C Queue Operations using Linked List

Stack

A queue is a data structure in which addition of new element takes place at end ( also called rear of the queue) and deletion of existing element are always take place at the front of the queue.

First In First Out (FIFO).

Linked list are not stored in adjacent locations and array occupy continuous locations.

C Program to Implement queue and operations using linked list

#include<stdio.h>  

//structure contains data and link
struct node
{
    int data;
    struct node *link;
};

// add queue element in the rear end
void addq(struct node **front, struct node **rear, int data)
{
    struct node *queue;

    queue = malloc( sizeof (struct node));
    queue->data = data;
    queue->link = NULL;
    if(*front == NULL)
        *front = queue;
    else
        (*rear)->link = queue;
    *rear = queue;
}

// displays the queue elements from front to rear end.
void display(struct node *head)
{
    printf("\n");
    // traversing entire linked list
    printf("\nfront==>");
    while(head != NULL)
    {
        printf("%2d-->", head->data);
        head = head->link;
    }
    printf("rear\n");
}

// displays the number of elements in the queue
int count(struct node *head)
{
    int c = 0;
    while(head != NULL)                                                         
    {                                                                           
        head = head->link;          
        c++;
    } 
    return c;
}

// deletes the front element in the queue.
int delq(struct node **front, struct node **rear)
{
    int data;
    struct node *queue;
    if(*front == NULL)
    {
        printf("\nqueue is empty!");
    }
    else
    {
        queue = *front;
        data = queue->data;
        *front = queue->link;
        free(queue);

        // if deletion leads to queue empty
        if(*front == NULL)
            *rear = NULL;
        return data;
    }
    return 0;
}


void main()                                                                     
{       
    int op, data;
    struct node *front, *rear;                                                           
    // Empty queue
    front = rear = NULL;                                                                 
                                                                                
    printf("\nNumber of elements in the queue: %d", count(front));          
    addq(&front, &rear, 1);                                                            
    addq(&front, &rear, 2);                                                            
    addq(&front, &rear, 3);                                                            
                                                                                
    display(front);       
    printf("\nNumber of elements in the queue: %d", count(front));

    do
    {
        printf("\nQueue\n---------------\n1.add queue \n2.delete queue\n3.display\n4.count\n0.exit");
        printf("\nEnter choice: ");
        scanf("%d", &op);
        switch(op)
        {
            case 1:
                printf("\nEnter element: ");                                    
                scanf("%d", &data);                                             
                addq(&front, &rear, data);
                break;
            case 2:
                printf("Deleted: %d", delq(&front, &rear));  
                break;
            case 3:
                display(front);
                break;
            case 4:
                printf("\nNumber of elements in the queue: %d", count(front));
                break;
        }
    }while(op>=1 && op<=4);
}      
                                                                                                                                                                                                                                                                                                                                                                                                           

$ cc queue.c 
queue.c: In function ‘addq’:
queue.c:14:13: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
     queue = malloc( sizeof (struct node));
             ^~~~~~
queue.c:14:13: warning: incompatible implicit declaration of built-in function ‘malloc’
queue.c:14:13: note: include ‘’ or provide a declaration of ‘malloc’
queue.c: In function ‘delq’:
queue.c:61:9: warning: implicit declaration of function ‘free’ [-Wimplicit-function-declaration]
         free(queue);
         ^~~~
queue.c:61:9: warning: incompatible implicit declaration of built-in function ‘free’
queue.c:61:9: note: include ‘’ or provide a declaration of ‘free’

$ ./a.out 

Number of elements in the queue: 0

front==> 1--> 2--> 3-->rear

Number of elements in the queue: 3
Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 3


front==> 1--> 2--> 3-->rear

Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 1

Enter element: 4

Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 3


front==> 1--> 2--> 3--> 4-->rear

Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 2
Deleted: 1
Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 3


front==> 2--> 3--> 4-->rear

Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 4

Number of elements in the queue: 3
Queue
---------------
1.add queue 
2.delete queue
3.display
4.count
0.exit
Enter choice: 0