Queue Data Structure Detailed Examples

Queue Data Structure

A queue is a fundamental data structure in computer science that follows the First-In-First-Out (FIFO) principle. The element that enters the queue first is the first to be removed and the element enters last is the last to be removed.

Operations in Queue

• Enqueue Operation
• Dequeue Operation
• Peek (or Front) Operation
• Size Operation
• Search Operation

Practical Queue Example in C Programming

#include <stdio.h>

#define MAX_SIZE 100

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

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

// Enqueue Operation
void enqueue(struct Queue* q, int item) {
    if (q->rear == MAX_SIZE - 1) {
        printf("Queue Overflow: Cannot enqueue more elements.\n");
        return;
    }

    if (q->front == -1) {
        q->front = q->rear = 0;
    } else {
        q->rear++;
    }

    q->arr[q->rear] = item;
    printf("%d enqueued to the queue.\n", item);
}

// Dequeue Operation
void dequeue(struct Queue* q) {
    if (q->front == -1) {
        printf("Queue Underflow: Cannot dequeue from an empty queue.\n");
        return;
    }

    int dequeuedItem = q->arr[q->front];

    if (q->front == q->rear) {
        q->front = q->rear = -1; // Reset the queue when the last element is dequeued
    } else {
        q->front++;
    }

    printf("%d dequeued from the queue.\n", dequeuedItem);
}

// Peek (or Front) Operation
void peek(struct Queue* q) {
    if (q->front == -1) {
        printf("Queue is empty.\n");
    } else {
        printf("Front element: %d\n", q->arr[q->front]);
    }
}

// Size Operation
int size(struct Queue* q) {
    if (q->front == -1) {
        return 0;
    }
    return q->rear - q->front + 1;
}

// Search Operation
int search(struct Queue* q, int item) {
    for (int i = q->front; i <= q->rear; i++) {
        if (q->arr[i] == item) {
            printf("%d found in the queue.\n", item);
            return 1;
        }
    }
    printf("%d not found in the queue.\n", item);
    return 0;
}

// Check if the queue is empty
int isEmpty(struct Queue* q) {
    return q->front == -1;
}

int main() {
    struct Queue myQueue;
    initQueue(&myQueue);

    enqueue(&myQueue, 10);
    enqueue(&myQueue, 20);
    enqueue(&myQueue, 30);

    peek(&myQueue);

    dequeue(&myQueue);
    peek(&myQueue);

    printf("Queue Size: %d\n", size(&myQueue));

    search(&myQueue, 20);
    search(&myQueue, 40);

    return 0;
}

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