[Haskell-cafe] Problem with percentage function?

[Tom Ellis]

On Sun, Apr 20, 2014 at 09:12:29PM +0000, Chapman, Anthony Sergio wrote:
> -- Turns a number into a percentage 
> toPerc :: Double -> Double
> toPerc x = 100*(myRound x 4)
> -- Rounds a number to s decimal points 
> myRound n s = fromIntegral (round (n * factor)) / factor
>     where factor = fromIntegral (10^s)
> 
> I would it to round to 2 decimal places no matter what the size of the input is
> 
> ei toPerc 0.22222222222222  --- = 22.22
>     toPerc 0.22342222222222  --- = 22.34 
> 
> The problem is that what I actually get is 22.220000000000002

The other replies, whilst correct, have been a bit oblique to your question. 
The upshot is you should do the rounding when displaying, not in the
floating point value itself.


import Text.Printf

toPerc :: Double -> String
toPerc = printf "%.2f" . (*100)

*Main> toPerc 0.22222222222222
"22.22"
*Main> toPerc 0.22342222222222
"22.34"


Tom