Math.PI comparision

[vu lee]

Any idea why this condition fail due to missing precision

    
  double pi = Math.PI;
  ....
  ....
  if(pi != Math.PI){
       System.out.println(" ????");
  }

Thanks

[Ernest Friedman-Hill]

Don't know what you mean; the code will compile, and the comparison will succeed. Do you observe otherwise? Tell us more details.

[Campbell Ritchie]

What happens depends on what is in between.

[vu lee]

In debug mode, I saw the value of one variable was 3.14159265358979323846 but the other was 3.1415926535897. It didn't happen all the time though.

[Campbell Ritchie]

Those figures from debug mode might not be precise. Is there any code which does arithmetic with pi in that block?

[vu lee]

Neither synchronized nor Lock, but the comparision occured in different thread. I'm in training, will post my code.

Thanks

[Rob Spoor]

You should hardly ever use == or != with flout or double comparison, just because of the missing precision. The following is better:

double epsilon = 0.000000001; // something small; the smaller the more accurate the comparison is
if (Math.abs(value1 - value2) < epsilon) // consider them as equal

Another option is using Double/Float.compare:

if (Double.compare(value1, value2) == 0) // consider them as equal

[Campbell Ritchie]

I tried that last night with a little arithmetic in between, and in all cases the two values appeared to be identical; it didn't print ???

But there might be something in between the lines shown which alters the value of pi; please tell us the details of what you are doing with pi in the interim.

And that is a splendid spelling error, Rob

[vu lee]

Thanks Rob. The Double.compare() has resolved my issues.