Calculate percentage of 64 int
I have the following code:
typedef __int64 BIG_INT;
typedef double CUT_TYPE;
#define CUT_IT(amount, percent) (amount * percent)
void main()
{
CUT_TYPE cut_percent = 1;
BIG_INT bintOriginal = 0x1FFFFFFFFFFFFFF;
BIG_INT bintAfter = CUT_IT(bintOriginal, cut_percent);
}
开发者_StackOverflow中文版
bintAfter's value after the calculation is 144115188075855872 instead of 144115188075855871 (see the "2" in the end, instead of "1"??).
On smaller values such as 0xFFFFFFFFFFFFF I get the correct result.
How do I get it to work, on 32bit app? What do I have to take in account? My aim is to cut a certain percentage of a very big number.I use VC++ 2008, Vista.
double
has a 52 bit mantissa, you're losing precision when you try to load a 60+ bit value into it.
Floating point calculations aren't guaranteed to be perfectly accurate, and you've defined CUT_TYPE
as double
.
See this answer for more info: Dealing with accuracy problems in floating-point numbers
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