Compare 2 doubles but without all the digits on C

Question

I want to compare 2 double but without all the digits. for example i have this double 1.548799778 and this 1.547990978 and I want to compare 1.xx from each. How can I "round" this double to 1.xx?? Thanks

Answer 1

One technique will be using the trunc function like this:

double d1, d2;
d1 = 1.548799778;
d2 = 1.547990978;
if (trunc(d1*100) == trunc(d2*100)) {
   // do your thing
}

Using 100 because you want two decimal places. You can use other numbers if you want more or less decimal places.

Answer 2

to compare that x and y are near enough, something like

(x==0.0 && y==0.0) || fabs(x-y)/max(fabs(x),fabs(y))<1e-6

to round x use floor(x) or ceil(x) as suggested by Jan Vorcak

Addenda: I'm not sure that it will work for NaN (the details give headache)

Answer 3

Another possibility is to just use a comparison with tolerance.

 if (fabs(a - b) <= tolerance) { .... }

choose your tolerance on how much precision you want.

For example, you can choose tolerance to be 0.001 to use about the first 2 digits.