Need help with random number generation

[loopdydoo]

I need help writing a bit of code that makes something have a 5% chance of happening, and then if it doesn't happen, it needs to have a boolean variable equivalent to true. Can anyone help me?

[keskiverto]

When you throw a dice, you can get any of the six faces. What is the chance to get a 6? ~17% In other words, you can get any number from a throw, but getting a 6 happens with a 17% chance.

Throw a "dice" that has enough faces -- N -- and you get numbers from 0 to N-1. If 1/N is 5%, then getting one particular value has the chance of 5%.

[loopdydoo]

@keskiverto so would this be correct?

hitchance =  rand () % 25;
if ((hitchance/5) % 2 == 0)
    {
	   hit = true;
    }

[Disch]

For percentages I usually generate a floating point number between 0-1 and compare the number to the desired percentage.


Using C++11 <random> lib:

// elsewhere:
std::mt19937    rng;

// percentage checker
bool randPercentageCheck(double pct)
{
    static const std::uniform_real_distribution<double> dist(0,1); // <- random number between 0-1

    return (dist(rng) < pct);  // <- returns true of the generated number is less than our given
        //   percentage
}


// usage
hit = randPercentageCheck( 0.05 );    // check for 5% 

With the old style rand() function, it's not as convenient to work with floating points... so the easiest way there is to do something like this:

hit = (rand() % 100) < 5;

Or if you want to turn that into a reusable function:

bool randPercentageCheck(int pct)
{
    return (rand() % 100) < pct;
}

// ...
hit = randPercentageCheck( 5 ); // 5% chance of hitting