Circular Queue

A fixed-size queue can wrap its head and tail indexes around the array.

circular_queue.c

#include <stdio.h>

int main(void) {
    int count = ;
    int queue[4] = {0, 0, 0, 0};
    int head = 0;
    int tail = 0;

    for (int i = 0; i < count; i++) {
        queue[tail] = (i + 1) * 10;
        tail = (tail + 1) % 4;
    }

    int first = queue[head];
    head = (head + 1) % 4;
    int second = queue[head];

    printf("first=%d second=%d tail=%d\n", first, second, tail);
    return 0;
}

#include <stdio.h>

int main(void) {
    int count = ;
    int queue[4] = {0, 0, 0, 0};
    int head = 0;
    int tail = 0;

    for (int i = 0; i < count; i++) {
        queue[tail] = (i + 1) * 10;
        tail = (tail + 1) % 4;
    }

    int first = queue[head];
    head = (head + 1) % 4;
    int second = queue[head];

    printf("first=%d second=%d tail=%d\n", first, second, tail);
    return 0;
}

#include <stdio.h>

int main(void) {
    int count = ;
    int queue[4] = {0, 0, 0, 0};
    int head = 0;
    int tail = 0;

    for (int i = 0; i < count; i++) {
        queue[tail] = (i + 1) * 10;
        tail = (tail + 1) % 4;
    }

    int first = queue[head];
    head = (head + 1) % 4;
    int second = queue[head];

    printf("first=%d second=%d tail=%d\n", first, second, tail);
    return 0;
}

tail wrap Modulo arithmetic moves the tail back to zero after the final slot.

first out Reading from the head removes the oldest queued value first.