bugGNU Octave - Bugs: bug #66514, spdiags does not...

 
 

bug #66514: spdiags does not "broadcast" elements over selected diagonals

Submitter:  Liang Tang <lt1234>
Submitted:  Tue 03 Dec 2024 02:20:23 AM UTC
   
 
Category:  Octave Function Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Confirmed Assigned to:  None
Originator Name:  lt1234 Open/Closed:  * Open
Release:  * 9.2.0 Operating System:  * Any
Fixed Release:  None Planned Release:  None
* Mandatory Fields

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Tue 03 Dec 2024 12:38:55 PM UTC, comment #2: 

Similar issue if the first argument is a scalar.

In Octave 9.2.0:

>> n=9; Z = sparse(n,n); A = spdiags([1], -1:1, Z)
A =

Compressed Column Sparse (rows = 9, cols = 9, nnz = 1 [1.2%])

  (2, 1) -> 1


In Matlab R2024b:

>> n=9; Z = sparse(n,n); A = spdiags([1], -1:1, Z); full(A)

ans =

     1     1     0     0     0     0     0     0     0
     1     1     1     0     0     0     0     0     0
     0     1     1     1     0     0     0     0     0
     0     0     1     1     1     0     0     0     0
     0     0     0     1     1     1     0     0     0
     0     0     0     0     1     1     1     0     0
     0     0     0     0     0     1     1     1     0
     0     0     0     0     0     0     1     1     1
     0     0     0     0     0     0     0     1     1

>>


 Markus Mützel <mmuetzel>
Group administrator
Tue 03 Dec 2024 12:35:37 PM UTC, comment #1: 

The output of the original code snippet in Matlab R2024a:

>> n=9; A = spdiags ([1 2 3], -1:1, n, n); full (A)

ans =

     2     3     0     0     0     0     0     0     0
     1     2     3     0     0     0     0     0     0
     0     1     2     3     0     0     0     0     0
     0     0     1     2     3     0     0     0     0
     0     0     0     1     2     3     0     0     0
     0     0     0     0     1     2     3     0     0
     0     0     0     0     0     1     2     3     0
     0     0     0     0     0     0     1     2     3
     0     0     0     0     0     0     0     1     2

>>



To put it in words: Matlab seems to "broadcast" elements of a row vector in the first argument to the diagonals that are selected in the second argument of `spdiag` if it is called with four input arguments.

Same for the three-input-argument form of `spdiag`.
In Matlab R2024a:

>> n=9; Z = sparse (n, n); A = spdiags ([1 2 3], -1:1, Z); full (A)

ans =

     2     3     0     0     0     0     0     0     0
     1     2     3     0     0     0     0     0     0
     0     1     2     3     0     0     0     0     0
     0     0     1     2     3     0     0     0     0
     0     0     0     1     2     3     0     0     0
     0     0     0     0     1     2     3     0     0
     0     0     0     0     0     1     2     3     0
     0     0     0     0     0     0     1     2     3
     0     0     0     0     0     0     0     1     2

>>


Since this would change behavior of Octave, any change should go to the default branch.

Markus Mützel <mmuetzel>
Group administrator
Tue 03 Dec 2024 02:20:23 AM UTC, original submission:  

Commands below emulates the matlab on-line example.  The result here will not match matlab's.  Thanks. 

> n=9;A = spdiags([1 2 3],-1:1,n,n);full(A)

ans =

   2   0   0   0   0   0   0   0   0
   1   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0
   0   0   0   0   0   0   0   0   0

Liang Tang <lt1234>

 

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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2024-12-03 mmuetzel StatusNone Confirmed
        Operating SystemMicrosoft Windows Any
        Summaryspdiags does not match one matlab on-line help example spdiags does not "broadcast" elements over selected diagonals

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