Re: oct file: slower than expected using fortran_vec

[John W. Eaton]

On 16-May-2011, Seb Astien wrote:

| It is a bit faster the second time, but it does not explain the gap
| between the two:
| The bigger the matrix, the bigger the gap between built-in function
| and the oct one.
| I suspect a copying taking it place somehwere.

Yes, because when you write

  NDArray A = args(0).array_value ();

you grab a second reference to the underlying array data.  Then when
you do

  const double *p = A.fortran_vec ();

you are forcing a copy.  The const on the LHS is not what determines
whether or not the const version of Array::fortran_vec is selected
over the non-const version.  That selection is based on whether the
method is called on a const object.

If you want to avoid the copy, then you should write

  const NDArray A = args(0).array_value ();
  const double *p = A.fortran_vec ();

or

  NDArray A = args(0).array_value ();  // could also declare A to be const
  const double *p = A.data ();

In the latter case, it does not matter whether A is const; no copy is
ever made with the Array::data method.

jwe