priority-queue.py

# Copyright (C) Deepali Srivastava - All Rights Reserved
# This code is part of DSA course available on CourseGalaxy.com  

class EmptyQueueError(Exception):
    pass

class Node:

    def __init__(self,value,pr):
        self.info = value 
        self.priority = pr
        self.link = None 
        
class PriorityQueue:

    def __init__(self):
        self.front = None

    def enqueue(self, data, data_priority):

        temp = Node(data, data_priority)
        
        #If queue is empty or element to be added has priority more than first element
        if self.is_empty() or data_priority < self.front.priority:
            temp.link = self.front
            self.front = temp
        else:
            p = self.front
            while p.link != None and p.link.priority <= data_priority:
                p = p.link
            temp.link = p.link
            p.link = temp
                 
    def dequeue(self):
        if self.is_empty():
            raise EmptyQueueError("Queue is empty")
        x = self.front.info
        self.front = self.front.link
        return x
    
    def is_empty(self):
        return self.front == None
            
    def display(self):
        if self.is_empty():
            print("Queue is empty")
            return

        print("Queue is : ")
        p = self.front 
        while p is not None:
            print(p.info , "     ", p.priority)
            p = p.link 
        print()
    
    def size(self):
        n = 0
        p = self.front 
        while p is not None:
            n+=1
            p = p.link
        return n
    
    
    
#########################################################################################

if __name__ == "__main__":
    qu = PriorityQueue()

    while True:
        print("1.Enqueue") 
        print("2.Dequeue") 
        print("3.Display all queue elements") 
        print("4.Display size of the queue")  
        print("5.Quit") 
         
        choice = int(input("Enter your choice : "))

        if choice == 1:
            x = int(input("Enter the element : "))
            pr = int(input("Enter its priority : "))
            qu.enqueue(x,pr) 
        elif choice == 2:
            x=qu.dequeue() 
            print("Element is : " , x) 
        elif choice == 3:
            qu.display() 
        elif choice == 4:
            print("Size of queue " , qu.size()) 
        elif choice == 5:
          break;
        else:
          print("Wrong choice") 
        print()