PHP Simple Math Calculation

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 =开发者_如何学JAVA 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;
}
?>

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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.

---

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

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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

---

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.

---

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."