Surprising float-int conversion in PHP

Question

After spending an hour trying to find out why a PHP script gave me incorrect output, it turned out that a loop ran one iteration short in one particular case.
To explain what is going on: a version number (2-digits, e.g. 1.5 or 2.0) is read from an XML-attribute and is multiplied by 100. The script later iterates over a defined range of version number multiples.

It turns out that 410 == 409, which gives a funny surprise if you compare a counter that increments in steps of 10 against that value.

Which brings me to my question: Am I fundamentally understanding something wrong? Certainly, 4.1, 100, and 410 should all be well-representable as float and should be well-convertible to int without rounding errors?

However, on my system (with PHP 5.3.2 CLI, Zend Engine 2.3.0), the following test case

<?
$a = 100 * 4.1;
$b = (string) $a;
$c = (int) $a;
$d = (int)(string) $a;

var_dump($a);
var_dump($b);
var_dump($c);
var_dump($d);
?>

outputs:

float(410)
string(3) "410"
int(409)
int(410)

I am now doing a (int)(string) conversion which works, but this is kind of a nasty hack that isn't pretty and doesn't quite feel right.
Is there a better (correct, no-hack) solution to get a precise result?

Answer 1

You usually can not see, if a value is well-convertible into a float just by looking at it. Read The warning in the manual for an example.

Answer 2

Your problem is in the first line:

$a = 100 * 4.1;

The float literal 4.1 does not, in fact, have the value 4.1, and so the result of multiplying it with 100 is not 410 but a little less. Apparently, casting to string rounds up, but casting to int rounds down.

To get the decimal math you expect, use the BC Math functions.