FIND on a 0x0 returns a 0x1

[John W. Eaton]

On 25-Mar-2009, Carsten Clark wrote:

| Here's a synopsis of some testing in MATLAB (monospaced display will help):
| 
| ZEROS          Input  Input    FIND  Output
|                type  ISVECTOR Output  type
| 0x0           0xOther  0       0x0     []
| 1x0             RV     1       1x0   basic RV
| 0x1             CV     1       0x1   basic CV
| 
| 0x0x0         0xOther  0       0x0     []
| 1x0x0         1xOther  0       0x1   basic CV
| 0x1x0         0xOther  0       0x0     []
| 1x1x0         1xOther  0       0x1   basic CV
| 0x0x1         0xOther  0       0x0     []
| 1x0x1           RV     1       1x0   basic RV
| 0x1x1           CV     1       0x1   basic CV
| 
| 1x0x1x1x1x1x1   RV     1       1x0   basic RV
| 1x0x1x1x1x0x1 1xOther  0       0x1   basic CV
| 
| 0x1x1x1x1x1x1   CV     1       0x1   basic CV
| 0x1x1x1x1x0x1 0xOther  0       0x0     []
| 
| 
| RV = row vector (1xN, 1xNx1, 1xNx1x1, ...)
| CV = col vector (Nx1, Nx1x1, Nx1x1x1, ...)
| 
| 
| Here's the raw MATLAB FIND output (note that [] is always 0x0 in MATLAB):
| 
| >> find(zeros(0,0))
| ans =
|      []
| >> find(zeros(1,0))
| ans =
|    Empty matrix: 1-by-0
| >> find(zeros(0,1))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(0,0,0))
| ans =
|      []
| >> find(zeros(1,0,0))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(0,1,0))
| ans =
|      []
| >> find(zeros(1,1,0))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(0,0,1))
| ans =
|      []
| >> find(zeros(1,0,1))
| ans =
|    Empty matrix: 1-by-0
| >> find(zeros(0,1,1))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(1,0,1,1,1,1,1))
| ans =
|    Empty matrix: 1-by-0
| >> find(zeros(1,0,1,1,1,0,1))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(0,1,1,1,1,1,1))
| ans =
|    Empty matrix: 0-by-1
| >> find(zeros(0,1,1,1,1,0,1))
| ans =
|      []
| 
| 
| Here's the result of MATLAB ISVECTOR on the same inputs:
| 
| >> isvector(zeros(0,0))
| ans =
|      0
| >> isvector(zeros(1,0))
| ans =
|      1
| >> isvector(zeros(0,1))
| ans =
|      1
| >> isvector(zeros(0,0,0))
| ans =
|      0
| >> isvector(zeros(1,0,0))
| ans =
|      0
| >> isvector(zeros(0,1,0))
| ans =
|      0
| >> isvector(zeros(1,1,0))
| ans =
|      0
| >> isvector(zeros(0,0,1))
| ans =
|      0
| >> isvector(zeros(1,0,1))
| ans =
|      1
| >> isvector(zeros(0,1,1))
| ans =
|      1
| >> isvector(zeros(1,0,1,1,1,1,1))
| ans =
|      1
| >> isvector(zeros(1,0,1,1,1,0,1))
| ans =
|      0
| >> isvector(zeros(0,1,1,1,1,1,1))
| ans =
|      1
| >> isvector(zeros(0,1,1,1,1,0,1))
| ans =
|      0
| 
| 
| Here's a set of rules deduced from the above for find(X), where X is
| empty.  Given the following definitions:
| 
| - a row vector is any matrix with size >= 0 down the 2nd dimension,
| and all other dimensions of size 1;
| 
| - a column vector is any matrix with size >= 0 down the 1st dimension,
| and all other dimensions of size 1;

Since Matlab (and Octave) strip trailing singleton dimensions, you
don't have to check all the trailing dimensions.  If you try to
construct an object with all trailing dimensions == 1, then they
should be stripped and the object should have only 2 dimensions.

| the following rules seem to apply when X is empty:
| 
| 1. If X is a row vector, return the basic empty row vector (1x0).
| 2. If X's 1st dim is 0 and X is *not* a column vector, return [] (0x0).
| 3. Return the basic empty column vector (0x1).

OK, how about the following change?

  http://hg.savannah.gnu.org/hgweb/octave/rev/12ca81f1fa99

Thanks,

jwe