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

Edward Kmett ekmett at gmail.com
Sat Aug 29 21:12:27 EDT 2009

```I have a version of this inside of the monoid library buried in the
Data.Ring.Semi.BitSet module:

do any better by walking the raw Integer internals you need to know the
'finger' size for the GMP for your platform, which isn't possible to do
portably.

-Edward Kmett

On Wed, Aug 26, 2009 at 10:42 AM, Uwe Hollerbach <uhollerbach at gmail.com>wrote:

> Here's my version... maybe not as elegant as some, but it seems to
> work. For base 2 (or 2^k), it's probably possible to make this even
> more efficient by just walking along the integer as stored in memory,
> but that difference probably won't show up until at least tens of
> thousands of digits.
>
> Uwe
>
> ilogb :: Integer -> Integer -> Integer
> ilogb b n | n < 0      = ilogb b (- n)
>          | n < b      = 0
>          | otherwise  = (up 1) - 1
>  where up a = if n < (b ^ a)
>                  then bin (quot a 2) a
>                  else up (2*a)
>        bin lo hi = if (hi - lo) <= 1
>                       then hi
>                       else let av = quot (lo + hi) 2
>                            in if n < (b ^ av)
>                                  then bin lo av
>                                  else bin av hi
>
> numDigits n = 1 + ilogb 10 n
>
> [fire up ghci, load, etc]
>
> *Main> numDigits (10^1500 - 1)
> 1500
> *Main> numDigits (10^1500)
> 1501
> _______________________________________________