Queue

Queue is a data structure which operates on FIFO(first in first out) principle.The element enqueued first,dequeues first.

front keeps track of front element in queue.
rear keeps track of last element in queue.

Lets see how we can implement a queue using an array.

Declaring the queue structure

typedef struct
{
    int front,rear,arr[SIZE];
}queue;

Initialize 

front = rear = -1

Check empty queue

front == -1 

              OR

rear == -1

Check full queue

q.rear == (SIZE-1)

Enqueue

if queue is empty 
    increment front and rear.Enqueue element at rear position.
else
    increment rear.Enqueue element at rear position.

void enqueue(int element)
{     
        if(isFull())
        {
            printf("\nQueue is full");
            return;
        }

        if(isEmpty())
            q.front++;
        q.arr[++q.rear] = element;    
}

Dequeue

Store front element in element.

if last element is left
    initialize the queue
else
    increment front

return element

int dequeue()
{
    int element = q.arr[q.front];

    if(isEmpty())
    {
        printf("\nQueue is empty.Can't dequeue");
        return -1;
    }
    else 
    {
        if(q.front == q.rear)       //last element in queue
            init();
        else
            q.front++;
        return element;
    }
}

Complete Code with output

More

Queue using linked list

Circular Queue