linked_list_queue.c

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

/* Each element is a node */
typedef struct Node {
    int data;
    struct Node *next;
} Node;

/* Queue holds both ends */
typedef struct {
    Node *front;
    Node *rear;
    int count;
} Queue;

void init(Queue *q) {
    q->front = NULL;
    q->rear = NULL;
    q->count = 0;
}

int isEmpty(Queue *q) {
    return q->front == NULL;
}

/* --- enqueue --- */
void enqueue(Queue *q, int value) {
    Node *new = (Node*)malloc(sizeof(Node));
    if(!new){
        printf("Error! Out of memory\n");
        return;
    }
    new->data = value;
    new->next = NULL;
    if(q->rear == NULL){
        q->front = new;
        q->rear = new;
    }
    else{
        q->rear->next = new;
        q->rear = new;
    }
    q->count++;
    printf("Added: %d\n", value);
}

/* --- dequeue --- */
int dequeue(Queue *q) {

    if(isEmpty(q)){
        printf("Error! Queue is empty\n");
        return -1;
    }
    Node *temp = q->front;
    int value = temp->data;
    q->front = q->front->next;

    if(q->front == NULL){
        q->rear = NULL;
    }
    free(temp);
    q->count--;
    return value;

}

void print(Queue *q) {
    printf("Queue (%d)" , q->count);
    for(Node *p = q->front; p; p = p->next){
        printf(" %d -> ", p->data);
    }
    printf("NULL\n");
}

/* frees all remaining nodes */
void clear(Queue *q) {
    while(!isEmpty(q)) {
        dequeue(q);
    }
}

int main(void) {
    Queue q;
    init(&q);

    enqueue(&q, 100);
    enqueue(&q, 200);
    enqueue(&q, 300);
    print(&q);

    printf("Dequeued: %d\n", dequeue(&q));
    print(&q);

    clear(&q);

    return 0;
}