[Haskell-cafe] Set of reals...?

[Jon Fairbairn]

On 2004-10-27 at 19:37+0900 Stijn De Saeger wrote:
> hello
> 
> Thanks for the explanation, at first it seemed like enumFromThenTo
> would indeed give me the functionality I am looking for. But then all
> of GHCi started acting weird while playing around... this is a
> copy-paste transcript from the terminal.
> 
> *S3> 0.5 `elem` [0.0,0.1..1.0]
> True
> *S3> 0.8 `elem` [0.6,0.7..1.0]
> False
> *S3> 0.8 `elem` [0.6,0.7..1.0]
> False
> *S3> [0.6,0.7..0.9]
> [0.6,0.7,0.7999999999999999,0.8999999999999999]
> *S3> 
> 
> ????????

> why would the floating point step size work the first time but not the
> second? confusing...

Doubles aren't real numbers, they're binary floating
point. 1/5 doesn't have a finite binary representation.

Also what gets printed out has been converted back to
decimal and rounded a bit. So don't expect too much from
them!

You probably want to use Double -> Bool, and possibly look
at infinite precision real arithmetic too.

 Jón

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk