changing probability of getting a random number

Question

I would like to generate a random number between 0 and 3 and I have the following in my code:

int random = rand() % 4;

This works fine but I would like it to generate 1, 2, and 3 most of the time and 0 only occasionally.

What is the best way to go about this? What are the names of common algorithms to address this problem?

Answer 1

Here's one way. Suppose you want 0, 1, 2, 3 to have a distribution of 5%, 20%, 30%, 45%.
You could do it like this:

double val = (double)rand() / RAND_MAX;

int random;
if (val < 0.05)       //  5%
    random = 0;
else if (val < 0.25)  //  5% + 20%
    random = 1;
else if (val < 0.55)  //  5% + 20% + 30%
    random = 2;
else
    random = 3;

Of course it doesn't have to be done with floating-point. I just did it this way since it's more intuitive.

Answer 2

You can use the discrete_distribution class from the random library.

#include <iostream>
#include <random>
#include <ctime>

int main()
{
    std::discrete_distribution<> dist({ 1.0, 4.0, 4.0, 4.0 });  
    std::mt19937 eng(std::time(0));
    for (int i=0; i<100; ++i)
        std::cout << dist(eng);
}

Demo: http://ideone.com/z8bq4

If you can't use C++11, these classes also exist in boost.