Round number to specified number of digits

Question

Is there a simple function to round a Double or Float to a specified number of digits? I've searched here and on Hoogle (for (Fractional a) => Int -> a -> a), but haven't found anything.

Answer 1

Not sure whether any standard function exists, but you can do it this way:

 (fromInteger $ round $ f * (10^n)) / (10.0^^n)

Answer 2

It depends on what you are going to do with the rounded number.

If you want to use it in calculations, you should use Data.Decimal from Decimal library.

If you want just to format the number nicely, you should use Text.Printf from the standard library (base package).

Answer 3

λ: ((/100) $ fromIntegral $ round (0.006 * 100)) == 0.006
λ: False

λ: ((/100) $ fromIntegral $ round (0.06 * 100)) == 0.06
λ: True