Re: Peculiar floating point numbers in GCC
<n.torrey.pines@gmail.com> wrote in message
news:1175893775.034501.82990@p77g2000hsh.googlegroups.com...
I understand that with floats, x*y == y*x, for example, might not
hold. But what if it's the exact same operation on both sides?
I tested this with GCC 3.4.4 (Cygwin), and it prints 0 (compiled with -
g flag)
#include <cmath>
#include <iostream>
int main() {
double x = 3.0;
double f = std::cos(x);
std::cout << (f == std::cos(x)) << std::endl;
}
Using -O3 instead optimizes something, and now both sides are equal.
VC++8.0 prints 1 in both debug and release.
What does the standard have to say?
It's not really the standard that's the issue I dont' think, it's just the
way floating point math works.
In your particular case, those statements are close together, initializing f
and the comparison, so the compiler may be optimizing and comparing the same
thing. It all depends on how the compiler optimizes. Even something like:
double f = std::cos(x);
double g = std::cos(x);
std::cout << ( f == g ) << std::endl;
may output 1 or 0, depending on compiler optimization.
You just cant count on floating point equality, it may work sometimes, not
others.
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