Question

i am not quite sure how this function in c++ works:

int rand_0toN1(int n) {
    return rand() % n;
}

Another tutorial on internet says to get a random number between a range you need to do something different however, with a being first number in range and n is number of terms in range:

int number = a + rand( ) % n;

I have read that it is supposed to return a random number between the value of 0 and n-1, but how does it do that? I understand that % means divide and give the remainder (so 5 % 2 would be 1) but how does that end up giving a number between 0 and n-1? Thanks for help in understanding this. I guess i don’t understand what the rand() function returns.

Answer 1

The modulo (remainder) of division by n > 0 is always in the range [0, n); that’s a basic property of modular arithmetic.

a + rand() % n does not return a number in the range [0, n) unless a=0; it returns an int in the range [a, n + a).

Note that this trick does not in general return uniformly distributed integers.