Apache Commons Math: how to perform sum along rows/columns?

I'm pretty new to Apache Common Math, so please pardon the trivial questions.

Based on the API doc, I'm unable to figure out how to perform column-wise or row-wise sum.

import org.apache.commons.math3.linear.RealMatrix;
import org.apache.commons.math3.linear.MatrixUtils;

double values = 1.0, 2.0, 3.0, 4.0, 5.0, 6.0;
RealMatrix mtx = MatrixUtils.createRealMatrix(values);


The above code generate a matrix like the following:

[[1, 2],
[3, 4],
[5, 6]]


What's the right syntax to perform column-wise sum? Which would give me:

[9, 12]


And how do I perform row-wise sum? Which would give me:

[3,
7,
11
]


For comparison, below is the syntax in Scala's Breeze library:

import breeze.linalg._
val mtx = DenseMatrix((1.0, 2.0), (3.0, 4.0), (5.0, 6.0))

// sum along the column direction
sum(mtx(::, *))
// Transpose(DenseVector(9.0, 12.0))


// sum along the row direction
sum(mtx(*, ::))
// DenseVector(3.0, 7.0, 11.0)


1 Answer
1

Seems there is no simple method to achieve that, how about generating a ones matrix to do the sum?, like:

//get the row sums
mtx.multiply(MatrixUtils.createRealMatrix(new double1, 1))
> Array2DRowRealMatrix3.0,7.0,11.0
//get the column sums
MatrixUtils.createRealMatrix(new double1, 1, 1).multiply(mtx)
> Array2DRowRealMatrix9.0,12.0


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