Queue Program in C using an Array and Link List

/*
 * C Program to Implement a Queue using an Array
 */
#include <stdio.h>

#define MAX 5
int queue_array[MAX];
int rear = - 1;
int front = - 1;

void insert();
void delete();
void display();

void main()
{
int choice;
while (1)
{
printf("1.Insert element to queue \n");
printf("2.Delete element from queue \n");
printf("3.Display all elements of queue \n");
printf("4.Quit \n");
printf("Enter your choice : ");
scanf("%d", &choice);
switch (choice)
{
case 1:
insert();
break;
case 2:
delete();
break;
case 3:
display();
break;
case 4:
exit(1);
default:
printf("Wrong choice \n");
} /*End of switch*/
} /*End of while*/
} /*End of main()*/

void insert()
{
int add_item;
if (rear == MAX - 1)
printf("Queue Overflow \n");
else
{
if (front == - 1)
/*If queue is initially empty */
front = 0;
printf("Inset the element in queue : ");
scanf("%d", &add_item);
rear = rear + 1;
queue_array[rear] = add_item;
}
} /*End of insert()*/

void delete()
{
if (front == - 1 || front > rear)
{
printf("Queue Underflow \n");
return ;
}
else
{
printf("Element deleted from queue is : %d\n", queue_array[front]);
front = front + 1;
}
} /*End of delete() */

void display()
{
int i;
if (front == - 1)
printf("Queue is empty \n");
else
{
printf("Queue is : \n");
for (i = front; i <= rear; i++)
printf("%d ", queue_array[i]);
printf("\n");
}
} /*End of display() */



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/*
 * 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;
front = rear = NULL;
clrscr();
while (1)
{
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");

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;
}
}
}

/* 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");
}