Code for implementing queue with ring buffer

Question

This is the part of a program for implementing queue with ring buffer in C. And I don't understand the line 8-9. What exactly does this line if(q->rear == q->max) q->rear = 0; mean?

if the rear index equals the max capacity... then assign zero to rear? T_T Help me please!

int Enque(IntQueue* q,int x)
{
    if (q->num >= q->max)
        return -1;
    else {
        q->num++;
        q->que[q->rear++] = x;
        if(q->rear == q->max)
            q->rear = 0;
        return 0;
    }
}

Answer 1

q->que[] is an array of integers. Individual array q->que[] integer elements are accessed by specifying their index in the array such as: q->que[n]; where n is a value from 0 to (q->max - 1).

q->rear represents an index into the array q->que[]. The value of q->rear may be anywhere from 0 through (q->max -1). Hence, if q->rear ever becomes equal to q->max, it would represent an index that is beyond the end of the q->que[] array, and (being a circular queue) must be positioned back to the beginning of the array (q->que[0]).

Hence, the logic of:

if (q->rear == q->max)
    q->rear = 0;