C comparing 2 doubles returns wrong result

Question

I am comparing two doubles like the following:

bool isbig = fabs(sin1 - sin(x)) > 0.000001;
if (isbig)
    printf("fabs=%f, max=%f", fabs(sin1-sin(x)), 0.000001);

Now, for some reason it prints in the output:

fabs=0.000001, max=0.000001

Why doesn't the comparison work? The print should only work if the left side is bigger than the right, but in the print we can see the sides are equal. Can somebody help? Thanks

By print more digits do you mean try comparing different numbers? — INTODAN, Mar 26 '21 at 19:42

score 2 · Answer 1 · answered Mar 26 '21 at 19:43

2

The %f format specifier to printf rounds to 6 decimal places by default, so it's entirely possible that fabs is something like 0.00000125 or 0.0000010064. Try something like %.16f to see more digits, and you should see fabs larger than max.

(And if you notice that max doesn't print as 0.00000100000000000000, go read Is floating point math broken?)

answered Mar 26 '21 at 19:43

Nate Eldredge

48,811
6
54
82

Thanks. Then how do I compare to the accuracy of the first 6 digits? – INTODAN Mar 26 '21 at 19:45
@INTODAN That is the subject of a totally different question. ;) – Some programmer dude Mar 26 '21 at 19:46

C comparing 2 doubles returns wrong result

1 Answers1