Mathnet numerics indexing parts of a matrix

Question

Using Math.Net Numerics, how can I index parts of a matrix?

For example, I have a collection of ints and I want to get a submatrix with the rows and columns chosen accordingly.

A[2:3,2:3] should give me that 2 x 2 submatrix of A where the row index and the column index is either 2 or 3

Read here, my decision of this problem [HERE](https://stackoverflow.com/a/51314889/10072967) — Nazar Ukolov, Jul 13 '18 at 10:02
Read here, my decision of this problem [HERE][1] [1]: https://stackoverflow.com/a/51314889/10072967 — Nazar Ukolov, Jul 13 '18 at 10:04

score 1 · Answer 1 · answered Jun 03 '15 at 09:48

1

Just use some thing like

var m = Matrix<double>.Build.Dense(6,4,(i,j) => 10*i + j);
m.Column(2); // [2,12,22,32,42,52]

to access the desired column use the Vector<double> Column(int columnIndex) extension method.

answered Jun 03 '15 at 09:48

MoonKnight

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Thanks. But what if I want just parts of the 2nd column? – elina Jun 03 '15 at 10:49
Well use the index notation for matricies `M[i, j]` for row and column `i` and `j`. I have just read your updated question and it makes no sense. You cannot get a sub-MATRIX od a vector. Vectors are essentally a single column/row matrix. MathNET is very well documented, my suggestion is to download the source code and look through the test project... – MoonKnight Jun 03 '15 at 11:36
Ok, thanks. Maybe not a Submatrix of a vector, but a submatrix of an array of ints? – elina Jun 03 '15 at 12:20
Math.Net's `subMatrix` method for matrices will easily give you submatrices. See examples at See https://numerics.mathdotnet.com/Matrix.html (Which is where this one seems to come from) – FXQuantTrader Oct 08 '19 at 03:01

score 0 · Answer 2 · answered Jul 20 '17 at 15:17

I suspect you were looking for something like this Extension method.

public static Matrix<double> Sub(this Matrix<double> m, int[] rows, int[] columns)
{
    var output = Matrix<double>.Build.Dense(rows.Length, columns.Length);
    for (int i = 0; i < rows.Length; i++)
    {
        for (int j = 0; j < columns.Length; j++)
        {
            output[i,j] = m[rows[i],columns[j]];
        }
    }

    return output;
}

I've omitted the exception handling to ensure rows and columns are not null.

Mathnet numerics indexing parts of a matrix

2 Answers2