Sparse to full matrix conversion not working...

The functions full(.) and issparse(.) are behaving strangely for me. In particular, issparse(.) seems unable to detetct sparse matrices correctly, and full(.) fails to convert sparse matrices to full matrices. For example, when I run the code below:

S = sparse(double(rand(200,200) < 1/2));
SS = speye(200);
A = full(S);
AA = full(SS);
d1=issparse(S)
d2=issparse(SS)
d3=issparse(A)
d4=issparse(AA)
whos

I get the following output:

d1 =

     0


d2 =

     0


d3 =

     0


d4 =

     0

  Name        Size              Bytes  Class     Attributes

  A         200x200            321096  double    sparse    
  AA        200x200              4808  double    sparse    
  S         200x200            321096  double    sparse    
  SS        200x200              4808  double    sparse    
  ans         1x1                   8  double              
  d1          1x1                   8  double              
  d2          1x1                   8  double              
  d3          1x1                   8  double              
  d4          1x1                   8  double 

So, issparse(.) tells me none of the matrices are sparse, while the "whos" command tells me that all four are sparse, when it should be the case that S and SS are sparse, while A and AA are not. Additionally, the Workspace window indicates that all four are sparse.

Any help with why this might be happening would be greatly appreciated. (The reason I need this is because a particular function I'm working with, schur(.), can only take full matrices as arguments.)

[Walter Roberson]

A and AA do not show up as sparse for me, and the issparse() tests return correct results for me. R2018b on Mac High Sierra

[Neil White]

Thanks for testing it. It seems like it's a problem that, for some reason, is specific to me.

Answer 1

[Steven Lord]

Something is definitely odd here. Your A and AA should be full matrices, not still sparse.

Let's make sure you're calling the correct issparse and full functions.

which -all issparse
which -all full

[Neil White]

Thanks for your response. It is odd. After running

which -all issparse
which -all full

output is

C:\ .. \MATLAB\issparse.m % comment: I deleted some of the actual path here
issparse is a built-in method                                              % matlab.lang.OnOffSwitchState method
issparse is a built-in method                                              % targetrepository.physical.Platform method
issparse is a built-in method                                              % targetrepository.toolchain.Endianess method
issparse is a built-in method                                              % targetrepository.toolchain.IntDivRoundTo method
issparse is a built-in method                                              % targetrepository.toolchain.LargestAtomicFloat method
issparse is a built-in method                                              % targetrepository.toolchain.LargestAtomicInteger method
issparse is a built-in method                                              % targetrepository.toolchain.ShiftRightIntArith method
built-in (C:\Program Files\MATLAB\R2018b\toolbox\matlab\sparfun\issparse)  % Shadowed
C:\ .. \MATLAB\full.m % comment: I deleted some of the actual path here
full is a built-in method                                              % matlab.lang.OnOffSwitchState method
full is a built-in method                                              % targetrepository.physical.Platform method
full is a built-in method                                              % targetrepository.toolchain.Endianess method
full is a built-in method                                              % targetrepository.toolchain.IntDivRoundTo method
full is a built-in method                                              % targetrepository.toolchain.LargestAtomicFloat method
full is a built-in method                                              % targetrepository.toolchain.LargestAtomicInteger method
full is a built-in method                                              % targetrepository.toolchain.ShiftRightIntArith method
built-in (C:\Program Files\MATLAB\R2018b\toolbox\matlab\sparfun\full)  % Shadowed

It does seem like I may have found a workaround by multiplying the sparse matrix by a full identity matrix, but it's still a little troubling that the functions aren't working correctly for me.

[Steven Lord]

C:\ .. \MATLAB\issparse.m % comment: I deleted some of the actual path here

C:\ .. \MATLAB\full.m % comment: I deleted some of the actual path here

Those are likely your culprit. From the tail end of those paths they don't look like the built-in issparse and full functions, but ones you've created or received from someone. Rename those files or delete them and see if you can reproduce the problem.

[Neil White]

Thanks! I just figured this out too. Thanks so much for your help! I'm not sure how they got there, but it seems to be working correctly now.