float equality

[Walter Bright]

Kevin Bealer wrote:
> 1. To solve the basic problem the original poster was asking -- if you are working
> with simple decimals and arithmetic you can get completely accurate representations
> this way.  For some cases like simple financial work this might work really well.
> e.g. where float would not be because of the slow leak of information with each
> operation.  (I assume real professional financial work is already done using a
> (better) representation.)

A reasonable way to do financial work is to use longs to represent pennies. 
After all, you don't have fractional cents in your accounts.

Using floating point to represent money is a disaster in the making.


> 2. To explain why the 'simple' task of representing something like .1 wasn't as easy
> as it looks.  In other words, why the people who designed float weren't just brain
> dead.  I think they really knew what they were doing but it shocks most people at
> first that a modern computer can't do what they see as grade school arithmetic.

There's just no getting around needing to understand how computer arithmetic works.


> I think for some purposes though, lossless domain specific representations can
> be a good tool -- if you can represent a problem in a way that is lossless you can
> maybe do better calculations over long series than working with 'double' and taking
> the accuracy hit.  This is necessarily an application specific technique though.

I.e. you have to know what you're doing :-)