Deque in Python

A deque stands for Double-Ended Queue. It is a special type of data structure that allows you to add and remove elements from both ends efficiently. This makes it useful in applications like task scheduling, sliding window problems and real-time data processing.

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.

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)

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))  

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)

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])

Operations on deque 

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

Deque in Python

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