Error Calculating with Decimals
Thu, 04/22/2010 - 15:45
I feel a bit odd/special/dumb since half of the topics in this forum are from me. Maybe I'm the only one that runs into these issues. ;)
I'm running into a very strange issue where very cut and dry math is getting distorted by PHP. Here's the function:
private function GetListPrice($stockingPrice)
{
$price = $stockingPrice / .7;
$decimal = $price - floor($price);
if ($price > 100)
return ceil($price);
else
{
if ($decimal <= .5)
return floor($price) + .5;
else
return floor($price) + 1;
}
}If I give this function a value of 175, it returns 251. It should be 250 (175 / .7 = 250). When I look at it with the debugger, $stockingPrice = 175, $price = 250 but $decimal is 2.8421709430404E-14, which is causing the ceiling function to return 251.
Any ideas why this is happening? It's jacking up our pricing! Thanks!
Unfortunately, that's just how computers represent numbers internally. You may want to try and avoid floating point (fractional) numbers whenever possible by doing this instead:
<?php$price = $stockingPrice * 10/7;
$decimal = $price - floor($price);
?>
In addition, you may want to account for floating point imprecision by only using ceil if $decimal is > 0.0000001, and otherwise using floor;
or better do a rounding.
<?php$price = round ( $stockingPrice/0.7 , 2 ) ;
?>
So you are sure, not to get unwanted fractions.
As VexedPanda said, it has to to with number representation of floats.
You don´t have the datatype decimal (16,2) in PHP.
http://en.wikipedia.org/wiki/Floating_point