C Program to Implement Queue using Linked List

This C Program implements queue using linked list. Queue is a particular kind of abstract data type or collection in which the entities in the collection are kept in order and the principal (or only) operations on the collection are the addition of entities to the rear terminal position, known as enqueue, and removal of entities from the front terminal position, known as dequeue. This makes the queue a First-In-First-Out (FIFO) data structure. Linked list is a data structure consisting of a group of nodes which together represent a sequence. Here we need to apply the application of linkedlist to perform basic operations of queue.

Here is source code of the C Program to implement queue using linked list. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

/*
 * C Program to Implement Queue Data Structure using Linked List
 */
#include <stdio.h>
#include <stdlib.h>
 
struct node
{
    int info;
    struct node *ptr;
}*front,*rear,*temp,*front1;
 
int frontelement();
void enq(int data);
void deq();
void empty();
void display();
void create();
void queuesize();
 
int count = 0;
 
void main()
{
    int no, ch, e;
 
    printf("\n 1 - Enque");
    printf("\n 2 - Deque");
    printf("\n 3 - Front element");
    printf("\n 4 - Empty");
    printf("\n 5 - Exit");
    printf("\n 6 - Display");
    printf("\n 7 - Queue size");
    create();
    while (1)
    {
        printf("\n Enter choice : ");
        scanf("%d", &ch);
        switch (ch)
        {
        case 1:
            printf("Enter data : ");
            scanf("%d", &no);
            enq(no);
            break;
        case 2:
            deq();
            break;
        case 3:
            e = frontelement();
            if (e != 0)
                printf("Front element : %d", e);
            else
                printf("\n No front element in Queue as queue is empty");
            break;
        case 4:
            empty();
            break;
        case 5:
            exit(0);
        case 6:
            display();
            break;
        case 7:
            queuesize();
            break;
        default:
            printf("Wrong choice, Please enter correct choice  ");
            break;
        }
    }
}
 
/* Create an empty queue */
void create()
{
    front = rear = NULL;
}
 
/* Returns queue size */
void queuesize()
{
    printf("\n Queue size : %d", count);
}
 
/* Enqueing the queue */
void enq(int data)
{
    if (rear == NULL)
    {
        rear = (struct node *)malloc(1*sizeof(struct node));
        rear->ptr = NULL;
        rear->info = data;
        front = rear;
    }
    else
    {
        temp=(struct node *)malloc(1*sizeof(struct node));
        rear->ptr = temp;
        temp->info = data;
        temp->ptr = NULL;
 
        rear = temp;
    }
    count++;
}
 
/* Displaying the queue elements */
void display()
{
    front1 = front;
 
    if ((front1 == NULL) && (rear == NULL))
    {
        printf("Queue is empty");
        return;
    }
    while (front1 != rear)
    {
        printf("%d ", front1->info);
        front1 = front1->ptr;
    }
    if (front1 == rear)
        printf("%d", front1->info);
}
 
/* Dequeing the queue */
void deq()
{
    front1 = front;
 
    if (front1 == NULL)
    {
        printf("\n Error: Trying to display elements from empty queue");
        return;
    }
    else
        if (front1->ptr != NULL)
        {
            front1 = front1->ptr;
            printf("\n Dequed value : %d", front->info);
            free(front);
            front = front1;
        }
        else
        {
            printf("\n Dequed value : %d", front->info);
            free(front);
            front = NULL;
            rear = NULL;
        }
        count--;
}
 
/* Returns the front element of queue */
int frontelement()
{
    if ((front != NULL) && (rear != NULL))
        return(front->info);
    else
        return 0;
}
 
/* Display if queue is empty or not */
void empty()
{
     if ((front == NULL) && (rear == NULL))
        printf("\n Queue empty");
    else
       printf("Queue not empty");
}

$ cc pgm4.c
$ a.out
 
1 - Enque
2 - Deque
3 - Front element
4 - Empty
5 - Exit
6 - Display
7 - Queue size
Enter choice : 1
Enter data : 14
 
Enter choice : 1
Enter data : 85
 
Enter choice : 1
Enter data : 38
 
Enter choice : 3
Front element : 14
Enter choice : 6
14 85 38
Enter choice : 7
 
Queue size : 3
Enter choice : 2
 
Dequed value : 14
Enter choice : 6
85 38
Enter choice : 7
 
Queue size : 2
Enter choice : 4
Queue not empty
Enter choice : 5

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