NMath User's Guide

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6.3 Value Operations on Matrices (.NET, C#, CSharp, VB, Visual Basic, F#)

The matrix classes have the following read-only properties:

● Cols gets the number of columns in a matrix.

● ColStride gets the step increment between successive elements in a column.

● Rows gets the number of rows in a matrix.

● RowStride gets the step increment between successive elements in a column.

● DataBlock gets a reference to the data block that a matrix is viewing.

For example, if A is a FloatComplexMatrix instance:

Code Example – C# matrix

int cols = A.Cols;

int rows = A.Rows;

FloatComplexDataBlock block = A.DataBlock;

Code Example – VB matrix

Dim Cols As Integer = A.Cols

Dim Rows As Integer = A.Rows

Dim Block As FloatComplexDataBlock = A.DataBlock

NOTE—As described in Section 4.1, use caution when accessing a data block refer­enced by a matrix. Other objects may be viewing the same data.

Accessing and Modifying Matrix Values

The matrix classes provide standard indexers for getting and setting element value at a specified row and column position in a matrix. Thus, A[i,j] always returns the element in the ith row and jth column of matrix A's view of the data.

NOTE—Indexing starts at 0.

Thus, this code sets the value in the lower right corner of the matrix to zero:

Code Example – C# matrix

var A = new DoubleMatrix( "2x2 [1 2 3 4]" );

A[1,1] = 0;

Code Example – VB matrix

Dim A As New DoubleMatrix("2x2 [1 2 3 4]")

A(1, 1) = 0

The matrix indexer is also overloaded to accept Range and Slice indexing objects. For instance:

Code Example – C# matrix

var A = new DoubleMatrix( 5, 5, 2);

var B = new DoubleMatrix( 2, 2, 1);

var s = new Slice( 0, 2 );





A[s,s] = B




Code Example – VB matrix


Dim A As New DoubleMatrix(5, 5, 2)




Dim B As New DoubleMatrix(2, 2, 1)




Dim S As New Slice(0, 2)








A(S, S) = B




You can also use the Set()
member function to set the data elements of a matrix to a specified value.
For instance, this code sets values in the last two columns of matrix
A to zero:


Code Example – C# matrix


int rows = 5, cols = 5;




var A = new DoubleMatrix( rows, cols, 0, 1 );








var col = new Slice( 3, 2 );




Slice row = Slice.All; 




A.Set( col, row, 0 );




Code Example – VB matrix


Dim Rows As Integer = 5




Dim Cols As Integer = 5




Dim A As New DoubleMatrix(Rows, Cols, 0, 1)








Dim Col As New Slice(3, 2)




Dim Row As Slice = Slice.All




A.Set(Col, Row, 0)




You can replace either slice with an integer value
indicating a particular row or column. Thus, this code changes the values
in the first column of A to -1:


Code Example – C# matrix


A.Set( 1, Slice.All, -1);




Code Example – VB matrix


A.Set(1, Slice.All, -1)




NOTE—Any
method that returns a vector view of the data referenced by a matrix
can be used to modify the values of matrix, since the returned vector
and the matrix share the data. See Section 6.6.


Clearing and Resizing a Matrix


The matrix classes provide two methods for changing
the size of a matrix after it has been created:


● Clear() resets the value of all data elements
to zero.


● Resize() changes the size of a matrix to the specified
number of rows and columns, adding zeros or truncating as necessary.


● ResizeAndClear() performs the same function as
Resize(), but also resets the value of all
remaining data elements to zero.





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