如何计算方差？

Question

如何用迭代器计算方差？

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

}

-此方法计算平均值

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

Answer 1

基于此answer，您可以这样做：

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);
}

但奇怪的是在mean 中使用了List。如果不需要此Iterator，则可以：

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);
}

还有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);

}