Calculating the sum for each array column

Question

I have a 2d-array data, as an example I choose those numbers:

int[][] data = {
        {1, 2, 3, 4},
        {1, 2, 3, 4},
        {1, 2, 3, 4}};

For each column I want to add the sum of numbers together and save them in seperate integers. This is the result im looking for:

c_zero = 3;
c_one = 6;
c_two = 9;
c_three = 12;

This is the code I have thus far:

int c_zero = 0;
int c_one = 0;
int c_two = 0;
int c_three = 0;

for (int a = 0; a < data.length; a++) {
    for (int b = 0; b < data.length; b++) {
        for (int[] row : data)
            for (int value : row) {
                if (int[] row == 0) { //this does not work
                    c_zero += value;
                }
                if (int[] row == 1) { //this does not work
                    c_one += value;
                }
                ...
            }
    }
}

How can I get the values for each row in a specific row?

Tim Biegeleisen · Accepted Answer · 2021-03-31 10:48:58Z

2

I would create a 1D integer array and use that to store the running sum for each column:

int[][] data = {{1,2,3,4},
                {1,2,3,4},
                {1,2,3,4}};
int[] colSums = new int[data[0].length];

for (int r=0; r < data.length; ++r) {
    for (int c=0; c < data[r].length; ++c) {
        colSums[c] += data[r][c];
    }
}

System.out.println(Arrays.toString(colSums)); // [3, 6, 9, 12]

answered Mar 31, 2021 at 10:48

Tim Biegeleisen

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score 1 · Accepted Answer · 2021-04-02 15:48:40Z

Using Java 8, you can apply the reduce method to the stream over the rows of a 2d array to sum the elements in the columns and produce a 1d array of sums.

// array must not be rectangular
int[][] data = {
        {1, 2, 3, 4},
        {1, 2, 3, 4},
        {1, 2, 3, 4, 5}};

int[] sum = Arrays.stream(data)
        // sequentially summation of
        // the elements of two rows
        .reduce((row1, row2) -> IntStream
                // iterating over the indexes of the largest row
                .range(0, Math.max(row1.length, row2.length))
                // sum the elements, if any, or 0 otherwise
                .map(i -> (i < row1.length ? row1[i] : 0)
                        + (i < row2.length ? row2[i] : 0))
                // array of sums by column
                .toArray())
        .orElse(null);

// output of an array of sums
System.out.println(Arrays.toString(sum));
// [3, 6, 9, 12, 5]

// output by columns
IntStream.range(0, sum.length)
        .mapToObj(i -> "Column " + i + " sum: " + sum[i])
        .forEach(System.out::println);
//Column 0 sum: 3
//Column 1 sum: 6
//Column 2 sum: 9
//Column 3 sum: 12
//Column 4 sum: 5

^{See also:

• Adding up all the elements of each column in a 2d array

• How to create all permutations of tuples without mixing them?}

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