PHP Simple Math Calculation

Question

can help me to see this calculation? It suppose echo "equal"... but it give me "not equal"

<?php
$tl_pax = 1;
$ct_pax = 2;
$at_pax = 2;

$a = 0.5;
$b = 0.2;
$c = 0.2;
$d = 0.2;
$e = 0.2;
$f = 0.2;
$g = 0.2;
$h = 0.9;

$sum = $a + $b + $c + $d + ($e * $tl_pax) + ($f * $ct_pax) + ($g * $at_pax) + $h;

$total = 3;

if($total == $sum){
    echo 'equal: ' . $sum . ' - ' . $total;
}
else{
    echo 'not equal: ' . $sum . ' - ' . $total;
}
?>

possible duplicate of [Floating point inaccuracy examples](http://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples) — Gordon, Aug 27 '11 at 18:45

Femaref · Answer 1 · 2011-08-27T18:47:25.187

5

This is the usual case of the rounding error associated with binary floating point numbers. There are numbers that can't be represented exactly in binary, and thus the result will be of by some margin. To read up on it, the wikipedia article about floating point numbers is great.

The usual pattern found in this case is to pick a delta and compare against it:

 if(abs($total - $sum) < 0.01)
   echo "equal";

You'll have to pick your delta appropiately according to the usecase.

edited Aug 27 '11 at 18:47

answered Aug 27 '11 at 18:38

Femaref

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2

Maybe you should make that `if(abs($total - $sum) < 0.01)` – Medo42 Aug 27 '11 at 18:42

genesis · Accepted Answer · 2011-08-27T18:50:00.063

3

Check if they have difference less than 0.00001

if(abs($total - $sum) < 0.00001){

http://sandbox.phpcode.eu/g/56905/6

This article shows you why is this happening

edited Aug 27 '11 at 18:50

answered Aug 27 '11 at 18:34

genesis

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Loses the credibility of saying equal (might really not be equal). – Vadiklk Aug 27 '11 at 18:36
but then they are not necessarilly equal. – Madara's Ghost Aug 27 '11 at 18:37
@Rikudo Second ... I'm searching for better solution – genesis Aug 27 '11 at 18:39
@Bart no it wouldn't... Press F5 – genesis Aug 27 '11 at 18:43
Okay, that's correct, but now you just copied what 4 people before you already recommended... – Bart Kiers Aug 27 '11 at 18:44
@Bart It's not 100% true, because I was first with this question. Also, I do check for very smaller number than others. Think what you want – genesis Aug 27 '11 at 18:46
@genesis, unless there is some kind of bug with your floating point arithmetic, 9+1 is exactly 10. – Medo42 Aug 27 '11 at 18:47
@genesis If `$total = 5` and `$sum = 1000` then `$total - $sum < 0.00001`. – Paul Aug 27 '11 at 18:49
What does it matter if your answer was first? Your first answer was incorrect, and after 4 other people answer correctly, you edit your own question making it look like the other people's answers. Mind as well delete your duplicate answer, IMO. – Bart Kiers Aug 27 '11 at 18:51
@Bart I have also live example, other don't (excluding Paul) & link to PHP documentation – genesis Aug 27 '11 at 18:52

score 2 · Answer 3 · answered Aug 27 '11 at 18:41

It's because your sum is really something like 2.9999999999999999999, due to floating pont arithmetic. PHP just hides that from you when you print it. See the example on floor((0.1+0.7)*10) here: http://php.net/manual/en/language.types.float.php

You should never compare a floating point number for equality. The proper way to compare floats is using a range like:

if($total-0.0000001 <= $sum && $sum <= $total+0.0000001){

You can see it in action here: http://codepad.org/kaVXM5g0

That line just means that $total must be within 0.0000001 of $sum to be considered equal. You can pick the number yourself, depending on the amount of precision you need.

Alternatively you can just round $sum in this case, but then you're basically doing the same thing just with a range from 2.5 - 3.499... instead of 2.9999999 - 3.0000001

score 1 · Answer 4 · answered Aug 27 '11 at 18:40

The difference is due to the limits of floating point precision.

Values like 0.9 (9/10) can't be written exactly as binary floating point numbers, just like 0.3333... (1/3) can't be written exactly as a decimal fraction. This means that e.g. $h holds an inexact, rounded representation of 0.9. As a result, your calculation yields something very close to 3, but not exactly 3.

score 1 · Answer 5 · answered Aug 27 '11 at 18:41

Floats are evil.

Quote from http://php.net/float

"So never trust floating number results to the last digit, and never compare floating point numbers for equality. If higher precision is necessary, the arbitrary precision math functions and gmp functions are available."

PHP Simple Math Calculation

5 Answers5