How to multiply 2 double[][] matrices using streams?

Question

I'm wondering what the most compact and efficient way to multiple 2 double[][] arrays matrices using streams. The approach should follow matrix multiplication rules as illustrated here:

Matrix Multiplication: How to Multiply Two Matrices Together

Here's one way to do it using for loops (this is the first matrix):

final int nRows = this.getRowDimension();
final int nCols = m.getColumnDimension();
final int nSum = this.getColumnDimension();

final double[][] outData = new double[nRows][nCols];
// Will hold a column of "m".
final double[] mCol = new double[nSum];
final double[][] mData = m.data;

// Multiply.
for (int col = 0; col < nCols; col++) {
    // Copy all elements of column "col" of "m" so that
    // will be in contiguous memory.
    for (int mRow = 0; mRow < nSum; mRow++) {
        mCol[mRow] = mData[mRow][col];
    }

    for (int row = 0; row < nRows; row++) {
        final double[] dataRow = data[row];
        double sum = 0;
        for (int i = 0; i < nSum; i++) {
            sum += dataRow[i] * mCol[i];
        }
        outData[row][col] = sum;
    }
}

The procedure should fit the following test data:

double[][] md1 = {{4d, 8d}, {0d, 2d}, {1d, 6d}};
double[][] md2 = {{5d, 2d, 5d, 5d}, {9d, 4d, 5d, 5d}};

double[][] mb1 = {{4d, 8d}, {0d, 2d}, {1d, 6d}};
double[][] mb2 = {{5d}, {9d}};

Answer 1

A more compact and readable solution is to create a Stream over the rows of the first matrix, map each row to the result of multiplying it with the second matrix column and collect that back into a double[][].

public static void main(String[] args) {
    double[][] m1 = {{4, 8}, {0, 2}, {1, 6}};
    double[][] m2 = {{5, 2}, {9, 4}};

    double[][] result = Arrays.stream(m1)
            .map(r -> IntStream.range(0, m2[0].length)
                    .mapToDouble(i -> IntStream.range(0, m2.length)
                            .mapToDouble(j -> r[j] * m2[j][i]).sum())
                    .toArray())
            .toArray(double[][]::new);

    System.out.println(Arrays.deepToString(result));
    // [[92.0, 40.0], [18.0, 8.0], [59.0, 26.0]]
}

This will calculate m1 * m2 and the result will be in result. For the multiplication of each row, we can't create a Stream with Arrays.stream of the second matrix since this would create a Stream over the rows when we need a Stream over the columns. To counteract that, we simply go back to using an IntStream over the indexes.