如何计算二维数组中每一列的平均值? [英] How to calculate the average value of each column in 2D array?

问题描述

我正在尝试计算2D数组中列的平均值，但是我无法弄清楚代码.该函数应返回每列的平均值.而且我无法在函数中打印结果.打印应该在主要功能中.

I am trying to calculate the average value of columns in 2D array, but I cannot figure out the code. The function should return the average value of each column. And I cannot print the result in the function. The print should be in main function.

static double average_columns(double matrix[][]) {
    int i, j, sum = 0, average=0;
    for (i = 0; i < matrix.length; i++) {
        for (j = 0; j < matrix[i].length; j++) {
            sum=(int) (sum+matrix[i][j]);
        }
        average=sum/matrix[i].length;
        sum=0;
    }
    return average;
}

推荐答案

您可以使用

You can use Stream.reduce method to summarise the elements in columns and then divide each sum by the number of columns:

double[][] matrix = {
        {1.11, 2.22, 3.33, 4.45, 5.56},
        {6.61, 7.76, 8.88, 9.9, 10.11},
        {11.1, 12.2, 13.3, 14.4, 15.5},
        {16.6, 17.7, 18.8, 19.9, 20.0},
        {21.1, 22.2, 23.3, 24.4, 25.5}};

double[] average = Arrays.stream(matrix)
        // summarize in pairs
        // the rows of the matrix
        .reduce((row1, row2) ->
                // iterate over the indices
                // from 0 to row length
                IntStream.range(0, row1.length)
                        // summarize in pairs the
                        // elements of two rows
                        .mapToDouble(i -> row1[i] + row2[i])
                        // an array of sums
                        .toArray())
        // stream containing
        // the resulting array
        // Stream<double[]>
        .stream()
        // iterate over the array of sums
        // Stream<double[]> to DoubleStream
        .flatMapToDouble(Arrays::stream)
        // divide each element by
        // the number of columns
        .map(i -> i / matrix.length)
        .toArray();

System.out.println(Arrays.toString(average));
// [11.304, 12.416, 13.522, 14.61, 15.334]

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^{另请参见:}将2d数组中每一列的所有元素相加

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