[Octave-bug-tracker] [bug #45622] Sparsity of sparse matrix when assigning all elements

[Amro]

URL:
  <http://savannah.gnu.org/bugs/?45622>

                 Summary: Sparsity of sparse matrix when assigning all
elements
                 Project: GNU Octave
            Submitted by: amro_octave
            Submitted on: Sat 25 Jul 2015 07:13:18 AM EEST
                Category: Interpreter
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: WTF, Matlab?!?
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.0.0
        Operating System: Any

    _______________________________________________________

Details:

I was reading bug #45589, and I found other incompatibilities with regard to
sparse matrices.

Consider the following example:


>> M = sparse(4,4);
>> M(:) = 1;
>> issparse(M)    % MATLAB -> false, Octave -> true
>> nnz(M)         % MATLAB -> 16,    Octave -> 16
>> nzmax(M)       % MATLAB -> 16,    Octave -> 16


In the latest MATLAB R2015a, the matrix "M" becomes full, while in Octave
4.0.0 it maintains its sparsity.

I guess MATLAB determined that it was more space-efficient to just convert the
matrix to full rather than just keep it in sparse storage, seeing that we fill
all the elements with non-zero values.

Note that the outcome is different from the following code:


M = sparse(4,4);
for i=1:numel(M)
    M(i) = 1;  % also try: 0
end


where the matrix "M" stays sparse in both MATLAB and Octave.

----

Another inconsistency:


>> A1 = sparse(4,4);
>> A1(:) = 0;
>> issparse(A1)    % MATLAB -> true, Octave -> true
>> nnz(A1)         % MATLAB -> 0,    Octave -> 0
>> nzmax(A1)       % MATLAB -> 1,    Octave -> 0

>> A2 = sparse(4,4);
>> A2(:) = zeros(size(A2));
>> issparse(A1)    % MATLAB -> false, Octave -> true
>> nnz(A1)         % MATLAB -> 0,     Octave -> 0
>> nzmax(A1)       % MATLAB -> 16,    Octave -> 0

>> A3 = sparse(4,4);
>> A3(:) = zeros(numel(A3),1);
>> issparse(A1)    % MATLAB -> false, Octave -> true
>> nnz(A1)         % MATLAB -> 0,     Octave -> 0
>> nzmax(A1)       % MATLAB -> 16,    Octave -> 0

>> whos A*


In Octave, all three cases are the same, and the resulting matrix remains
sparse.
However in MATLAB, "A1" is the only one that stays sparse, and the other two
"A2" and "A3" are converted to full. Also note that the "A1" storage (nzmax)
is expanded to one.

So it seems that "A(:) = scalar" is treated differently from "A(:) =
vector/matrix" in MATLAB, even if scalar/vector/matrix are all zeros...





    _______________________________________________________

Reply to this item at:

  <http://savannah.gnu.org/bugs/?45622>

_______________________________________________
  Message sent via/by Savannah
  http://savannah.gnu.org/