C Lesson 39 – Queues | Dataplexa

Queues in C

After learning stacks, we now study another important linear data structure called a Queue.

Queues follow a different rule than stacks and are used heavily in real-world systems.

What Is a Queue?

A queue is a linear data structure that follows:

FIFO – First In, First Out

The element inserted first is removed first.

Real-World Example

Think of a queue at a ticket counter:

First person in line is served first
New people join from the back

This is exactly how a queue works.

Basic Queue Operations

Enqueue – Insert an element at the rear
Dequeue – Remove an element from the front
Front – View the front element

Queue Implementation Using Array


#include <stdio.h>
#define SIZE 5

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

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

void dequeue() {
    if (front == -1 || front > rear) {
        printf("Queue Underflow\n");
        return;
    }
    printf("Dequeued: %d\n", queue[front++]);
}

int main() {
    enqueue(10);
    enqueue(20);
    enqueue(30);
    dequeue();
    dequeue();
    return 0;
}

Limitations of Array Queue

Fixed size
Unused spaces after dequeuing

To overcome this, linked lists are used.

Queue Using Linked List


#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node *next;
};

struct Node *front = NULL;
struct Node *rear = NULL;

void enqueue(int value) {
    struct Node *newNode = malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = NULL;

    if (rear == NULL) {
        front = rear = newNode;
        return;
    }

    rear->next = newNode;
    rear = newNode;
}

void dequeue() {
    if (front == NULL) {
        printf("Queue Underflow\n");
        return;
    }

    struct Node *temp = front;
    printf("Dequeued: %d\n", temp->data);

    front = front->next;
    if (front == NULL)
        rear = NULL;

    free(temp);
}

int main() {
    enqueue(5);
    enqueue(15);
    enqueue(25);
    dequeue();
    dequeue();
    return 0;
}

Where Queues Are Used

CPU scheduling
Printer spooling
Network traffic
Task scheduling

Mini Practice

Create a queue using array
Implement enqueue and dequeue
Display front element

Quick Quiz

Q1. What does FIFO mean?

Q2. Which operation inserts an element?

Q3. Which operation removes an element?

Q4. Which queue implementation removes size limitation?

Q5. Are queues used in operating systems?

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