Queue in C

What is a Queue?

A queue inserts elements from one end (rear) and removes from the other (front). It works like a real-life line of people.

• Insertion → Enqueue
• Deletion → Dequeue
• Front → first element
• Rear → last element

Applications of Queue

• CPU Scheduling
• Printer Queue
• Call center systems
• Network packet management
• Breadth-First Search (BFS)

Types of Queues

• Simple Queue – basic FIFO
• Circular Queue – prevents wasted space
• Priority Queue – elements with priority
• Double-Ended Queue (Deque) – insert/delete from both ends

Queue Representation (Using Array)

A simple queue uses:

• front → index of first item
• rear → index of last item
• Array → stores elements
• Condition for full → rear == size - 1
• Condition for empty → front == -1

Queue Operations – 5 Examples

Example 1 – Enqueue Operation

#include <stdio.h>
#define SIZE 5

int queue[SIZE];
int front = -1, rear = -1;

void enqueue(int val){
    if(rear == SIZE-1){
        printf("Queue Full\n");
        return;
    }
    if(front == -1) front = 0;
    queue[++rear] = val;
}

int main(){
    enqueue(10);
    enqueue(20);
    printf("Inserted: %d %d", queue[0], queue[1]);
    return 0;
}

Example 2 – Dequeue Operation

#include <stdio.h>
#define SIZE 5

int queue[SIZE];
int front = -1, rear = -1;

int dequeue(){
    if(front == -1 || front > rear){
        printf("Queue Empty\n");
        return -1;
    }
    return queue[front++];
}

int main(){
    queue[0]=10; queue[1]=20; front=0; rear=1;
    printf("Removed: %d", dequeue());
    return 0;
}

Example 3 – Display Queue

#include <stdio.h>
#define SIZE 5

int q[SIZE] = {10,20,30};
int front = 0, rear = 2;

void display(){
    for(int i=front;i<=rear;i++)
        printf("%d ", q[i]);
}

int main(){
    display();
    return 0;
}

Example 4 – Check Full / Empty

#include <stdio.h>
#define SIZE 5

int front=-1, rear=-1;

int isEmpty(){
    return (front==-1 || front>rear);
}

int isFull(){
    return (rear==SIZE-1);
}

int main(){
    printf("Empty: %d\n", isEmpty());
    return 0;
}

Example 5 – Simple Queue Menu

#include <stdio.h>
#define SIZE 5

int q[SIZE];
int front=-1, rear=-1;

void enqueue(int x){
    if(rear==SIZE-1){ printf("Full\n"); return; }
    if(front==-1) front=0;
    q[++rear]=x;
}

void dequeue(){
    if(front==-1 || front>rear){ printf("Empty\n"); return; }
    printf("Removed %d\n", q[front++]);
}

int main(){
    enqueue(5);
    enqueue(10);
    dequeue();
    return 0;
}