function tapply_mean($val, $fac)
{
// resembles a special case of R's tapply function
$levels = array();
foreach (range(0, count($fac) - 1) as $i) {
$val_i = $val[$i];
$fac_i = $fac[$i];
if (in_array($fac_i, array_keys($levels))) {
$levels_fac_i = $levels[$fac_i];
$levels_fac_i[] = $val_i;
$levels[$fac_i] = $levels_fac_i;
} else {
$levels[$fac_i] = array($val_i);
}
}
$means = array();
foreach (array_keys($levels) as $key) {
$means[$key] = array_mean($levels[$key]);
}
return $means;
}
function array_regression_a($arr1, $arr2)
{
return array_mean($arr2) - array_regression_b($arr1, $arr2) * array_mean($arr1);
}
