Question

I have a script which calls this function more than 100k times, so I am looking for anyway to squeeze a bit more performance out of it.

Can you suggest optimisations or an alternate method for calculating standard deviation in PHP?

function calcStandardDev($samples){


    $sample_count = count($samples);

    for ($current_sample = 0; $sample_count > $current_sample; ++$current_sample) $sample_square[$current_sample] = pow($samples[$current_sample], 2);

    return sqrt(array_sum($sample_square) / $sample_count - pow((array_sum($samples) / $sample_count), 2));

}

Answer 1

foreach by referance is faster than for, an you already have a loop, you can calculate “sum” in this loop. and $x*$x is so faster then pow($x,2);
there are some functions comparations. hope to help.

Your Function microtime = ~ 0.526

Second Function = ~ 0.290

  <?php
    function calcStandardDev($samples)
    {


        $sample_count = count($samples);

        for ($current_sample = 0; $sample_count > $current_sample; ++$current_sample) 
            $sample_square[$current_sample] = pow($samples[$current_sample], 2);

        return sqrt(array_sum($sample_square) / $sample_count - pow((array_sum($samples) / $sample_count), 2));

    }

    function calcStandardDev2($samples)
    {
        $sample_count = count($samples);

        $sum_sample_square  = 0;
        $sum_sample         = 0;

        foreach ($samples as &$sample) 
        {
            $sum_sample         += $sample;
            $sum_sample_square  += $sample * $sample; 
        }

        return sqrt($sum_sample_square / $sample_count - pow($sum_sample / $sample_count,2));

    }

     function calcStandardDev3($samples)
    {
        $sample_count = count($samples);

        $sum_sample_square  = 0;
        $sum_sample         = 0;

        foreach ($samples as &$sample) 
        {
            $sum_sample         += $sample;
            $sum_sample_square  += pow($sample ,2); 
        }

        return sqrt($sum_sample_square / $sample_count - pow($sum_sample  / $sample_count,2));

    }

    echo "<pre>";
    $samples = range(2,100000);

    $start  = microtime(true);
    echo calcStandardDev($samples)."\r\n";
    $end  = microtime(true);
    echo $end - $start ."\r\n";  
    echo "-------\r\n";

    $start  = microtime(true);
    echo calcStandardDev2($samples)."\r\n";
    $end  = microtime(true);
    echo $end - $start."\r\n";
    echo "-------\r\n";

    $start  = microtime(true);
    echo calcStandardDev3($samples)."\r\n";
    $end  = microtime(true);
    echo $end - $start;
    echo "-------\r\n";
?>