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Date: Fri, 23 Jul 2004 13:32:38 +0200
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To: George Whiffen <george@ishop.co.uk>
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Subject: Re: fp, rounding, decimal arithmetic - definitive account?
From: cschneid@cschneid.com (Christian Schneider)

Hey George,

My point is: I'm not against improving accuracy of floating point 
arithmetic in PHP (although it could lead to some weird compatibility 
problems with older version) but I wouldn't advise people who really, 
really have to rely on their numbers to be correct to use floats anyway.


George Whiffen wrote:
 > Exactly my point.  When coding php you need floating point you can
 > trust or it is useless.  As I said, bc is probably the best currently

You can never trust floating point if you need both big numbers and 
precision at the same time. Floating point is always a trade off between 
the two.

 > If I'm wrong, and it's really only bc that can be trusted to add 0.1
 > and 0.1, then shouldn't we be using that internally, rather than
 > pretending to have a useable fp decimal arithmetic?

Floating point (even the current PHP implementation) is fine if you're 
happy enough with approximations which is the case 99% of the time.

 > P.S. I'm interested that you have money numbers so big that 15 digits
 > are not enough.  It can't be gross world income which still isn't up

I store cent values as integer strings and I possibly have to deal with 
values bigger than 214 million bucks (where PHP starts to convert ints 
to float) so I decided to go for BC Math. I didn't even care to check 
what the range/precision limits of floats are as I wanted something 
absolutely predictable, no matter what numbers are put into the system 
by our customers.

Just because you're paranoid doesn't mean they're not after you :-)
- Chris