In Data Structure a priority queue is a queue in that each element has some priority and all elements are processed based on their priorities. and the element with high priority is processed before the element with low priority.
if the two elements have the same priority then we follow the FIFO rule means the element that comes first will place first and deleted first. and the element that has the highest priority is always at the front of the queue. and has been inserted first and also deleted first.
Here in the above example, we implement a priority queue using a linked list. in each node, we have three parts. in the first part, we store the priority of the element and in the second part we store the value of the element in the third part, we store the linked part of the next element.
if we want to insert a new element that has priority 3 then we need to insert the element after node three.
Python program to Implement Priority Queue.
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 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()
