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

Or better numDigits = length . show It's probably even faster. 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]

2009/8/22 Roberto López :

...

Ok. I wonder if someone could help me with this problem...

I want to calculate the number of digits of a positive integer. I was thinking of ...

       numDigits n = truncate (logBase 10 n) + 1

But (logBase 10 1000) = 2.9999999999999996 so numDigits 1000 = 2.

Maybe adding a small amount

       numDigits n = truncate ((logBase 10 n) + 0.0000000000000005) + 1

Prelude> numDigits 100 3 Prelude> numDigits 1000 4 Prelude> numDigits 10000 5 Prelude> numDigits 10000 5 Prelude> numDigits 100000 6 Prelude> numDigits 1000000 7 Prelude> numDigits 10000000 8 Prelude> numDigits 100000000 9 Prelude> numDigits 1000000000 9                                          <---- This is wrong!!!! Prelude> numDigits 10000000000 11

Is there a reliable way to calculate the number of digits by means of logBase?

Regards!

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-- Eugene Kirpichov Web IR developer, market.yandex.ru

-- Eugene Kirpichov Web IR developer, market.yandex.ru