C++ Program to Implement Priority Queue

The queue which is implemented as FIFO where insertions are done at one end (rear) and deletions are done from another end (front). The first element that entered is deleted first.

Queue operations are

• EnQueue (int data): Insertion at rear end

• int DeQueue(): Deletion from front end

But a priority queue doesn’t follow First-In-First-Out, but rather than each element has a priority based on the basis of urgency.

Items with the same priority are processed on First-In-First-Out service basis.

An item with higher priority is processed before other items with lower priority.

Class Descriptions

Begin
   class Priority_Queue has following functions:
   function insert() to insert items at priority queue with their priorities:
      1) If queue is empty insert data from the left end of the queue.
      2) If queue is having some nodes then insert the new node at the end of those nodes having priority
         same with the new node and also before all the nodes having priority lesser than the
         current priority of the new node.
      function del() to delete items from queue.
   If queue is completely empty, print underflow otherwise delete the front element and update front.
End

Example

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
using namespace std;
struct n // node declaration {
   int p;
   int info;
   struct n *l;
};
class Priority_Queue {
   private:
      //Declare a front pointer f and initialize it to NULL.
      n *f;
   public:
      Priority_Queue() //constructor {
         f = NULL;
      }
      void insert(int i, int p) {
         n *t, *q;
         t = new n;
         t->info = i;
         t->p = p;
         if (f == NULL || p < f->p) {
            t->l= f;
            f = t;
         } else {
            q = f;
            while (q->l != NULL && q->l->p <= p)
               q = q->l;
               t->l = q->l;
               q->l = t;
         }
      }
      void del() {
         n *t;
         if(f == NULL) //if queue is null
            cout<<"Queue Underflow\n";
         else {
            t = f;
            cout<<"Deleted item is: "<<t->info<<endl;
            f = f->l;
            free(t);
         }
      }
      void show() //print queue {
         n *ptr;
         ptr = f;
         if (f == NULL)
            cout<<"Queue is empty\n";
         else {
            cout<<"Queue is :\n";
            cout<<"Priority Item\n";
            while(ptr != NULL) {
               cout<<ptr->p<<" "<<ptr->info<<endl;
               ptr = ptr->l;
            }
         }
      }
};
int main() {
   int c, i, p;
   Priority_Queue pq;
   Do//perform switch opeartion {
      cout<<"1.Insert\n";
      cout<<"2.Delete\n";
      cout<<"3.Display\n";
      cout<<"4.Exit\n";
      cout<<"Enter your choice : ";
      cin>>c;
      switch(c) {
         case 1:
            cout<<"Input the item value to be added in the queue : ";
            cin>>i;
            cout<<"Enter its priority : ";
            cin>>p;
            pq.insert(i, p);
            break;
         case 2:
            pq.del();
            break;
         case 3:
            pq.show();
            break;
         case 4:
            break;
         default:
         cout<<"Wrong choice\n";
      }
   }
   while(c != 4);
   return 0;
}

Output

1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 1
Input the item value to be added in the queue : 7
Enter its priority : 2
1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 1
Input the item value to be added in the queue : 6
Enter its priority : 1
1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 1
Input the item value to be added in the queue : 3
Enter its priority : 3
1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 1
Input the item value to be added in the queue : 4
Enter its priority : 3
1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 3
Queue is :
Priority Item
1 6
2 7
3 3
3 4
1.Insert
2.Delete
3.Display
4.Exit
Enter your choice : 4