How can I multiply a matrix by a vector using JAMA?

Question

I'm trying to create a vector from an array of doubles. I then want to multiply this vector by a matrix. Does anyone know how I can achieve this? Below is a really simple example that I would like to get working.

// Create the matrix (using JAMA)
Matrix a = new Matrix( [[1,2,3],[1,2,3],[1,2,3]] );

// Create a vector out of an array
...

// Multiply the vector by the matrix
...

Solution 1

Here is simple example of wanted operation:

double[][] array = {{1.,2.,3},{1.,2.,3.},{1.,2.,3.}}; 
Matrix a = new Matrix(array);   
Matrix b = new Matrix(new double[]{1., 1., 1.}, 1);     
Matrix c = b.times(a);  
System.out.println(Arrays.deepToString(c.getArray()));

Result:

[[3.0, 6.0, 9.0]]

In other words that is:

Solution 2

Why can't you use Matrix's arrayTimes method? A vector is just a 1 x n matrix (I think) so can't you initialize a second matrix with just 1 dimension and use arrayTimes?

Matrix a = new Matrix( [[1,2,3],[1,2,3],[1,2,3]] );
Matrix b = new Matrix( [[1,2,3]] ); // this is a vector
Matrix c = a.arrayTimes(b.transpose); // transpose so that the inner dimensions agree

This is what I think would work from reading the doc.

Comments

[Grzegorz Szpetkowski]

@Jon: Note that Matrix class API (math.nist.gov/javanumerics/jama/doc/Jama/Matrix.html) is your friend to get more options :)

[Joehot200]

Hi! Sorry to be an idiot, but could you please give us an example of what happens when [1 1 1] is something like [2 5 3] instead?

[Grzegorz Szpetkowski]

@Joehot200: Matrix multiplication is fairly easy to learn by yourself, this should make your clear how to do it. Essentially you multiply "rows by columns", each product is then summed up and placed accordingly.

[Joehot200]

Right. Got it. Thanks. I can't believe that I had got all the way to learning sines, cosines, tangent, vector multiplication, etc, and yet never encountered matrix-vector multiplication.