php, bankers rounding, and my sanity.

stevesweetland

Ok then.

Here we have a number.

$net = 129;

Here we have its VAT;

$vat = $net * 0.175;

So then, if i go

print $vat;

i get 22.575. which is correct.

but, its not quite correct, i need to round it to two decimal places. so

$vat = round($vat,2);

i'll get 22.58, right? WRONG! php makes it 22.57.... try it!

i need a way to do a bankers rounding type function. people on php.net (on the 'round' manual page) have a couple posted, and iv tried them, and they dont work. the best one looks to be Hitlers Pet Gerbils function, but it just dont work.

and i need it to, really... to save not only my invoicing system, but also my sanity.

etully

Hmmm... works for me.

<?php

$net = 129;
$vat = $net * 0.175;
print "$vat\n";
$vat = round($vat,2);
print "$vat\n";

?>

[localhost ~]$ php testvat.php
22.575
22.58

[localhost ~]$

If I were you, I'd write my own rounding function:

<?php

function bankers_round($x) {
if ((floor($x*1000)-(10*floor($x*100))) > 4) { return ((floor($x*100)+1)/100); }
else { return (floor($x*100)/100); }
} 

echo bankers_round(129 * 0.175) . "\n\n";

?>

returns:

[localhost ~]$ php testvat.php
22.58

[localhost ~]$ vi testvat.php

stevesweetland

etully wrote:

If I were you, I'd write my own rounding function:

<?php

function bankers_round($x) {
if ((floor($x*1000)-(10*floor($x*100))) > 4) { return ((floor($x*100)+1)/100); }
else { return (floor($x*100)/100); }
} 

echo bankers_round(129 * 0.175) . "\n\n";

?>

returns:

that looks brilliant... ill try it

Installer

That function fails occasionally, e.g. when the value passed to it is 0.245 or 1.015. I think the problem is with too many floating point operations, each one potentially introducing a small error due to the inexactitude of the c fp library.

This one uses string and bcmath operations and returns correct results in my testing.

function my_round($num)
{
    if (!strpos($num, '.')) {
        return $num;
    }
    $num = '0' . $num;
    $num_arr = explode('.', $num);
    if (empty($num_arr[1]) || (strlen($num_arr[1]) < 3)) {
        return $num;
    }
    $units = ltrim($num_arr[0], '0');
    $cents = substr($num_arr[1], 0, 2);
    $extra = rtrim(substr($num_arr[1], 2), '0');
    $mils = substr($extra, 0, 1);
    if (($mils < 5) || (($extra == 5) && !($cents % 2))) {
        return $units . '.' . $cents;
    } else {
        return ltrim(bcadd(($units . '.' . $cents), '.01', 2), '0');
    }
}

Edit: The line

if (!strpos($num, '.'))

should, of course, be

if (strpos($num, '.') === false)

for well-known reasons.

etully

Nice catch Installer.