float multiplication

[kafine]

hi

i've seen this problem when i took a java class:

$x = 1.8;
$y = 1.2;
if($x>$y*1.5) exit();

the above code will exit even though $x = $y*1.5
the difference between the two is : 2.2204460492503E-16
does anyone know why this happens? and more importantly,
any ideas on how to deal with this?
thanks!
keith

 

[fortytwo]

Does this happen in PHP? It could be a Java thing.

 

[kafine]

yeah, sorry. this just happened in php3. I just meant that I remember having the same problem in Java and wasn't too sure how to deal with it then. the big problem is that I have these checks all over my production site. I guess the only thing to do is to round the variable unless you can think of a better solution.
But it seems like whenever you multiply a float by another float, the answer is not precise. In my case the difference was trivial (like 2E-16) but that's enough to return true in the above if statement.
thanks,
keith

 

[fortytwo]

What do you get if you set the output of the calculation to a variable and print it?

I had a word with one of the c programmers and they said that it was possible that the numbers were just too large and the variables couldn't hold so many decimal places.