C Program to Implement the multiple priority queue.

A priority queue is a data structure where each element is served according to its priority rather than just the order it arrived. A multiple priority queue implements this with several separate queues — one per priority level — so that higher-priority items are always removed before lower-priority ones. This C program builds a simple menu-driven multiple priority queue with three priority levels using arrays.

How It Works

We keep an array of three queue structures (priority 1 is highest). Inserting an element adds it to the queue for its priority. Deleting always removes from the highest-priority non-empty queue first, then the next, and so on.

The Program

#include <stdio.h>
#include <stdlib.h>

#define MAX 5      /* capacity of each priority queue */
#define LEVELS 3   /* number of priority levels */

struct pqueue {
    int front, rear;
    int items[MAX];
};

int is_empty(struct pqueue *q) { return q->front > q->rear; }
int is_full(struct pqueue *q)  { return q->rear == MAX - 1; }

void insert(struct pqueue *q)
{
    int x;
    if (is_full(q)) {
        printf("Queue overflow.\n");
        return;
    }
    printf("Enter the element to insert : ");
    scanf("%d", &x);
    q->items[++(q->rear)] = x;
}

void delete_item(struct pqueue q[])
{
    for (int i = 0; i < LEVELS; i++)
        if (!is_empty(&q[i])) {
            printf("Deleted %d (from priority %d)\n",
                   q[i].items[q[i].front++], i + 1);
            return;
        }
    printf("All queues are empty.\n");
}

void display(struct pqueue q[])
{
    for (int i = 0; i < LEVELS; i++) {
        printf("Priority %d : ", i + 1);
        if (is_empty(&q[i]))
            printf("(empty)");
        else
            for (int j = q[i].front; j <= q[i].rear; j++)
                printf("%d ", q[i].items[j]);
        printf("\n");
    }
}

int main(void)
{
    struct pqueue q[LEVELS];
    int ch, p;

    for (int i = 0; i < LEVELS; i++) {
        q[i].front = 0;
        q[i].rear = -1;
    }

    while (1) {
        printf("\n--- MENU ---\n1.Insert  2.Delete  3.Display  4.Exit\n");
        printf("Enter your choice : ");
        if (scanf("%d", &ch) != 1)
            break;

        switch (ch) {
        case 1:
            printf("Enter priority (1-%d) : ", LEVELS);
            scanf("%d", &p);
            if (p < 1 || p > LEVELS) {
                printf("Invalid priority.\n");
                break;
            }
            insert(&q[p - 1]);
            break;
        case 2: delete_item(q); break;
        case 3: display(q);     break;
        case 4: exit(0);
        default: printf("Invalid choice.\n");
        }
    }
    return 0;
}

How the Program Works

• Each priority level is a fixed-size array queue with its own front and rear indices.
• Insert asks for a priority (1–3) and appends the element to that queue, guarding against overflow.
• Delete scans the queues from highest to lowest priority and removes from the first non-empty one — this is what gives the structure its priority behaviour.
• Display prints the contents of all three queues.
• This version cleans up the original implicit-int main(), the missing newline escapes (n instead of \n), and a stray character, and adds input validation.

Sample Output

--- MENU ---
1.Insert  2.Delete  3.Display  4.Exit
Enter your choice : 1
Enter priority (1-3) : 2
Enter the element to insert : 50

Enter your choice : 2
Deleted 50 (from priority 2)

For a thorough treatment of queues and other data structures in C, The C Programming Language by Kernighan and Ritchie is the classic reference — find it on Amazon.

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