[Haskell-cafe] How to calculate de number of digits of an
integer? (was: Is logBase right?)
derek.a.elkins at gmail.com
Sat Aug 22 13:31:31 EDT 2009
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]
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.
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