[gobicse]

Program to illustrate queue using linked list concept:

/*Program to illustrate queue using linked list concept*/
#include /*Link section*/
struct node /*Structure used to represent elements as nodes*/
{
int val;
struct node *next;
}*rear = NULL,*front = NULL,*temp = NULL;

void enqueue(int); /*Function declaration section*/
int dequeue();
void disp();

/*Main Function starts here*/
void main()
{
int opt,item,dele;
/*Loop which processes infinitely and ends when forced by the user*/
do
{
clrscr();
printf("Enter the required option:\n1.Enqueue\n2.Dequeue\n3.View the queue\n4.Exit:");
scanf("%d",&opt); /*Required option is received here*/
switch(opt)
{
case 1:
printf("Enter the value to enqueue:");
scanf("%d",&item);
enqueue(item); /*Element added to the queue */
break;

case 2:
/*Element removed from the queue pointed by the front pointer*/
dele = dequeue();
if(dele == 0)
{
disp();
continue;
}
getch();
break;

case 3:
disp(); /*Displays the queue when called upon*/
break;
}
}while(opt != 4); /*Checks for the entered option*/
}

void enqueue(int item)
{
if(rear == NULL)
{
/*Reserving memory space using malloc*/
rear = (struct node *)malloc(sizeof(struct node));

/*Assigning the first value to the rear pointer*/
rear->val = item;

/*Assigning the next of rear pointer to NULL*/
rear->next = NULL;

/*Assigning the front pointer to the rear pointer*/
front = rear;
}
else
{
/*Reserving memory space using malloc*/
rear->next = (struct node *)malloc(sizeof(struct node));

/*Assigning the value of the next element of rear pointer*/
rear->next->val = item;

/*Assigning the next of next element of rear pointer to NULL*/
rear->next->next = NULL;

/*Moving rear by one node*/
rear = rear->next;
}
}

/*Function used to dequeue an element*/
int dequeue()
{
int dequeued; /*Variable used to store the dequeued element*/
if(front == NULL) /*Checking for the queue to be NULL*/
{
printf("\nQueue is empty\n"); /*Printing the queue is empty*/
return;
}
dequeued = front->val; /*Assigning the dequeued value to a variable*/

/*Printing the dequeued element*/
printf("The dequeued item is %d",front->val);

temp = front; /*Temp initialized to front*/
front = front->next; /*Element is deleted by moving the front pointer*/
free(temp); /*Temp is made free*/
if(front == NULL) /*Checking for the queue to be NULL*/
rear = NULL;
return dequeued; /*Dequeued value is returned here*/
}

/*Function to display the queue elements*/
void disp()
{
if(front == NULL) /*Checking for the queue to be NULL*/
{
printf("The queue is empty");
getch();
return;
}
temp = front; /*Temp initialized to front pointer*/
printf("\nQueue elements are:\n");




/*Loop to display the queue elements*/
while(temp != NULL)
{
printf("%d\n",temp->val);
temp = temp->next; /*Temp is passed to the next node by pointers*/
}
getch();
}


Output:

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:3
The queue is empty

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:1
Enter the value to enqueue:15

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:1
Enter the value to enqueue:69

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:3


Queue elements are:
15
69

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:1
Enter the value to enqueue:39

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:1
Enter the value to enqueue:25

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:1
Enter the value to enqueue:30

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:3

Queue elements are:
15
69
39
25
30



Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2
The dequeued item is 15

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2
The dequeued item is 69

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:3

Queue elements are:
39
25
30

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2
The dequeued item is 39

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2
The dequeued item is 25


Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2
The dequeued item is 30

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:2

Queue is empty

Enter the required option:
1.Enqueue
2.Dequeue
3.View the queue
4.Exit:4
-------