Queue is a linear data structure that stores items in a First In First Out (FIFO) manner. The item that is added first will be removed first. Queues are widely used in real-life scenarios, like ticket booking, or CPU task scheduling, where first-come, first-served rule is followed.
There are various ways to implement a queue in Python by following ways:
1. Using list - Inefficient
Lists can be used as queues, but removing elements from front requires shifting all other elements, making it O(n).
Python
q = []
q.append('a')
q.append('b')
q.append('c')
print("Initial queue:", q)
print("Elements dequeued from queue:")
print(q.pop(0))
print(q.pop(0))
print(q.pop(0))
print("Queue after removing elements:", q)
OutputInitial queue: ['a', 'b', 'c']
Elements dequeued from queue:
a
b
c
Queue after removing elements: []
Explanation: We added elements using append() and removed from the front using pop(0). After removing all elements, queue is empty.
2. Using collections.deque - Efficient
deque (double-ended queue) is preferred over a list for queues because both append() and popleft() run in O(1) time.
Python
from collections import deque
q = deque()
q.append('a')
q.append('b')
q.append('c')
print("Initial queue:", q)
print("Elements dequeued from the queue:")
print(q.popleft())
print(q.popleft())
print(q.popleft())
print("Queue after removing elements:", q)
OutputInitial queue: deque(['a', 'b', 'c'])
Elements dequeued from the queue:
a
b
c
Queue after removing elements: deque([])
Explanation: popleft() efficiently removes the first element without shifting, making deque ideal for queues.
3. Using queue.Queue - Efficient and Thread Safe
Python’s queue module provides a thread-safe FIFO queue. You can specify a maxsize. Key Methods are:
- put(item) / put_nowait(item) – Add an element.
- get() / get_nowait() – Remove an element.
- empty() – Check if the queue is empty.
- full() – Check if the queue is full.
- qsize() – Get current size of the queue.
Python
from queue import Queue
q = Queue(maxsize=3)
print("Initial size:", q.qsize())
q.put('a')
q.put('b')
q.put('c')
print("Is full:", q.full())
print("Elements dequeued from the queue:")
print(q.get())
print(q.get())
print(q.get())
print("Is empty:", q.empty())
q.put(1)
print("Is empty:", q.empty())
print("Is full:", q.full())
OutputInitial size: 0
Is full: True
Elements dequeued from the queue:
a
b
c
Is empty: True
Is empty: False
Is full: False
Explanation: queue.Queue class handles thread-safe operations. You can check fullness or emptiness before adding or removing elements.
To practice problems related to Queue, refer to this article Queue Data Structure
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