C Program: Queue implementation using structure

Last update on August 01 2025 13:03:22 (UTC/GMT +8 hours)

7. Queue Implementation with Structures

Write a C program that implements a simple queue using a structure. The structure should contain an array representing the queue and front and rear indices. Include functions for enqueue and dequeue operations.

Sample Solution:

C Code:

#include <stdio.h>

// Define the structure "Queue"
#define MAX_SIZE 100

struct Queue {
    int arr[MAX_SIZE];
    int front, rear;
};

// Function to initialize the queue
void initializeQueue(struct Queue *queue) {
    queue->front = -1;
    queue->rear = -1;
}

// Function to check if the queue is empty
int isEmpty(struct Queue *queue) {
    return (queue->front == -1 && queue->rear == -1);
}

// Function to check if the queue is full
int isFull(struct Queue *queue) {
    return (queue->rear == MAX_SIZE - 1);
}

// Function to enqueue an element into the queue
void enqueue(struct Queue *queue, int value) {
    if (isFull(queue)) {
        printf("Queue is full. Cannot enqueue.\n");
        return;
    }

    if (isEmpty(queue)) {
        // If the queue is empty, set front and rear to 0
        queue->front = 0;
        queue->rear = 0;
    } else {
        // Increment rear circularly
        queue->rear = (queue->rear + 1) % MAX_SIZE;
    }

    // Enqueue the value
    queue->arr[queue->rear] = value;
    printf("Enqueued: %d\n", value);
}

// Function to dequeue an element from the queue
int dequeue(struct Queue *queue) {
    int value;

    if (isEmpty(queue)) {
        printf("Queue is empty. Cannot dequeue.\n");
        return -1; // Assuming -1 represents an error or empty queue
    }

    // Dequeue the value
    value = queue->arr[queue->front];

    if (queue->front == queue->rear) {
        // If the queue has only one element, set front and rear to -1
        queue->front = -1;
        queue->rear = -1;
    } else {
        // Increment front circularly
        queue->front = (queue->front + 1) % MAX_SIZE;
    }

    printf("Dequeued: %d\n", value);
    return value;
}

int main() {
    // Declare and initialize a queue
    struct Queue myQueue;
    initializeQueue(&myQueue);

    // Enqueue some elements
    enqueue(&myQueue, 100);
    enqueue(&myQueue, 200);
    enqueue(&myQueue, 300);

    // Dequeue elements
    dequeue(&myQueue);
    dequeue(&myQueue);

    // Enqueue more elements
    enqueue(&myQueue, 400);
    enqueue(&myQueue, 500);

    // Dequeue remaining elements
    dequeue(&myQueue);
    dequeue(&myQueue);

    return 0;
}

Output:

Enqueued: 100
Enqueued: 200
Enqueued: 300
Dequeued: 100
Dequeued: 200
Enqueued: 400
Enqueued: 500
Dequeued: 300
Dequeued: 400

Explanation:

In the exercise above:

The "Queue" structure is defined with an array '(arr)', 'front', and 'rear' indices.
Functions like "initializeQueue()", "isEmpty()", and "isFull()" are used for queue management.
The "enqueue()" function adds an element to the rear of the queue.
The "dequeue()" function removes an element from the front of the queue.
The "main()" function demonstrates the usage of the queue operations.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to implement a simple queue using a structure, including functions for enqueue, dequeue, and display operations.
Write a C program to implement a circular queue using a structure and handle overflow and underflow conditions.
Write a C program to implement a queue using a structure, then reverse the order of the elements in the queue.
Write a C program to implement a priority queue using a structure where each element has an associated priority, and perform insertion and deletion based on priority.

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