Re: problem plotting "N-d object"

[Sergei Steshenko]

----- Original Message -----
> From: Ben Abbott <address@hidden>
> To: Sergei Steshenko <address@hidden>
> Cc: "address@hidden" <address@hidden>
> Sent: Friday, February 17, 2012 6:06 PM
> Subject: Re: problem plotting "N-d object"
> 
> On Feb 17, 2012, at 10:15 AM, Sergei Steshenko wrote:
> 
>>  Hello,
>> 
>>  here is a screen session:
>> 
>>  "
>>  octave:3> foo(1,1,:) = [1 2 3]
>>  foo =
>> 
>>  ans(:,:,1) =  1
>>  ans(:,:,2) =  2
>>  ans(:,:,3) =  3
>> 
>>  octave:4> plot(foo(1,1,:));
>>  error: transpose not defined for N-d objects
>>  error: called from:
>>  error:  
> /home/qemu/AFSWD/20111122/octave-3.4.2/share/octave/3.4.2/m/plot/private/__plt__.m
>  
> at line 179, column 8
>>  error:  
> /home/qemu/AFSWD/20111122/octave-3.4.2/share/octave/3.4.2/m/plot/private/__plt__.m
>  
> at line 104, column 17
>>  error:  
> /home/qemu/AFSWD/20111122/octave-3.4.2/share/octave/3.4.2/m/plot/plot.m at 
> line 
> 194, column 9
>>  octave:4> plot(foo(1,1,:)(:));
>>  octave:5>            
>> 
>>  ".
>> 
>>  I don't understand why 'octave' complains about N-d objects - 
> to me "foo(1,1,:)" looks like a 1-d object. Because the first two of 
> the three indexes are fixed in the expression.
>> 
>> 
>>  And adding "(:)" solves the problem.
>> 
>>  Just wondering - should it be that hard for end user ? Does Matlab behave 
> the sane way ?
>> 
>>  Thanks,
>>    Sergei.
> 
> For either plot(foo) or plot(foo(1,1,:), Matlab gives ...
> 
>     plot (foo)
>     Error using plot
>     Data may not have more than 2 dimensions
> 
> Even so, maybe an appropriate solution is to check if the numberic inputs 
> have 
> ndim > 2 and attempt to apply squeeze before throwing an error ?
> 
> If you like this idea, please open a feature request in the tracker.
> 
>     https://savannah.gnu.org/bugs/?group=octave
> 
> Ben


First of all, I still don't understand why both Matlab/Octave consider the data 
to have more than 2 dimensions - this is because in my 3-d object two indexes 
are _fixed_, they are _constants_.

Because of my math background I find it senseless to say that a 3-d cube 
projection onto a 2-d plane is still 3-d - it is just 2-d, and, likewise, that 
2-d cube image of a cube on a plane being projected onto a 1-d straight line 
would be just 1-d.


...


You know what, I dare to extend the subject/issue.


IMO in Octave/Matlab world the difference between line and column vectors is a 
nuisance in many many cases, and I mention vectors because of "transpose not 
defined" (specifically, "transpose") in the original error message.

I think Octave should automatically apply '(:)' operation whenever row/column 
trait doesn't matter. And probably the 'plot' family of functions is a good 
place to begin with applying this - as you wrote "maybe an appropriate solution 
is to check if the numberic inputs have ndim > 2 and attempt to apply squeeze 
before throwing an error".

Thanks,
  Sergei.




>