Random Function With Probability

Question

I have an array containing the following data:

(
    [Blue] => 15.3
    [Red] => 64.7
    [Green] => 20.0
)

The probability of Blue being picked is 15.3%, Red has a high chance of being picked with 64.7%, and Green has a 20% chance.

I need to create a function that will pick one of these colours at random, but also factoring in probability.

Answer 1

I am pretty sure that this function should do the trick for you, you can adjust it for points after the decimal if needed:

Edit: Did a touch of actual testing and modified the code ever so slightly (as well as adding loads of output for you to see what is happening:

<?php 
function chance($input=array())
{
    echo 'The Max Value can be: '.(array_sum($input)*10).'<br>';
    $number=rand(0,array_sum($input)*10);
    echo 'Checking for: '.$number.'<br>';
    $starter=0;
    foreach($input as $key => $val)
    {
        $starter+=$val*10;
        echo 'Current value being tested against is: '.$starter.' which is '.$key.'<br>';
        if($number<=$starter)
        {
            $ret=$key;
            break;
        }

    }

    return $ret;
}

$array=array('Blue' => 15.3, 'Red' => 64.7, 'Green' => 20.0);

for($i=0;$i<10;$i++)
{
    echo chance($array).'<br><br>';
}
?>

Example Output:

The Max Value can be: 1000
Checking for: 355
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 63
Current value being tested against is: 153 which is Blue
Blue

The Max Value can be: 1000
Checking for: 692
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 803
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Current value being tested against is: 1000 which is Green
Green

The Max Value can be: 1000
Checking for: 360
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 174
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 117
Current value being tested against is: 153 which is Blue
Blue

The Max Value can be: 1000
Checking for: 769
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 462
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

The Max Value can be: 1000
Checking for: 418
Current value being tested against is: 153 which is Blue
Current value being tested against is: 800 which is Red
Red

Answer 2

Using mt_rand you can pick a min and a max. So you use 0 to 100 for instance.

$randomValue = (mt_rand ( 0 , 1000 ) / 10)

Then, just pick the breach appropriate to your percentage like this:

if($randomValue <= 15.3) { $pick = 'Blue';}
elseif($randomValue <= 64.7) { $pick = 'Red';}
else { $pick = 'Green';}

Using your array you could easily to this in a function. Just sort your array by value and then handle appropriately.

function getRandomColor($colorsWithProbability) {
  sort($colorsWithProbability);
  $last = '';

  foreach($colorsWithProbability as $color => $probability) {
    $last = $color;
    if($randval <= $probability) {
      return $color;
    }
  }
  return $last;
}