Problem with Floats! (in PHP)

Question

What's wrong with php floats?

$v = 5.2 * 3;

if($v==15.6) {
    echo 'Everything is fine =)';
} else {
    echo 'Okay something is wrong :S';
    var_dump($v); // float 15.6
}

Check this out too:

$v = 5.2 * 3;

if($v>15.6 AND $v<15.60000000001) {
    echo 'We are doomed :S';
    var_dump($v); // float 15.6
} else {
    echo 'Everything is fine =)';
}

I guess it has something to do with the internal representation or some obscure low level stuff? If that's the case, how do I get around this issue? Anything else I should know before using this to calculate real money transactions?

possible duplicate of [Real vs. Floating Point vs. Money](http://stackoverflow.com/questions/273371/real-vs-floating-point-vs-money) — Piskvor left the building, Nov 25 '10 at 11:35
*(reference)* http://de.php.net/manual/en/function.bccomp.php — Gordon, Nov 25 '10 at 11:35
This should be helpful: http://docs.sun.com/source/806-3568/ncg_goldberg.html — Piskvor left the building, Nov 25 '10 at 11:36

score 9 · Accepted Answer · answered Nov 25 '10 at 11:39

9

I am sure this is a duplicate, so I'm making this a CW

$v = 5.2 * 3;
if (bccomp($v, 15.6) === 0) {
    echo 'Everything is fine =)';
} else {
    echo 'Okay something is wrong :S';
    var_dump($v); // float 15.6
}

will give 'Everything is fine =)'

answered Nov 25 '10 at 11:39

Gordon

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score 4 · Answer 2 · answered Nov 25 '10 at 11:33

It has to do with the internal representation indeed :). Never compare float values. I think there will exists php classes/modules that work around this problem, but you can also store your money values as integers and multiply them by 100. Before display you can divide them again :)

score 2 · Answer 3 · answered Nov 25 '10 at 11:33

This has to do with floats in general and is not limited to PHP.

When using floats you should check for intervals with a specific precision, just as you did in your last example:

if(abs($v - 15.6) < 0.0001)
{
    // Yey :)
}

You can run into a lot of problems when using exact values.

score 2 · Answer 4 · edited May 23 '17 at 12:00

2

the value you are getting is 15.600000000000001 and it is diferent to 15.6, this topic has been widely discussed, you can take a look to this threads:

HTH, Regards

edited May 23 '17 at 12:00

Community

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answered Nov 25 '10 at 11:33

SubniC

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score 2 · Answer 5 · answered Nov 25 '10 at 11:36

2

The PHP page on floats has a warning against comparing for equality and also links to this page which links to bccomp.

answered Nov 25 '10 at 11:36

crdx

1,412
13
26

score 2 · Answer 6 · answered Nov 25 '10 at 11:37

2

If you really need to compare floats check for being in bounds of an allowed deviation instead of an exact match, like

$DEVIATION = 0.0001;
if(abs($v-15.6)<$DEVIATION) ...

answered Nov 25 '10 at 11:37

Ray

396
3
10

score 1 · Answer 7 · answered Nov 25 '10 at 11:44

$v = 5.2 * 3;

if((string)$v == "15.6") {
    print "yes";
}

It's always good to check PHP website as most developers have come across problems before PHP.net Floating Point Number this comment shows a function for comparing floats.

<?php 
    function IsFloatEqual($x,$y,$precision=0.0000001) 
    { 
        return ($x+$precision >= $y) && ($x-$precision <= $y); 
    } 
?>

score 1 · Answer 8 · answered Nov 25 '10 at 12:07

1

Just a minor addition for anywone looking:

the BC* functions are the only true way to work with floats in PHP!

answered Nov 25 '10 at 12:07

Gekkie

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Problem with Floats! (in PHP)

8 Answers8

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