C in 100 Seconds: Queue

Queue — Enqueue, Dequeue, Circular Array

A queue is first in, first out (FIFO). The first item added is the first one removed. We build a circular queue using an array and modulo arithmetic.

The Queue Struct

typedef struct {
  int items[MAX];
  int front;
  int rear;
  int count;
} Queue;

front is where we dequeue from, rear is where we enqueue to, count tracks how many items are in the queue.

Enqueue — Add to Rear

void enqueue(Queue *q, int value) {
  if (is_full(q)) { printf("Queue full!\n"); return; }
  q->rear = (q->rear + 1) % MAX;
  q->items[q->rear] = value;
  q->count++;
}

The % MAX makes it circular — when rear reaches the end of the array, it wraps back to zero.

Dequeue — Remove from Front

int dequeue(Queue *q) {
  if (is_empty(q)) { printf("Queue empty!\n"); return -1; }
  int value = q->items[q->front];
  q->front = (q->front + 1) % MAX;
  q->count--;
  return value;
}

Read from front, advance front circularly, decrement count.

Circular Wrap

After dequeuing items from the front, those array slots are free. When we enqueue again, rear wraps around and reuses them. No wasted space.

Student Code

Try it yourself: episode47/queue.c