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Assuming a 1 x M (A) and N x M (B) SimpleMatrix objects in ejml, is there a simple way to subtract A from B? I was looking for a way to repeat the rows of A to be the size of B, but didn't find the method to do this easily.

SimpleMatrix A = new SimpleMatrix(1, 2);
SimpleMatrix B = new SimpleMatrix(2, 2);

A.set(1.0);

B.setRow(0, 0, 2.0, 2.0);
B.setRow(1, 0, 4.0, 4.0);

// Throws java.lang.IllegalArgumentException
// The 'a' and 'b' matrices do not have compatible dimensions
SimpleMatrix C = B.minus(A);

// Expecting
// 1 1
// 3 3

Many answers using matlab (here and here), but I couldn't find a simple syntax for ejml.

grovduck
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1 Answers1

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According to docs:

Will concat A and B along their columns and then concat the result with C along their rows. [A,B;C]

So you could define an equation which will construct a matrix from repeated rows with like (I don't know the N value of B matrix): 

A.equation("A = [A,A,A]")

or

A.equation("A = [A,A,A]", "A")

The other option is to use SimpleBase.concatColumns(SimpleBase...), it looks like this:

A = A.concatColumns(A,A)

Assuming A is 1xM it will produce 3xM matrix and store it in A. If you desire to construct such array dynamically you could just concatenate "A," N times (without trailing coma of course) or pass N - 1 times matrix A to function.

UPDATE

Sorry, its late I wrongly assumed A is row vector, as it is column vector use comas instead of semicolons, as described in docs.

Jakub Licznerski
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    Thanks much. I assume you mean "comma" instead of "colon" in your answer (for row concatenation). I'm surprised you have to use an equation to accomplish this - I would have thought there was some kind of a repeat method. – grovduck Jan 17 '18 at 01:05
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    oooh, you've had the gut! there are methods `concatRows` and `concatColumns` but defined for `SimpleBase` (superclass for `SimpleMatrix`) so I didn't notice them at first glance... I'll update my answer accordingly – Jakub Licznerski Jan 17 '18 at 01:12