Re: Representing a double as a Fast scaled decimal
Daniel May / SpryWare, LLC <>
9 Oct 2008 10:48AM ETDimity,
There are two ways to look at this problem. If you know ahead of time that you are going to always use a fixed number of decimal precision (again, let's say your business rules require a precision of 4 decimal points), and you have already either rounded your float to those 4 points of precision, or truncation is okay for your application, then
the conversion is simple:
float x = 123.4500
int64_t mantissa = (int64_t)(x * 10000);
int32_t exponent = 4;
The more complicated case is when you are trying to generically convert a floating point number to a FAST scaled decimal with
a.) not losing any precision, and
b.) optimizing the exponent as to keep the mantissa as small as possible.
For this, take a look at modf() in the standard C library. It breaks a float into the whole and fractional parts. You can then cast or convert the float whole and fractional parts to integers. Next, you would remove any unnecessary precision from the fractional integer by using a mod and divide by 10 while there are trailing zeros left.
Finally, to create the FAST scaled decimal mantissa, you must determine the FAST exponent by inspecting the size of the fractional integer, and then adjust the whole integer by that factor, and add back the fractional part.
I am working on C/C++ an example for you...
/Daniel
