How to choose randomly in a certain ratio

Question

I want to choose randomly* between two alternatives with unequal probability.

For instance, when the user presses a button, 25% of the time it would make sound A and

Answer 1

Assuming your random number generator gives back a double value between 0.0 and 1.0, you just do a comparison to the exact ratio you want. In the case of 3 out of 5, you'd check to see if the random value was less than 0.6.

if(rand < 0.6) {
    playSound(A);
}
else {
    playSound(B);
}