[Octave-bug-tracker] [bug #63080] Allow matrix power for sparse empty matrices

[Arun Giridhar]

URL:
  <https://savannah.gnu.org/bugs/?63080>

                 Summary: Allow matrix power for sparse empty matrices
                 Project: GNU Octave
               Submitter: arungiridhar
               Submitted: Mon 19 Sep 2022 07:04:30 AM EDT
                Category: None
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Unexpected Error or Warning
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: dev
         Discussion Lock: Any
        Operating System: Any


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Follow-up Comments:


-------------------------------------------------------
Date: Mon 19 Sep 2022 07:04:30 AM EDT By: Arun Giridhar <arungiridhar>
Taking the matrix power of an empty sparse matrix fails in certain legal cases
even though passing an empty full matrix works fine:

A = sparse ([]);
A ^ 1
A ^ 0
A ^ -1


Oddly, non-integer exponents work even for sparse empty inputs, like A ^ 1.3
or A ^ -1.3.

The problem turned out to be this input validation in sparse-xpow.cc:

   if (nr == 0 || nc == 0 || nr != nc)
     error ("for A^b, A must be a square matrix.  Use .^ for elementwise
power.");


The following change fixes it for me:

diff -r 6646f2b5a3d1 libinterp/corefcn/sparse-xpow.cc
--- a/libinterp/corefcn/sparse-xpow.cc  Sat Sep 17 04:22:38 2022 -0400
+++ b/libinterp/corefcn/sparse-xpow.cc  Mon Sep 19 06:46:27 2022 -0400
@@ -66,9 +66,12 @@ xpow (const SparseMatrix& a, double b)
   octave_idx_type nr = a.rows ();
   octave_idx_type nc = a.cols ();
 
-  if (nr == 0 || nc == 0 || nr != nc)
+  if (nr != nc)
     error ("for A^b, A must be a square matrix.  Use .^ for elementwise
power.");
 
+  if (nr == 0 && nc == 0)
+    return a;
+
   if (! xisint (b))
     error ("use full(a) ^ full(b)");


I will push this after I get a bug number.







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