Generating random integer from a range

Question

I need a function which would generate a random integer in given range (including border values). I don\'t unreasonable quality/randomness requirements, I have four requirem

Answer 1

Notice that in most suggestions the initial random value that you have got from rand() function, which is typically from 0 to RAND_MAX, is simply wasted. You are creating only one random number out of it, while there is a sound procedure that can give you more.

Assume that you want [min,max] region of integer random numbers. We start from [0, max-min]

Take base b=max-min+1

Start from representing a number you got from rand() in base b.

That way you have got floor(log(b,RAND_MAX)) because each digit in base b, except possibly the last one, represents a random number in the range [0, max-min].

Of course the final shift to [min,max] is simple for each random number r+min.

int n = NUM_DIGIT-1;
while(n >= 0)
{
    r[n] = res % b;
    res -= r[n];
    res /= b;
    n--;
}

If NUM_DIGIT is the number of digit in base b that you can extract and that is

NUM_DIGIT = floor(log(b,RAND_MAX))

then the above is as a simple implementation of extracting NUM_DIGIT random numbers from 0 to b-1 out of one RAND_MAX random number providing b < RAND_MAX.