How to [portably] get DBL_EPSILON in C/C++

Question

I am using gcc 3.4 on linux (AS 3) and trying to figure out to get DBL_EPSILON number, or at least a decent approximation.

How to get it programmatically?

How can I do that?

Answer 1

It should be in "float.h". That is portable, it's part of the C and C++ standards (albeit deprecated in C++ - use <cfloat> or sbi's answer for "guaranteed" forward compatibility).

If you don't have it, then since your doubles are IEEE 64-bit, you can just steal the value from someone else's float.h. Here's the first one I found:

http://opensource.apple.com/source/gcc/gcc-937.2/float.h

#define DBL_EPSILON 2.2204460492503131e-16

The value looks about right to me, but if you want to be sure on your compiler, you could check that (1.0 + DBL_EPSILON) != 1.0 && (1.0 + DBL_EPSILON/2) == 1.0

Edit: I'm not quite sure what you mean by "programmatically". It's a standard constant, you aren't supposed to calculate it, it's a property of the implementation given to you in a header file. But I guess you could do something like this. Again, assuming IEEE representation or something like it, so that DBL_EPSILON is bound to be whatever power of 0.5 represents a 1 in the last bit of precision of the representation of 1.0:

double getDblEpsilon(void) {
    double d = 1;
    while (1.0 + d/2 != 1.0) {
        d = d/2;
    }
    return d;
}

Answer 2

In C++ it's std::numeric_limits<double>::epsilon().