F# Math.Net Matrix.mapRows to create new matrix with different size

Question

I have a function that manipulates a Vector<float> resulting a new Vector<float> with different length, an example would be appending a number in front of the vector

let addElementInfront (x:Vector<float>) =
    x.ToArray() 
    |> Array.append [|x.[0]|]
    |> vector

Now I want to apply this to all the rows of a (2x2) matrix and I would expect a (2x3) matrix, I tried to use the Matrix.mapRows of MathNet.Numerics.LinearAlgebra but it gives me error that the size needs to be the same.

Just wonder if MathNet has any other function to map rows that results a different size matrix.

Thanks.

Answer 1

It seems you are trying to duplicate the first column of the matrix. For example:

1.0; 2.0         1.0; 1.0; 2.0
3.0; 4.0 becomes 3.0; 3.0; 4.0

If this is true, then the code could be:

let m = matrix [ [ 1.0; 2.0 ]
                 [ 3.0; 4.0 ] ]
m
|> Matrix.prependCol (m.Column 0)

UPDATE

Because the assumption above is not true.

So you can get the seq of the matrix rows, then transform it as usual with Seq.map, and finally make the result matrix:

let transform f m =
    m
    |> Matrix.toRowSeq
    |> Seq.map f
    |> matrix

// or even shorter in F# idiomatic style:
let transform f =
    Matrix.toRowSeq >> Seq.map f >> matrix

// test

let addElementInFront (x : Vector<float>) =
    x.ToArray()
    |> Array.append [| x.[0] |]
    |> vector

matrix [ [ 1.0; 2.0 ]
         [ 3.0; 4.0 ] ]
|> transform addElementInFront