Find number of elements in a Circular Queue

Question

How do I find the number of items in a circular queue? |front - rear| doesn't always work.

Is there one formula to know how many elements are there in a circular queue using front, rear and size of the array?

Answer 1

actually the size would be,

size = front > rear ? (MAX - front + rear + 1) : (rear - front + 1);

or one can go for a generic formula:

size = abs(abs(MAX - front) - abs(MAX -rear));//this works in every situation

Answer 2

Assuming you implement it using an array with size N so there are pointers pointing to the front and rear. Use the following formula:

size = front > rear ? (front - rear) : (front+N -  rear);