# Python Program to Implement Queues using Stacks

PythonServer Side ProgrammingProgramming

When it is required to implement a queue using a stack, a queue class can be defined, where two stack instances can be defined. Different operations can be performed on the queue that are defined as methods in this class.

Below is a demonstration of the same −

## Example

Live Demo

class Queue_structure:
def __init__(self):
self.in_val = Stack_structure()
self.out_val = Stack_structure()

def check_empty(self):
return (self.in_val.check_empty() and self.out_val.check_empty())

def enqueue_operation(self, data):
self.in_val.push_operation(data)

def dequeue_operation(self):
if self.out_val.check_empty():
while not self.in_val.check_empty():
deleted_val = self.in_val.pop_operation()
self.out_val.push_operation(deleted_val)
return self.out_val.pop_operation()

class Stack_structure:
def __init__(self):
self.items = []

def check_empty(self):
return self.items == []

def push_operation(self, data):
self.items.append(data)

def pop_operation(self):
return self.items.pop()

my_instance = Queue_structure()

while True:
print('enqueue <value>')
print('dequeue')
print('quit')
my_input = input('What operation would you like to perform ?').split()

operation = my_input[0].strip().lower()
if operation == 'enqueue':
my_instance.enqueue_operation(int(my_input[1]))
elif operation == 'dequeue':
if my_instance.check_empty():
print('The queue is empty')
else:
deleted_elem = my_instance.dequeue_operation()
print('The deleted element is : ', int(deleted_elem))
elif operation == 'quit':
break

## Output

enqueue <value>
dequeue
quit
What operation would you like to perform ?enqueue 45
enqueue <value>
dequeue
quit
What operation would you like to perform ?enqueue 23
enqueue <value>
dequeue
quit
What operation would you like to perform ?enqueue 78
enqueue <value>
dequeue
quit
What operation would you like to perform ?dequeue
The deleted element is : 45
enqueue <value>
dequeue
quit
What operation would you like to perform ?quit

## Explanation

• A ‘Queue_structure’ is defined, that defines two instances of Stack.

• It has a method named ‘check_empty’ that checks to see if the queue is empty.

• Another method named ‘enqueue_operation’ is defined, that helps add elements to the queue.

• Another method named ‘dequeue_operation’ is defined, that deletes an element from the queue.

• Another ‘Stack_structure’ class is created.

• It initializes an empty list.

• It has a method named ‘check_empty’ that checks to see if the stack is empty.

• Another method named ‘push_operation’ is defined, that helps add elements to the queue.

• Another method named ‘pop_operation’ is defined, that deletes an element from the queue.

• A ‘Queue_structure’ instance is created.

• Three options are given- enqueue, dequeue, and quit.

• Based on the option given by user, the operations are performed, and relevant output is displayed on the console.

Published on 15-Apr-2021 12:46:28