Elegant way to generate a random value regarding p

Question

I have N values (integer). I'd like to know what is the most elegant way to randomly pick one of those values regarding a percentage. For example, for a 3 values example:

• Value 1 has 30% chance to get picked
• Value 2 has 12% chance to get picked
• Value 3 has 45% chance to get picked

I need this for a program i'm developing with Java but a pseudo code algorithm or a code in any other language would be ok.

Answer 1

Proposed algorithm:

• generate a random number (n) between 0 and 1 (assuming your random generator is well distributed)
• if n < 0.30 return value 1
• if n < 0.42 return value 2
• else if n < 0.87 return value 3
• else say Hello (your numbers don't add up to 100%)

Answer 2

One way of doing this without calculating values to use is

double d = Math.random() * 100;
if ((d -= 30) < 0) return 1;
if ((d -= 12) < 0) return 2;
if ((d -= 45) < 0) return 3;
return 4;