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## [Octave-bug-tracker] [bug #55831] Incorrect sparse QR factorization

 From: anonymous Subject: [Octave-bug-tracker] [bug #55831] Incorrect sparse QR factorization Date: Tue, 5 Mar 2019 02:37:58 -0500 (EST) User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.121 Safari/537.36

```URL:
<https://savannah.gnu.org/bugs/?55831>

Summary: Incorrect sparse QR factorization
Project: GNU Octave
Submitted by: None
Submitted on: Tue 05 Mar 2019 07:37:57 AM UTC
Category: Octave Function
Severity: 3 - Normal
Priority: 5 - Normal
Item Group: Incorrect Result
Status: None
Assigned to: None
Originator Name:
Originator Email:
Open/Closed: Open
Discussion Lock: Any
Release: 5.1.0
Operating System: Microsoft Windows

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Details:

The following code:
[Q,R] = qr(sparse([1,0;0,0]));
outputs
Q = [-1 0
0 0]

obviously Q is not unitary
it seems to me that each column with all zeros in a sparse input return a
column with all zeros in the Q matrix

the size of Q is also incorrect when the input is sparse and an economy size
factorization is requested. e.g.:
[Q,R] = qr(sparse(5,2),0);
outputs Q with size 5x5 rather than 5x2. Even though Q is 5x5, R is 2x2.

btw, is there a reason why Octave does not return a sparse Q like Matlab does
when the input is sparse? I know Q is usually dense, but in some cases it may
be sparse (e.g., economy factorization of a tall and sparse matrix). This
could be useful to avoid running out of memory, for instance.

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<https://savannah.gnu.org/bugs/?55831>

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