Comparison Operators Confusion

nullspirit

I am a bit confuse, I am having this problem with comparison operators which I can't think of the reason. Initially, i tot it was because I was using string type, then as I play around with the type, i realise, something else is wrong.

Running the code below on my pc return TRUE. $d > $c, where both are of the same type, and has a value of 5. Can someone please enlighten me?

$a = '8.80';
$b = '3.80';
$c = '5.00';

#settype($a, 'float');
#settype($b, 'float');
#settype($c, 'float');

$d = $a - $b;

if ($d > $c) {
	print 'c = '.$c.' of type '.gettype($c).'<br>';
	print 'd = '.$d.' of type '.gettype($d).'<br><br>';
	print 'yes';
} else {
	print 'c = '.$c.' of type '.gettype($c).'<br>';
	print 'd = '.$d.' of type '.gettype($d).'<br><br>';
	print 'no';
}

mtmosier

You might want to read the manual page about floats. Especially the note on percision.

nullspirit

i read the manual a bit. Seems like we should not be comparing floats with comparison operators. But then, how do we do it? 😕

mtmosier

You can certainly use comparison operators on floats. You just can't expect two floats to be exactly equal, which is what you're doing here.

Instead of just reading a bit of the manual page you should read the whole thing. The note on percision suggests using bc or gmp functions. Also the user comments have many suggestions on how to do it.

nullspirit

I see thanks. I read through the user post as well, but some of the thigns looks very alien to me. Let me go read it again and see what I'll find.

Roger_Ramjet

Floating point numbers are beloved by computer scientists and engineers of all kinds, and of little use or value to the rest of us. They are always only' approximate' values which is OK when doing complex maths but wrong when doing arithmetic. How wrong can be easily seen when you add 40+60 in floating point and end up with 99.89. 😕

Not a lot of use for most commercial websites which are trying to add up money and getting that wrong is serious stuff. The only solution is to use bcmath functions. If your host does not have them installed then MOVE. All the other common work-arounds are booby-traps waiting to happen.

nullspirit

Thanks. 🙂 I will try remember that.

For now, I wouldn't want to worry about server. And since my project is not too complex now, I am trying to convert all money related fields in database to record cents instead of the normal 1.60, 4.58..... I will try to avoid playing with floating numbers from now, until i have time to look into bcmath functions.

Roger_Ramjet

Yes, using price*100 to get integer value in cents/pence is a common work around. Just be carefull to divide it again when you display the total, and especially when you pass the details to your payment system. Charging someone 100 times the real price is not likely to win you many friends and could get your bank very upset with you.

Mysql does have the decimal column type so you can store prices acccurately, and you can do your arithmetic in your queries. You may find this siuts you better.