Question

I have a double[] and it has a value which is NaN. When I add up the array’s elements, the NaN makes the result NaN too.

How did it happen? How can I prevent this NaN from being added to the rest of my array?

Answer 1

NaN stands for Not-a-Number. It can arise in a variety of ways, for example as a result of 0./0., sqrt(-1), or as the result of a calculation involving other NaNs.

The easiest way to check whether v is a NaN is by using Double.isNaN(v):

public static double sum(double arr[]) {
  double sum = 0.0;
  for (double val : arr) {
    if (!Double.isNaN(val)) {
      sum += val;
    }
  }
  return sum;
}

edit: @Stephen C makes a good point in the comments: before deciding to ignore that NaN, it would be prudent to understand where it came from. It could be that it is the result of a bug elsewhere in your code, and by blindly ignoring the NaN you could simply be masking the bug instead of fixing it.