[Octave-bug-tracker] [bug #46533] numel() returns negative values for large diagonal matrices

[Rik]

URL:
  <http://savannah.gnu.org/bugs/?46533>

                 Summary: numel() returns negative values for large diagonal
matrices
                 Project: GNU Octave
            Submitted by: rik5
            Submitted on: Wed 25 Nov 2015 12:00:05 PM PST
                Category: Libraries
                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: dev
        Operating System: Any

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Details:

Sample code:


octave:2> x = eye (1e6, 1e6);
octave:3> typeinfo (x)
ans = diagonal matrix
octave:4> numel (x)
ans = -727379968


The problem is that the simple calculation of the number of elements is
rows*columns which is 1e12 in this example.  The ordinary octave_idx_type is
2^31, or ~2e9, so the value rolls over into negative numbers.

Diagonal matrices are a sort of compressed, sparse implementation of a matrix.
 Perhaps they should do what the existing sparse code does which is return an
error message.


octave:1> x = speye (1e6,1e6);
octave:2> numel (x)
error: out of memory or dimension too large for Octave's index type


See the related bug #42425.





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