Re: [Haskell-cafe] How to calculate de number of digits of an integer? (was: Is logBase right?)

2009/8/22 Eugene Kirpichov :

Use 'round' instead of 'truncate'.

Prelude> let numDigits = (+1) . round . logBase 10 . fromIntegral Prelude> map (numDigits . (10^)) [0..9] [1,2,3,4,5,6,7,8,9,10]

round won't work because 999 is close to 1000. You simply need to use logBase 10 as a guess and then check the answer, e.g. numDigits n | n < n' = e | otherwise = e + 1 where e = ceiling $ logBase 10 $ fromIntegral n n' = 10^e This will need to special case 0 which it currently doesn't do.