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What is the difference between a stack and a queue in Python?

In Python, a stack and a queue are both data structures used to organize and manipulate collections of elements. However, they have distinct characteristics and operate on different principles:

Stack:

  • A stack is a Last-In-First-Out (LIFO) data structure.
  • Elements are added and removed from only one end, known as the "top" of the stack.
  • The last element added to the stack is the first one to be removed.
  • Common stack operations include:
    • Push: Adds an element to the top of the stack.
    • Pop: Removes and returns the element from the top of the stack.
    • Peek: Returns the element at the top of the stack without removing it.
  • Python's list type can be used as a basic stack by using the append() method to push elements and the pop() method to remove elements from the end.

Queue:

  • A queue is a First-In-First-Out (FIFO) data structure.
  • Elements are added to one end, known as the "rear" or "back" of the queue, and removed from the other end, known as the "front" or "head" of the queue.
  • The first element added to the queue is the first one to be removed.
  • Common queue operations include:
    • Enqueue or Push: Adds an element to the rear of the queue.
    • Dequeue or Pop: Removes and returns the element from the front of the queue.
    • Peek: Returns the element at the front of the queue without removing it.
  • Python's collections.deque class can be used as a basic queue. The append() method adds elements to the rear, and the popleft() method removes elements from the front.

Here's an example to demonstrate the difference between a stack and a queue in Python:

# Stack example
stack = []
stack.append(1)
stack.append(2)
stack.append(3)
print(stack.pop())  # Output: 3

# Queue example
from collections import deque
queue = deque()
queue.append(1)
queue.append(2)
queue.append(3)
print(queue.popleft())  # Output: 1

In this example, when using a stack, the last element added (3) is the first one to be removed. In contrast, when using a queue, the first element added (1) is the first one to be removed.

Stacks are often used for tasks such as tracking function calls, evaluating expressions, or implementing undo/redo operations. Queues, on the other hand, are useful for handling tasks that follow a First-In-First-Out (FIFO) order, such as handling incoming requests or implementing a breadth-first search algorithm.

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