kron with a single sparse matrix- another corner case

[augustm]

Octave-3.0.0. on linux 64 bit.


I like to build finite difference operators (with several fields) from sparse
representations of derivative operators, which are then replicated with the
"kron" function.

It is perhaps natural that
kron(full, sparse) gives a sparse result. 

However kron promotes to full by defaults. A trivial example creates
an empty sparse matrix and replicates twice using kron.


octave:1> j=sparse( [], [] , [], 10,10) ;
octave:2> whos j

*** local user variables:

  Prot Name        Size                     Bytes  Class
  ==== ====        ====                     =====  ===== 
   rwd j          10x10                        44  double

Total is 0 elements using 44 bytes

octave:3> jj=kron ( [1 0], j );
octave:4> whos jj

*** local user variables:

  Prot Name        Size                     Bytes  Class
  ==== ====        ====                     =====  ===== 
   rwd jj         10x20                      1600  double

Total is 200 elements using 1600 bytes


The work around is to use
jj=kron ( sparse([1 0]), j );


Note Matlab uses the suggested behaviour. This was found on porting over
a code previously run under matlab.

TM, Paris.


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