Question

Why is variance big enough of random by probability result?

Test code:

function probability($chances) {

    asort($chances);
    $sum    = array_sum($chances);
    $random = mt_rand(1, $sum);

    foreach($chances as $key => $chance) {
        if($random < $chance)
            return $key;
    }

    return $key;

}

$chances['case1'] = 10;
$chances['case2'] = 30;
$chances['case3'] = 60;

$result = array();

for($i = 0; $i < 100000; $i++)
    @$result[probability($chances)]++;

asort($result);
$sum = array_sum($result);

echo "Case\tCount\tOrig\tResult\n";

foreach($result as $key => $value)
    echo "$key\t$value\t".$chances[$key]."%\t".round($value / $sum * 100)."%\n";

Result:

Case    Count   Orig    Result
case1   14913   10%     15%
case2   33099   30%     33%
Case3   51988   60%     52%

Is it possible to adjust it somehow? I was trying to use mt_srand(), but it doesn’t help.

Info:

$ php -v
PHP 5.3.10-1ubuntu3.2 with Suhosin-Patch (cli) (built: Jun 13 2012 17:20:55) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
    with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans
    with Suhosin v0.9.33, Copyright (c) 2007-2012, by SektionEins GmbH

$ uname -a
Linux desktop 3.2.0-26-generic-pae #41-Ubuntu SMP Thu Jun 14 16:45:14 UTC 2012 i686 i686 i386 GNU/Linux

Answer 1

$sum    = max($chances);

max() dosn’t sum, use array_sum() insted

I got this result:

Case    Count   Orig    Result
case1   11068   10%     11%
case2   29672   30%     30%
case3   59260   60%     59%

From running this version of your code:

<?php

function probability($chances)
{
    asort($chances);
    $sum    = array_sum($chances);
    $random = mt_rand(1, $sum);

    foreach($chances as $key => $chance)
    {
        $random -= $chance;
        if($random <= 0)
        {
            return $key;
        }
    }

    return $key;
}

$chances['case1'] = 10;
$chances['case2'] = 30;
$chances['case3'] = 60;

$result = array();

for($i = 0; $i < 100000; $i++)
{
    @$result[probability($chances)]++;
}

asort($result);
$sum = array_sum($result);

echo "Case\tCount\tOrig\tResult\n";

foreach($result as $key => $value)
{
    echo "$key\t$value\t".$chances[$key]."%\t".round($value / $sum * 100)."%\n";
}
?>