C Queue Definition: Array & Linked List Guide

In C language, queues can be defined using arrays or linked lists. Here is an example of defining a queue using arrays:

#include <stdio.h>

#define MAX_SIZE 100

struct Queue {
  int items[MAX_SIZE];
  int front;
  int rear;
};

struct Queue* createQueue() {
  struct Queue* queue = (struct Queue*)malloc(sizeof(struct Queue));
  queue->front = -1;
  queue->rear = -1;
  return queue;
}

int isEmpty(struct Queue* queue) {
  if (queue->rear == -1) {
    return 1;
  }
  return 0;
}

int isFull(struct Queue* queue) {
  if (queue->rear == MAX_SIZE - 1) {
    return 1;
  }
  return 0;
}

void enqueue(struct Queue* queue, int value) {
  if (isFull(queue)) {
    printf("Queue is full\n");
  } else {
    if (isEmpty(queue)) {
      queue->front = 0;
    }
    queue->rear++;
    queue->items[queue->rear] = value;
    printf("%d enqueued to queue\n", value);
  }
}

void dequeue(struct Queue* queue) {
  if (isEmpty(queue)) {
    printf("Queue is empty\n");
  } else {
    printf("%d dequeued from queue\n", queue->items[queue->front]);
    queue->front++;
    if (queue->front > queue->rear) {
      queue->front = queue->rear = -1;
    }
  }
}

int main() {
  struct Queue* queue = createQueue();

  enqueue(queue, 10);
  enqueue(queue, 20);
  enqueue(queue, 30);

  dequeue(queue);
  dequeue(queue);
  dequeue(queue);
  dequeue(queue);

  return 0;
}

In the above example, we used a struct to define a queue and implemented enqueue and dequeue operations. We also defined a createQueue function for creating a queue, an isEmpty function to check if the queue is empty, and an isFull function to check if the queue is full. By using these functions, we can manipulate the queue.