Question

I’m using java and referring to the ‘double’ datatype. To keep it short, I’m reading some values from standard input that I read in my code as doubles (I would much rather use something like BigInteger but right now it’s not possible).

I expect to get double values from the user but sometimes they might input things like: 999999999999999999999999999.9999999999 which I think is beyond an accurate representation for double and get’s rounded to 1.0E27 (probably).

I would like to get some pointers on how to detect whether a specific value would be unable to be accurately represented in a double and would require rounding (so that I could refuse that value or other such actions).

Thank you very much

Answer 1

Most values entered by humans won’t be exactly representable with a double.

For instance, do you want to prevent the user from entering 0.1? That’s not exactly representable as a double.

To find the scale of the error, you could do something like:

BigDecimal userAsDecimal = new BigDecimal(userInput); double userAsDouble = double.parseDouble(userInput); BigDecimal doubleToDecimal = new BigDecimal(userAsDouble); BigDecimal error = userAsDecimal.subtract(userAsDouble).abs(); 

Then check whether ‘error’ is tolerably small.