Question

I’m trying to design an odds system that goes from 1-100, however it also uses 0-1 for rarer odds.

I was told I should use a floating point format, but I don’t know that.

Basically I have..

if (mt_rand(1,1000)/100 == $odds) {} else if (mt_rand(1,100) == $odds) {}

however that only yields the same probability.

I looked up floating point format in the PHP manual, but the answers there couldn’t help me.

Answer 1

See Odds to understand how to convert your odds to a probability. If you have odds of 4:1 then there is a 1/5 == 0.2 probability of the event. If your odds are .2:1 then there is a 5/6 (about .833) probability of the event happening. In general, if the odds are m:n against then the probability is n/(m+n).

Now, if you want to simulate whether an event occurs or not, you need to get a random floating point number between 0 and 1 then check if this is less than the probability of the event. You can use something like mt_rand(0,1000)/1000 to get a random number between 0 and 1.

Examples:

$odds1 = 4; // 4:1
$prob1 = 1/($odds1+1); // 1/5

if( mt_rand(0,1000)/1000 <= $prob1 ) {
// event happened
}

$odds2 = .2; // .2:1
$prob2 = 1/($odds2+1);  // 5/6

if( mt_rand(0,1000)/1000 <= $prob2 ) {
// event happened
}