[Octave-bug-tracker] [bug #60089] the sparse QR factorizion does not give economy sized Q

[Andreas Wachtel]

URL:
  <https://savannah.gnu.org/bugs/?60089>

                 Summary: the sparse QR factorizion does not give economy
sized Q
                 Project: GNU Octave
            Submitted by: awachtel
            Submitted on: Sun 21 Feb 2021 07:18:16 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: aWachtel
        Originator Email: 
             Open/Closed: Open
                 Release: 4.2.2
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

according to the docs
  [...] = qr (..., 0)
If the final argument is the scalar 0 and the number of rows is larger than
the number of columns, then an "economy" factorization is returned, omitting
zeroes of R and the corresponding columns of Q.  That is, R will have only
'size (A,1)' rows.  In this case, P  is a vector rather than a matrix.


Z = [magic(3); speye(3)];
[Q,~] = qr(Z, 0);


I would expect 3 orthogonal columns, but I get 6, the full Q if you will. On
the other hand, the following lines give 3 orthogonal columns.


Z = [magic(3); speye(3)];
[Q,~] = qr(full(Z), 0);


I was not sure how to classify this item, it is a consistency error in
behavior rather than anything else.




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