Weighted Randomization In PHP

Do you need to make a random decision, but you want some options to have a higher likely-hood of getting picked? The weighted_random function below accomplishes this and its one I've been using for a while.

The example below will choose 'rain' 80% of the time and 'sun' 20% of the time. The sum of the weights(keys) is arbitrary, so I chose 100 because it's easier to think of percentages that way. However you could use any integer value for the weighting.

$options = array(80=>'rain',20=>'sun');
echo weighted_random(array_keys($options), array_values($options));

function weighted_random($weights, $values){ 
    $count = count($values); 
    $i = 0; 
    $n = 0; 
    $num = mt_rand(0, array_sum($weights)); 
    while($i < $count){
        $n += $weights[$i]; 
        if($n >= $num){
            break; 
        }
        $i++; 
    } 
    return $values[$i]; 
}

Now supposed you just want to toss a coin, but you want to determine the likely-hood of it returning True of False. Here's a simpler wrapper function that makes this easy. You simply pass in a value between 0 and 100 and it will return True that percentage of the time. So, if you pass in 50, it will return True 50% of the time, 80=80%, 35=35%, and so on.

echo percentage_decision(50);

function percentage_decision($percentage){
    $percentage = ($percentage>100)?100:$percentage;
    $percentage = ($percentage<0)?0:$percentage;
    $margin = 100-$percentage;
    $weights = array(1=>$percentage,0=>$margin);
    return (weighted_random(array_keys($weights), array_values($weights)) == 1);
}

I hope someone finds this helpful. If you you know of a drastically more efficient way of accomplishing this, please post your suggestions :)

Comments

[Kane]

Thanks for the code, exactly what I was looking for! Although there's a tiny problem with it. Your keys and values are in the wrong order. And you never actually used the variable $options ;)

Should be:
$options = array('rain' => 80, 'sun' => 20);
echo weighted_random(array_keys($options), array_values($options));

[Aaron]

Thanks for catching that. I updated fixed the array name & I also swapped the function arguments from (values, weights) to (weights, values).

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