Queue

Data Structures O(1) enqueue/dequeue O(n)

FIFO container implemented as a circular buffer: enqueue at the rear, dequeue from the front.

front

rear

Pick an operation to walk through the code.

InvariantItems leave from `front` and arrive at `rear`, both wrapping around, so order is preserved with no shifting.

queue.c

#define MAX 100
 
typedef struct {
    int items[MAX];
    int front, rear, size;
} Queue;
 
void enqueue(Queue *q, int value) {
    if (q->size == MAX) return;      /* full */
    q->rear = (q->rear + 1) % MAX;
    q->items[q->rear] = value;
    q->size++;
}
 
int dequeue(Queue *q) {
    if (q->size == 0) return -1;     /* empty */
    int value = q->items[q->front];
    q->front = (q->front + 1) % MAX;
    q->size--;
    return value;
}

How Queue works

First in, first out. Enqueue at the rear, dequeue from the front; a circular buffer wraps both indices around a fixed array so no element ever moves.

When to use it

Task scheduling, BFS, buffering between producers and consumers.