Can .oct files do in-place matrix operations?

[John W. Eaton]

On 12-May-1998, Dirk Laurie <address@hidden> wrote:

| I have some old Matlab 3.5 files in which row and column rotations
| are done in-place using .mex files.  I.e. the .mex file for
|   rotate(A,ij,R,job)
| does the equivalent of
|      A(ij,:)=R*A(ij,:);   (job=1)
| or   A(:,ij)=A(:,ij)*R;   (job=2) 
| or   both (job=3), 
| where R is a 2x2 matrix and ij is a two-component index vector.
| 
| This speeded up my program by about 50%, I recall.
| 
| Now I get the impression that Octave will not allow me to do
| this by an .oct file.  It seems to require my returning the
| changed A as an octave_value, which as far as I can make out
| involves two full matrix assignments: one to make the local
| copy of A that I modify, and one to assign it back to my
| original matrix.  Since rotation is an O(n) operation I will
| probably not gain by doing this, or am I wrong?

You may still get some improvement in performance, but arguments
passed to functions defined by .oct files still have pass-by-value
semantics, though they are only copied if necessary (if they are
modified in the function).  In either case (.m or .oct file) only
one copy should be performed:

  funcction y = f (x)
    y = x;             # no data copied -- only reference count updated 
    y(1) = 2;          # copy forced.
  endfunction


jwe