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Algorithm for probability when looping

By: nezzario on Jan 17th, 2013  |  syntax: PHP  |  size: 1.62 KB  |  views: 38  |  expires: Never
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  1. <?php
  2.   // Based on code from:
  3.   // http://stackoverflow.com/questions/14389180/algorithm-for-probability-when-looping-over-a-randomly-ordered-array
  4.   // http://math.stackexchange.com/questions/280792/determining-probability-in-a-random-loop/280948#280948
  5.   // -------------
  6.  
  7.   // Taken from php.net/shuffle user notes
  8.   function shuffle_assoc($array) {
  9.     $keys = array_keys($array);
  10.     shuffle($keys);
  11.     foreach($keys as $key) {
  12.       $new[$key] = $array[$key];
  13.     }
  14.     return $new;
  15.   }
  16.  
  17.   $i = 1000000; // How many tests to perform
  18.  
  19.   // This is my rule list.  Each key is a simple color
  20.   // and each value is a probability represented as a percent
  21.   $rules = array(
  22.     'black' => 20,
  23.     'white' => 10,
  24.     'red' => 40,
  25.     'green' => 5,
  26.     'blue' => 25,
  27.   );
  28.  
  29.   // Initialize the scores array with all 0's
  30.   // The "outs" will be used when the probability does not
  31.   // occur in any of the rules
  32.   $scores = array('outs' => 0);
  33.   foreach($rules as $k => $v) {
  34.     $scores[$k] = 0;
  35.   }
  36.  
  37.   $count = count($rules);
  38.  
  39.   for($x = 0; $x < $i; $x++) {
  40.     $rules = shuffle_assoc($rules);
  41.  
  42.     $accumulated_probability = 0;
  43.     $rand = mt_rand(1,100);
  44.     foreach($rules as $k => $probability) {
  45.       $actual_probability = $probability + $accumulated_probability;
  46.       $accumulated_probability = $probability + $accumulated_probability;
  47.  
  48.       if($rand > $actual_probability) {
  49.         continue;
  50.       } else {
  51.         $scores[$k]++;
  52.         continue 2;
  53.       }
  54.     }
  55.     $scores['outs']++;
  56.   }
  57.  
  58.  
  59.   foreach($scores as $k => $v) {
  60.     echo "$k: " . (($v/$i)*100) . "% ($v/$i)\n";
  61.   }
  62.  
  63. ?>
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