Conversion from string to double - Possibility and errors

Question

I am aware that the string 2.34 would never be equal to the double 2.34. No matter what library or algorithm you tried (lexical_cast,atof). Also 2.3400 can not be represented as double type. Instead it will be equal to 2.3399999999999999 . A little background I am working on an application that passes of values to an external application using its api. Think of it as some sort of a trading application. The user can pass values using the applications api or the user can pass value by using the application directly.Now when the user uses the application directly and the user types in 2.34 the value is processed as 2.34 however when I use the API which requires double as a parameter I pass 2.34 and it passes of as 2.3399999999999999 which is not acceptable. My question is how would the application be handling this and is there a way to store 2.34000.. in a double so that I could pass it to an API ?

Answer 1

If you need to pass decimal values through an API which takes double but you need to get the exact values, there isn't much of a problem: As long as you don't use more than std::numeric_limits<double>::digits10 digits, you can recover the original decimal value although not necessarily the same representation (trailing fractional zeros will be lost). To do so, you need to convert the original decimal string into the closest representation as double and later use a suitable algorithm to restore the best decimal representation again. The parsing and formatting functions from the C and C++ standard libraries will do that correctly for you.

Note that you shouldn't try to do any arithmetic on the double values when you want to restore the original decimal values: the result of double arithmetic will use binary rounding and the values won't be the closest decimal values. However, as long as you only transfer the double values, there is no problem.

Answer 2

Since you mention "trading application" I will conclude that the numbers represent currencies. If that is the case you are probably dealing with a fixed number of fractional digits as well. In that case you can scale your floating point numbers by multiplying them by 10 ^ number_of_fractional_digits, essentially making them integer values. Floating point numbers can accurately store integer values (as long as they do not exceed the floating point type's range).

Another possibility - if the assumptions above are correct - would be to use Binary-coded decimals.

Answer 3

The one way to work around floating point precision issues is using a well made fraction class. You may code one for yourself or use the ones provided by common math libraries. Such classes will represent your 2.34 as 234/100 internally, which will lead higher amount of memory consumption compared to a single float.