float equality

[Jonathan M Davis]

On Saturday 19 February 2011 04:06:38 spir wrote:
> Hello,
> 
> What do you think of this?
> 
> unittest {
>      assert(-1.1 + 2.2 == 1.1);          // pass
>      assert(-1.1 + 2.2 + 3.3 == 4.4);    // pass
>      assert(-1.1 + 3.3 + 2.2 == 4.4);    // fail
>      assert(-1.1 + 3.3 == 2.2);          // fail
> }
> 
> There is approxEquals in stdlib, right; but shouldn't builtin "==" be
> consistent anyway?

== is completely consistent. The floating point values in question are _not_ 
equal. The problem is that floating point values are inexact. There are values 
which a floating point _cannot_ hold, so it holds the closest value that it can. 
This can quickly and easily lead to having two floating points which seem like 
they should be equal but which aren't. == checks that they're equal, which is 
exactly what it should do (especially in a systems language).

The thing is, of course, that actual equality sucks for floating point values. 
What's really needed in something like approxEquals. So, approxEquals really 
_should_ be used for comparing floating points and == should only be used on them 
when that is actually what you need (which would be extremely rare). However, 
you can't really make == do approxEquals, because that means that == isn't 
really exact equality anymore and because then it makes it hard to actually 
compare floating point values for equality if that's what you actually want. 
Making == do approxEquals instead would make == inconsistent, not consistent.

This is just one of those cases where the programmer needs to know what they're 
doing and not use ==.

- Jonathan M Davis