internal round off error with log?

kevinsoccer

Perhaps someone can point me in the right direction to where I can find out what is going wrong.

if I take the log of some integer number with a known integer return, say 8, I think that there is an internal round off error. Here is why.

$return = log( 8, 2); // should be 3, right?
$floor = floor( $return ); shouldn't this still be 3?

now I'll print the two values, and here's what I'm getting:

print "<br>return = '$return'<br>floor = '$floor'";
output: return = '3'
output: floor = '2'

now I'll change the code in step 1:
1a. $return = log( 8.000000001, 2 )

the final output is now
output: return = '3'
output: floor = '3'

Is there something that I should know about? I'm using PHP 4.3.8

Thanks for any help or suggestions.

laserlight

I dont have such an error with PHP 4.3.9, but floating point operations do suffer from such problems with precision even on other platforms and with other programming languages anyway.

kevinsoccer

Alright, it's as much as I expected. Thank you for your help.

Installer

If you do math work, you should check out BCMath Arbitrary Precision Mathematics Functions. For example, instead of:

$floor = floor( $return );

you could use:

$floor = bcadd($return, 0, 0); // returns 3

(True, the same result could be gotten with, e.g., "explode()", but this is just an example.)

kevinsoccer

Well, I wasn't actually doing anything intensive, other than trying to implement my own debugging function. (If you want to gander at what I was doing, take a look at <http://cs.earlham.edu/~hunteke/AlumniDB/debug.inc>.)

Unfortunately, I have to quit work on the project, due to my semester abroad coming up. However, I thank you for the help, and will take a look at BCMath for later purposes. I was shying away from BCMath for a couple of reasons however. One is for performance reasons. I haven't tested it myself, but somewhere along the line, I understood that while it afforded some excellent functionality and precision, most of the functions cost a fair amount of performance. Being a relative newbie to PHP, I listened to the hype. Perhaps I was wrong, but there you have.

TFTH,

Kevin

kevinsoccer

P.S. If you do gander at my work, suggestions, tips, and "YOU SHOULD DO THIS AND NOT THAT"s are welcome and appreciated. How better way to learn than by criticism!?