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

On Sat, Aug 22, 2009 at 12:31 PM, Derek Elkins wrote:

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.

Note that logBase 10 will start failing for large Integers (or rather fromIntegral will.) Writing an integer log using a binary search would be relatively easy, reasonably efficient, and would work for all Integers.