Re: subsetting matrices

[John W. Eaton]

On 26-Oct-1999, address@hidden <address@hidden> wrote:

| On Tue, 26 Oct 1999, John W. Eaton wrote:
| 
| > On 26-Oct-1999, address@hidden <address@hidden> wrote:
| > 
| > | One of the beautiful things about octave is the ability to subset a matrix
| > | like
| > | 
| > |   Q = A(a:b,c:d);
| > | 
| > | Now I need to do this in a dld (.oct file).  Anyone know how to do this?
| > 
| > It depends.  What is the type of A?  Is it an octave_value object or a
| > Matrix, or what?
| 
| It's a matrix.  I need to perform some operations on blocks of a matrix.
| This operation gets done very frequently in the optimization I'm doing so
| I'm trying to speed it up with a .oct file.

Here is an example of one way to do it:

  #include <octave/config.h>

  #include <octave/oct.h>

  DEFUN_DLD (foo, args, ,
    "y = foo (x, a, b, c, d): perform y = x(a:b, c:d)")
  {
    octave_value retval;

    double a = args(1).double_value ();
    double b = args(2).double_value ();

    double c = args(3).double_value ();
    double d = args(4).double_value ();

    Matrix x = args(0).matrix_value ();

    // This should work
    //
    // Matrix y = x.index (Range (a, b), Range (c, d));
    //
    // but for now, you have to write this:

    idx_vector i (Range (a, b));
    idx_vector j (Range (c, d));

    retval = Matrix (x.index (i, j));

    return retval;
  }

jwe



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