java - How can I calculate the variance?

Question

how can I calculate the variance with iterator?

public static double variance(final Iterator<Integer> array) {

}

-this method calculate the average

 private static double mean(final List<Integer> array) {
        return array.stream()
        .mapToInt(Integer::intValue)
        .average()
        .orElse(Double.NaN);
    }

Answer 1

Based on this answer you can do:

public static double variance(final Iterator<Integer> iterator) {
  List<Integer> array = new ArrayList<>();
  iterator.forEachRemaining(array::add);
  double mean = mean(array);
  double temp = 0;
  for(double a :array)
    temp += (a-mean)*(a-mean);
  return temp/(array.size()-1);
}

But it is strange that in mean is used List. If this Iterator is not needed than it can be:

public static double variance(final List<Integer> array) {
  double mean = mean(array);
  double temp = 0;
  for(double a :array)
    temp += (a-mean)*(a-mean);
  return temp/(array.size()-1);
}

And with stream

public static double variance(final Iterator<Integer> iterator) {
  List<Integer> array = new ArrayList<>();
  iterator.forEachRemaining(array::add);
  double mean = mean(array);

  return array.stream()
          .map(a -> (a-mean)*(a-mean))
          .reduce(0d, Double::sum)/(array.size()-1);

}