Here’s a very useful function to retrieve a randomly weighted key from an associative array in PHP.

Sometimes you need to fetch random elements from a collection but you need some elements to have a little more chance than others to be fetched (business rules or whatever…)

srand(time()); //dont call this inside the function or you'll get same result always
function getRandomWeightedKey($weighted_elements, $default_key) {
  $weighted_ranges = array();
  $weight_offset = 0;

  //if you know weights are not gonna change you could
  //move this part of the code to another place and cache it
  //for better performance. Specially if you have a lot of weighted elements
  foreach ($weighted_elements as $key => $weight) {
    $weighted_ranges[]=array('key'=>$key,
                             'range'=>array(intval($weight_offset),intval($weight_offset+$weight)));
    $weight_offset += $weight;
  }

  //get a random number between the 0 and the maximum
  $r = rand(0,$weight_offset);

  foreach ($weighted_ranges as $range) {
    if ($r >= $range['range'][0] && $r < $range['range'][1]) {
      return $range['key'];
    }
  }

  //this shouldn't happen but Mr. Justin Case.
  return $default_key;
}

//how to use it. (weights don't necessarily need to add to 100,
//but it helps me to visualize things basing weights on 100
$weights = array("a" => 25, "b" => 25, "c" => 50);

$randomKey = getRandomWeightedKey($weights, "c");

It should be trivial porting this code to the language of your choice. Enjoy.

This entry was posted on Tuesday, March 15th, 2011 at 7:10 pm and is filed under Code. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.