queue tag

I'm studying mobile Q, but there's something I don't understand (indicated by <--)

#include <stdio.h>int a[4];int front = -1;int rear = -1;void enQueue(int in){ int i, j; If (ear < 3) { // <----Why should it be less than part 3 here?`` `` rear++; a[rear] = in; printf(a...