Rounding in PHP

Crazy Squirrel · 16 Feb 2007 at 14:22

Whats the deal with PHP rounding?

I've tried 4.3.2 on a win2k IIS box and 4.4.2 on a win2k apache box, and seem to have strange results using round($x , 2):

27.115 goes to 27.12
27.125 goes to 27.13
125.245 goes to 125.25
125.244 goes to 125.24
37.535 goes to 37.53 <----- Errrr why?

It's not using bankers rounding, 5 is being treated as go up, but that last one is being odd.....bug?

Different version of PHP?

Dj_Jestar · 16 Feb 2007 at 14:49

To do with the way Computers store number. I haven't got the details to hand.. but the short version is "It sucks. Such is life."

(53.535 may be rounded up from 53.5349, so it is closer to 53.53 than it is 53.54)

Crazy Squirrel · 16 Feb 2007 at 14:53

I was thinking it might be a precision problem, my issue is that I need it to match up with the calculations done by Visual Basic, which doesn't seem to have a precision problem with it's currency datatype. Hmm

Dj_Jestar · 16 Feb 2007 at 14:58

Code:

<?php

function safe_round ($val, $precision)
{
    $val = floatval(strval($val));
    return round ($val, $precision);
}

?>

Try that.

Crazy Squirrel · 16 Feb 2007 at 15:02

safe_round(37.535, 2) = 37.53

Dj_Jestar · 16 Feb 2007 at 15:09

Balls.

Hmm.. I've seen this elsewhere, but my maths sucks, and it looks bit.. odd:

Code:

<?php

        function safe_round($num, $round, $precision = 4)
        {
                $x = round($num, $round + $precision);
                return round($x, $round);
        }

?>

Crazy Squirrel · 16 Feb 2007 at 15:14

Thanks for trying, that doesn't seem to work either

Augmented · 16 Feb 2007 at 15:34

There appears to be a solution on the man page for round(), amazingly from a gerbil, and Hitler's pet gerbil at that:

Code:
In response to boonedocks at gmail dot com

Your calculations are slightly incorrect. You only use the formula when the 1/1000 position is equal to 5, otherwise you follow standard rounding proceedures.

I wrote a quick snippet which works with the input as a string, uses inline substrings and substitutions. Works VERY fast.

Code:

<?php function bankers_round ($moneyfloat = null) { $money_str = sprintf("%01.3f", round($moneyfloat, 3)); // convert to rounded (to the nearest thousandth) string $thous_pos = strlen($money_str)-1; // Thousandth string position $hundt_pos = strlen($money_str)-2; // Hundredth string position if ($money_str[$thous_pos] === "5") $money_str[$thous_pos] = ((int)$money_str[$hundt_pos] & 1) ? "9" : "0"; // use a bitwise test, its faster than modulus // The above statement assists round() by setting the thousandth position to 9 or zero if the hundredth is even or odd (respectively) return round($money_str, 2); // Return rounded value }

?>

Hitlers Pet Gerbil

I've not actually looked at it properly to see if's any good.

Crazy Squirrel · 16 Feb 2007 at 15:48

That's doing it to bankers rounding which I don't want but yes the principle seems to work. I've tweaked it so if the 1/1000 position is 5 then change it to a 9 and then round, else just round. Playing with it now and so far so good......

Crazy Squirrel · 16 Feb 2007 at 16:13

With a bit of quick tinkering i've got this working, though it needs a tidy up and is probably quite slow.

Code:

function super_round($moneyfloat = null)
{
	$a = strpos($moneyfloat, ".");
	if ($a > 0) {
		$a = $a + 3;
		if (substr($moneyfloat,$a,1) == "5") {
			$moneyfloat = sprintf("%01.3f", $moneyfloat);
			$moneyfloat[$a] = "9";
		}
	}
	return round($moneyfloat,2);
}

37.535 = 37.54
37.533 = 37.53
37.53499999999 = 37.53