Double equals 0 problem in C

Question

I was implementing an algorithm to calculate natural logs in C.

double taylor_ln(int z) {
    double sum = 0.0;
    double tmp = 1.0;

    int i = 1;
    whi         


        

Answer 1

Don't use exact equality operations when dealing with floating point numbers. Although your number may look like 0, it likely to be something like 0.00000000000000000000001.

You'll see this if you use %.50f instead of %f in your format strings. The latter uses a sensible default for decimal places (6 in your case) but the former explicitly states you want a lot.

For safety, use a delta to check if it's close enough, such as:

if (fabs (val) < 0.0001) {
    // close enough.
}

Obviously, the delta depends entirely on your needs. If you're talking money, 10^-5 may be plenty. If you're a physicist, you should probably choose a smaller value.

Of course, if you're a mathematician, no inaccuracy is small enough :-)