Am 21.11.2010 um 18:54 schrieb Helmut Hartl: > While stabilizing my bullet physics port i saw some differences in > floating point behaviour between c++ and fpc32 and fpc 64 bit. > > In question is a simple dot product. > > The function : > result := a*d+b*e+c*f; > > Gives "different" results( 6.3846106530 vs 6.3846111610 ) depending > on the usage of intermediate variables for the final result and the fpc > version > 32 bit / 64 bit, while the C++ version does not show this behaviour and always > gives the same result. (XCode 3.2.1/ GCC on OSX) > > I only show the (extracted) compilable fpc exampe: > > program fpc_floatmul; > var a,b,c,d,e,f : single; > i,j,k : single; > result_same_as_c : single; > begin > a:= 1; > b:= -5.65984446; > c:= -3.4953573; > d:= 8.48976051; > e:= 1.5; > f:= -1.82659933; > i := a * d; > j := b * e; > k := c * f; > result_same_as_c := i + j + k; > writeln(result_same_as_c:10:10); > writeln(a*d+b*e+c*f:10:10); // different result > end. > > Running the above program gives > 1) C++ / FPC 64 Bit > 6.3846106530 > 6.3846106530 > 2) FPC32 Bit > 6.3846106530 > 6.3846111610 > > Is this explainable or wrong behaviour ?
Single has 7 significant digits. So it makes no sense to write more since you can't trust them. If you use in your case 10:6 instead of 10:10 in writeln() you'll get in all cases '6.384611'. And it makes no sense to store a value with more than these number of digits into a Single type. So perhaps the equation in the last writeln() is done internally in a higher precision than Single. When I use a calculator that can handle all you specified digits than I get 6,384611122290609 which is closer to the last result. What do you get when you replace Single and float by Double? Michael_______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
