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

The ration 3:5 is equivalent to 37.5% of the time or 0.375 (3 times it's A, 5 times it's B, so 3/8 is 37.5%). So you can calculate it like this:

random() < 0.375 ? "A" : "B"

From

http://en.wikipedia.org/wiki/Ratio

If there are 2 oranges and 3 apples, the ratio of oranges to apples is shown as 2:3, whereas the fraction of oranges to total fruit is 2/5.