Attempts to simulate the addition or removal of a value from a mean

modify-mean.php

<?php

/**
 * modify_mean
 *
 * Add or subtract a value from a mean
 * As if the original mean were calculate with/without it
 *
 * @param int $mean The current mean
 * @param int $points The number of data points
 * @param string $modifier The value to be added or subtracted, e.g. "-10"
 *
 * @return array Keys 'mean' and 'points' contain the resulting mean and number of points
 */

function modify_mean($mean, $points, $modifier) {

    // Calculate total of values
    $total = $mean * $points;

    // Add or subtract?
    if (substr($modifier, 0, 1) === '-') {

        // Subtract
        $point = substr($modifier, 1);
        $points--;
        $total = $total - $point;
        $mean = $total / $points;

        return array(
            'mean' => $mean,
            'points' => $points,
        );

    } else {

        // Add
        $point = substr($modifier, 1);
        $points++;
        $total = $total + $point;
        $mean = $total / $points;

        return array(
            'mean' => $mean,
            'points' => $points,
        );

    }

}