From help-octave-request at bevo dot che dot wisc dot edu Wed Dec 18 14:19:31 2002
Subject: A bit puzzled...
From: "Albert F. Niessner" <Al dot Niessner at jpl dot nasa dot gov>
To: Paul Kienzle <pkienzle at users dot sourceforge dot net>
Cc: Octave Help <help-octave at bevo dot che dot wisc dot edu>
Date: 18 Dec 2002 15:19:12 -0500


Thanks for your responses to my posts, but your replies have me a bit
puzzled.

In one of them
(http://www.octave.org/mailing-lists/help-octave/2002/1164), you wrote:
"The data is not actually copied until you do A.fortran_vec() to access
the data for modification."

However, in the other
(http://www.octave.org/mailing-lists/help-octave/2002/1163) you wrote:
"If the operation is not in place, or if you need a working vector,
allocate it beforehand:

    octave_value_list retval;
    const Matrix A(args(0).matrix_value());
    const Matrix B(args(1).matrix_value());
    Matrix C(A.rows(),B.columns());
    F77_FUNC(f,F)(A.data(),B.data(),C.fortran_vec());
    retval(0) = C;
    return retval;"

But, if the data is being copied at the C.fortran_vec() routine, then
retval(0) = C cannot possibly contain the answer. In fact, this is
exactly what I am seeing. If I make changes to allow for a copy
operation taking place I get a segmentation fault. I think it is from
retval(0) = fvec where 'double *fvec = C.fortran_vec();'. I will walk
through the core dump when I get a chance, but it may take a while.

I then walked through some of the octave code and I saw in Array.h that
the template defines fortran_vec() as 'return data()' which is defined
as 'return rep->data' which I think is T *data. So, I am not sure why
your example does not work or why you suggest a copy operation takes
place.

I have read liboctave.ps and "Da Coda Al Fine" and did not find the
answers there either.

Al Niessner



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