Java流。标准偏差

Question

outPut.stream()
            .collect(Collectors.groupingBy(e -> e.getCar(),
                    Collectors.averagingDouble(e -> (e.getHigh() - e.getLow()))))
            .forEach((car,avgHLDifference) -> System.out.println(car+ "\t" + avgHLDifference));

Answer 1

可以对此任务使用自定义收集器来计算平方和。buit-inDoubleSummaryStatistics收集器不跟踪它。这是专家组在本线程中讨论过的，但最终没有实现。计算平方和时的困难是对中间结果进行平方时的潜在溢出。

static class DoubleStatistics extends DoubleSummaryStatistics {

    private double sumOfSquare = 0.0d;
    private double sumOfSquareCompensation; // Low order bits of sum
    private double simpleSumOfSquare; // Used to compute right sum for non-finite inputs

    @Override
    public void accept(double value) {
        super.accept(value);
        double squareValue = value * value;
        simpleSumOfSquare += squareValue;
        sumOfSquareWithCompensation(squareValue);
    }

    public DoubleStatistics combine(DoubleStatistics other) {
        super.combine(other);
        simpleSumOfSquare += other.simpleSumOfSquare;
        sumOfSquareWithCompensation(other.sumOfSquare);
        sumOfSquareWithCompensation(other.sumOfSquareCompensation);
        return this;
    }

    private void sumOfSquareWithCompensation(double value) {
        double tmp = value - sumOfSquareCompensation;
        double velvel = sumOfSquare + tmp; // Little wolf of rounding error
        sumOfSquareCompensation = (velvel - sumOfSquare) - tmp;
        sumOfSquare = velvel;
    }

    public double getSumOfSquare() {
        double tmp =  sumOfSquare + sumOfSquareCompensation;
        if (Double.isNaN(tmp) && Double.isInfinite(simpleSumOfSquare)) {
            return simpleSumOfSquare;
        }
        return tmp;
    }

    public final double getStandardDeviation() {
        return getCount() > 0 ? Math.sqrt((getSumOfSquare() / getCount()) - Math.pow(getAverage(), 2)) : 0.0d;
    }

}

然后，可以将该类与

Map<String, Double> standardDeviationMap =
    list.stream()
        .collect(Collectors.groupingBy(
            e -> e.getCar(),
            Collectors.mapping(
                e -> e.getHigh() - e.getLow(),
                Collector.of(
                    DoubleStatistics::new,
                    DoubleStatistics::accept,
                    DoubleStatistics::combine,
                    d -> d.getStandardDeviation()
                )
            )
        ));

这将把输入列表收集到一个映射中，其中的值对应于相同键的High-Low的标准偏差。