Question

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How to generate a random number from within a range – C

I saw the following code from programming pearls

int randint(int l, int u)
{   return l + (RAND_MAX*rand() + rand()) % (u-l+1);
}

Can anyone help me explain it?

Can we just use

return l + rand() % (u-l+1);

Thanks,

Answer 1

The problem with using rand() % n to get a number between 0 and n-1 is that it has some bias when n is not an exact divisor of RAND_MAX. The higher the value of n, the stronger this bias becomes.

To illustrate why this happens, let’s imagine that rand() would be implemented with a six-sided die. So RAND_MAX would be 5. We want to use this die to generate random numbers between 0 and 3, so we do this:

x = rand() % 4

What’s the value of x for each of the six outcomes of rand?

0 % 4 = 0
1 % 4 = 1
2 % 4 = 2
3 % 4 = 3
4 % 4 = 0
5 % 4 = 1

As you can see, the numbers 0 and 1 will be generated twice as often as the numbers 2 and 3.

When your use case doesn’t permit bias, this is a better way to calculate a random number:

 (int)((double)rand() / (double)RAND_MAX * (double)n)