C Program to Implement Priority Queue to Add and Delete Elements

/*
 * C Program to Implement Priority Queue to Add and Delete Elements
 */
#include <stdio.h>
#include <stdlib.h>

#define MAX 5

void insert_by_priority(int);
void delete_by_priority(int);
void create();
void check(int);
void display_pqueue();

int pri_que[MAX];
int front, rear;

void main()
{
    int n, ch;

    printf("\n1 - Insert an element into queue");
    printf("\n2 - Delete an element from queue");
    printf("\n3 - Display queue elements");
    printf("\n4 - Exit");

    create();

    while (1)
    {
        printf("\nEnter your choice : ");  
        scanf("%d", &ch);

        switch (ch)
        {
        case 1:
            printf("\nEnter value to be inserted : ");
            scanf("%d",&n);
            insert_by_priority(n);
            break;
        case 2:
            printf("\nEnter value to delete : ");
            scanf("%d",&n);
            delete_by_priority(n);
            break;
        case 3:
            display_pqueue();
            break;
        case 4:
            exit(0);
        default:
            printf("\nChoice is incorrect, Enter a correct choice");
        }
    }
}

/* Function to create an empty priority queue */
void create()
{
    front = rear = -1;
}

/* Function to insert value into priority queue */
void insert_by_priority(int data)
{
    if (rear >= MAX - 1)
    {
        printf("\nQueue overflow no more elements can be inserted");
        return;
    }
    if ((front == -1) && (rear == -1))
    {
        front++;
        rear++;
        pri_que[rear] = data;
        return;
    }  
    else
        check(data);
    rear++;
}

/* Function to check priority and place element */
void check(int data)
{
    int i,j;

    for (i = 0; i <= rear; i++)
    {
        if (data >= pri_que[i])
        {
            for (j = rear + 1; j > i; j--)
            {
                pri_que[j] = pri_que[j - 1];
            }
            pri_que[i] = data;
            return;
        }
    }
    pri_que[i] = data;
}

/* Function to delete an element from queue */
void delete_by_priority(int data)
{
    int i;

    if ((front==-1) && (rear==-1))
    {
        printf("\nQueue is empty no elements to delete");
        return;
    }

    for (i = 0; i <= rear; i++)
    {
        if (data == pri_que[i])
        {
            for (; i < rear; i++)
            {
                pri_que[i] = pri_que[i + 1];
            }

        pri_que[i] = -99;
        rear--;

        if (rear == -1)
            front = -1;
        return;
        }
    }
    printf("\n%d not found in queue to delete", data);
}

/* Function to display queue elements */
void display_pqueue()
{
    if ((front == -1) && (rear == -1))
    {
        printf("\nQueue is empty");
        return;
    }

    for (; front <= rear; front++)
    {
        printf(" %d ", pri_que[front]);
    }

    front = 0;
}

OUTPUT

1 - Insert an element into queue
2 - Delete an element from queue
3 - Display queue elements
4 - Exit
Enter your choice : 1

Enter value to be inserted : 20

Enter your choice : 1

Enter value to be inserted : 45

Enter your choice : 1

Enter value to be inserted : 89

Enter your choice : 3
 89  45  20
Enter your choice : 1

Enter value to be inserted : 56

Enter your choice : 3
 89  56  45  20
Enter your choice : 2

Enter value to delete : 45

Enter your choice : 3
 89  56  20
Enter your choice : 4