Q:

How does one write a repeatable pseudo random number generator?

For example:
If rprng(seed,index) is my function, then for any pair of (seed,index), I should always get the same value for a given (seed,index).
For example:
rprng(4,2) = 17
rprng(4,5) = 21
rprng(4,2) = 17 

A:

int rprng(int seed, int index){
    srand(seed);
    while(index-- >0)
        rand();
    return rand();
}

