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How to use Queue in Python

hasansajedi profile image Hasan Sajedi ・2 min read

This Article will help you to understand a Queue data structure and how to implement it.

A Queue is a simple data structure concept that can be easily applied in our day to day life, like when you stand in a line to buy coffee at Starbucks.
Let's make a few observations based on this example:

Queue

Now, let’s look at the above points programmatically:

  • Queues are open from both ends meaning elements are added from the back and removed from the front
  • The element to be added first is removed first (First In First Out - FIFO) If all the elements are removed, then the queue is empty and if you try to remove elements from an empty queue, a warning or an error message is thrown.
  • If the queue is full and you add more elements to the queue, a warning or error message must be thrown.

Algorithm

  1. Declare a list and an integer MaxSize, denoting a virtual maximum size of the Queue
  2. Head and Tail are initially set to 0
  3. Size method
    • Calculates the number of elements in the queue -> Size = Tail - Head
  4. Reset method:
    • Resets Tail and Head to 0
    • Creates a new Queue (initializes queue to a new list)
  5. Enqueue operation:
    • Check if Size is less than the MaxSize:
      • If yes, append data to Queue and then increment Tail by 1
      • If no, print Queue full message
  6. Dequeue operation:
    • Check if Size is greater than 0:
      • If yes, pop the first element from the list and increment Head by 1
      • If no:
        • Call Reset method
        • Print Queue empty message
class Queue:

    #Constructor
    def __init__(self):
        self.queue = list()
        self.maxSize = 8
        self.head = 0
        self.tail = 0

    #Adding elements
    def enqueue(self,data):
        #Checking if the queue is full
        if self.size() >= self.maxSize:
            return ("Queue Full")
        self.queue.append(data)
        self.tail += 1
        return True     

    #Deleting elements 
    def dequeue(self):
        #Checking if the queue is empty
        if self.size() <= 0:
            self.resetQueue()
            return ("Queue Empty") 
        data = self.queue[self.head]
        self.head+=1
        return data

    #Calculate size
    def size(self):
        return self.tail - self.head

    #Reset queue
    def resetQueue(self):
        self.tail = 0
        self.head = 0
        self.queue = list()

q = Queue()
print(q.enqueue(1))#prints True
print(q.enqueue(2))#prints True
print(q.enqueue(3))#prints True
print(q.enqueue(4))#prints True
print(q.enqueue(5))#prints True
print(q.enqueue(6))#prints True
print(q.enqueue(7))#prints True
print(q.enqueue(8))#prints True
print(q.enqueue(9))#prints Queue Full!
print(q.size())#prints 8        
print(q.dequeue())#prints 8
print(q.dequeue())#prints 7 
print(q.dequeue())#prints 6
print(q.dequeue())#prints 5
print(q.dequeue())#prints 4
print(q.dequeue())#prints 3
print(q.dequeue())#prints 2
print(q.dequeue())#prints 1
print(q.dequeue())#prints Queue Empty
#Queue is reset here 
print(q.enqueue(1))#prints True
print(q.enqueue(2))#prints True
print(q.enqueue(3))#prints True
print(q.enqueue(4))#prints True

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Note: Element 9 was not added to the Queue and hence the size of the Queue remains 8

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