Incorrect result when performing simple math calculations

walanski

I'm getting weird, very small negative numbers when subtracting what should be the same two numbers, but only sometimes depending on the multiplier.

So, in this example, 9.95 is multiplied by 1 thought 100 and is subtracted by the same number, or in this case 1 times the same number.

Here's some sample code that demonstrates the issue:

for($i = 1; $i < 100; $i++)
{
$discountAmount = 100;
$orderTotal = 9.95 $i;
$discountAmount = $discountAmount/100; //convert to percent:
$discountTotal = $orderTotal $discountAmount;
$discountTotal = $discountTotal + 0.0001;
$discountTotal = round($discountTotal,2);
$orderTotal = $orderTotal - $discountTotal;
echo " ". $i . ": " . $orderTotal;
}

Here's a link that shows the results:

http://www.northerncoloradorentals.com/test/temp2.php

If I comment out both of these lines of code it works fine:

$discountTotal = $discountTotal + 0.0001;
$discountTotal = round($discountTotal,2);

But if I leave either one in, I have the same problem.

I don't think this is a case of not being able to represent certain floating point numbers...but I'm not entirely certain.

Any ideas would be greatly appreciated. Thanks.

LordShryku

It definately has something to do with the floating point value. Use bcsub if you can. Here's a small test I did with the first failure, which was 3.

$orderTotal     = (9.95 * 3);
$discountAmount = 1;
$discountTotal  = round((($orderTotal * $discountAmount) + 0.0001), 2);
echo $discountTotal."\n";
echo  bcsub($orderTotal, $discountTotal)."\n";
echo ($orderTotal - $discountTotal)."\n";

And here's the results:

29.85                <-- discountTotal
0                    <-- Final result with bcsub
-3.5527136788005E-15 <-- Final result without bcsub

walanski

Hi,

Thanks for the insight...I found one other way to fix this issue, after mulling yours over a bit and also considering it's a floating point problem. If I round the initial order total (9.95 x i) to 2 decimals, it works:

for($i = 1; $i < 100; $i++)
{
$discountAmount = 100;
$orderTotal = round(9.95 $i, 2); // round result of multiplication
$discountAmount = $discountAmount/100; //convert to percent:
$discountTotal = $orderTotal $discountAmount;
$discountTotal = $discountTotal + 0.0001;
$discountTotal = round($discountTotal,2);
$orderTotal = $orderTotal - $discountTotal;
echo " ". $i . ": " . $orderTotal; //. " \n " . $tempTotal;
if($orderTotal < 0)
{
$orderTotal = 0;
echo " ". $i . ": " . $orderTotal;
}
}

Thanks for your reply.