

dq.pop()

​

# Remove elements from the left

dq.popleft()  

​

print("After pop and popleft:", dq)

​

# clear() - Removes all elements from the deque

dq.clear()  # deque: []

print("After clear():", dq)

Output:

After extend([50, 60, 70]): deque([5, 10, 20, 30, 40, 50, 60, 70])
After extendleft([0, 5]): deque([5, 0, 5, 10, 20, 30, 40, 50, 60, 70])
After remove(20): deque([5, 0, 5, 10, 30, 40, 50, 60, 70])
After pop and popleft: deque([0, 5, 10, 30, 40, 50, 60])
After clear(): deque([])

Accessing Item and length of deque

• Indexing: Access elements by position using positive or negative indices.
• len(): Returns the number of elements in the deque.

import collections

dq = collections.deque([1, 2, 3, 3, 4, 2, 4])

# Accessing elements by index
print(dq[0])  
print(dq[-1]) 

# Finding the length of the deque
print(len(dq))  

import collections

​

dq = collections.deque([1, 2, 3, 3, 4, 2, 4])

​

# Accessing elements by index

print(dq[0])  

print(dq[-1]) 

​

# Finding the length of the deque

print(len(dq))

1
4
7

Count, Rotation and Reversal of a deque

• count(value): This method counts the number of occurrences of a specific element in the deque.
• rotate(n): This method rotates the deque by n steps. Positive n rotates to the right and negative n rotates to the left.
• reverse(): This method reverses the order of elements in the deque.

from collections import deque

# Create a deque
dq = deque([10, 20, 30, 40, 50, 20, 30, 20])

# 1. Counting occurrences of a value
print(dq.count(20))  # Occurrences of 20
print(dq.count(30))  # Occurrences of 30

# 2. Rotating the deque
dq.rotate(2)  # Rotate the deque 2 steps to the right
print(dq)

dq.rotate(-3)  # Rotate the deque 3 steps to the left
print(dq)

# 3. Reversing the deque
dq.reverse()  # Reverse the deque
print(dq)

from collections import deque

​

# Create a deque

dq = deque([10, 20, 30, 40, 50, 20, 30, 20])

​

# 1. Counting occurrences of a value

print(dq.count(20))  # Occurrences of 20

print(dq.count(30))  # Occurrences of 30

​

# 2. Rotating the deque

dq.rotate(2)  # Rotate the deque 2 steps to the right

print(dq)

​

dq.rotate(-3)  # Rotate the deque 3 steps to the left

print(dq)

​

# 3. Reversing the deque

dq.reverse()  # Reverse the deque

print(dq)

3
2
deque([30, 20, 10, 20, 30, 40, 50, 20])
deque([20, 30, 40, 50, 20, 30, 20, 10])
deque([10, 20, 30, 20, 50, 40, 30, 20])

Deque in Python

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