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

Derek Elkins derek.a.elkins at gmail.com
Sat Aug 22 13:33:47 EDT 2009

```On Sat, Aug 22, 2009 at 12:31 PM, Derek Elkins<derek.a.elkins at gmail.com> wrote:
> 2009/8/22 Eugene Kirpichov <ekirpichov at gmail.com>:
>> 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.
```

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