Simulating probability

Question

Stupid stupid question but here we are..

so, I want to generate some random events for an iPhone game but I guess this could be applied to any game written with a framework that suppports calls to "C" functions as I am using CCRANDOM that calls the random(void) function from the C stdlib.h.

I have written a function "verifyEvents" that I call each time I load a particular view (game scene).

To associate a probability to an event I assigned a value to each event (e.g. kill player: 0.05, give extra bonus 0.08).

In the function I generate a random number using CCRANDOM_0_1() and if the number is less than the assigned value I trigger the event (below there is the code).

Is this the best approach or do you use something else in your games?

-(void) verifyEvents
{
    float value = CCRANDOM_0_1() ;
    float eventPValue = 0.05f;

    if(value<eventPValue){
        CCLOG(@"EVENT!");
    }
}

PS: CCRANDOM_0_1() is defined as following:

#define CCRANDOM_0_1() ((random() / (float)0x7fffffff ))

Answer 1

Yes, your general approach is fine (define a subset of some range as the "activation" range and produce a random number, if that number falls into the activation range you trigger something, otherwise you don't) -- provided your method of random number generation produces a sufficient distribution for your needs.

As is noted in the comments, there are various implementation details that could factor in to whether or not your RNG is sufficient.

This isn't directly related to your question, but this article on probability in games was linked recently in the #gamedev IRC channel on Afternet, and I thought you might find it interesting reading.

Answer 2

Since I can't comment, I have to point to people suggesting what RAND_MAX should be, that RAND_MAX is implementation defined, and you can easily determine it with a little program.

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char ** argv)
{
    printf("RAND_MAX = %i\n", RAND_MAX);
    return 1;
}`

On my machine this gives RAND_MAX = 2147483647, much like what the gcc manual says. I would suggest an approach of

#define CCRANDOM_0_1() ((random() / (float)RAND_MAX ))

but actually, I would make this a function

float CCRANDOM_0_1() { return (random() /  (float)RAND_MAX) }

and also realize you probably want to seed the random number generator too. I wouldn't worry about the quality of your random number generator unless it becomes an apparent issue.

Back to your original question, I am not sure I fully understanding what you are trying to ask. Are you asking us if this is the best place to do this? They best way to do this? I think your code is generally fine, although I tend to be a "if it works" type programmer.