Why, oh why does isempty return 0? Questions related to dataset access and return from strfind.

Question

Marshall 2014-9-29

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评论： Stephen23 2014-9-30

Hello all,

I have a dataset array for example called measurement which contains a subfield called tempSequence containing single digits stored as strings (eg. '1', '2', etc.) I want to perform a check for the contents of this field and am using a combination of isempty and strfind and can't seem to get a TRUE return.

Specifically, I want to check that the field contains both a '1' and a '5' like so:

    if(  ~isempty(strfind(measurement.TempSequence, '1')) && isempty(strfind(messung.TempSequence, '5')) )
        % Both 1 and 5 are present, perform operation on different subfield.
    end

If tempSequence is as follows:

K>> measurement.TempSequence
ans = 
      '3'
      '4'
      '5'
      '2'

And strfind returns:

K>> strfind(measurement.TempSequence, '1')
ans = 
      []
      []
      []
      []

Why why why:

K>> isempty(strfind(measurement.TempSequence, '1'))
ans =
       0

Is it something to do with it being a dataset and therefore has attached metadata? Am I using strfind in a dumb way? What am I missing?

Why:

K>> isempty(strfind(measurement.TempSequence, 'foobar'))
ans =
       0

Thank you for reading! Marshall

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Answer 1

Matt J 2014-9-29

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编辑：Matt J 2014-9-29

在 MATLAB Online 中打开

The output of strfind is a cell array. To be considered empty, a cell array must have no cells whatsoever,

    >> isempty({})
    ans =
         1

This is different from a cell array containing cells with empty matrices

    >> c={[]};
    >> isempty(c)
    ans =
         0
    >> isempty(c{1})
    ans =
         1

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G A 2014-9-29

在 MATLAB Online 中打开

>> c={[]}
>> isempty(cell2mat(c))
ans =
       1

Marshall 2014-9-30

Ah, that makes sense. Strfind makes much more sense now (and I even got my code working, so that's a plus).

Thanks so much for the answers!

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Answer 2

Stephen23 2014-9-29

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编辑：Stephen23 2014-9-29

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Expanding on Matt J's answer, if you wish to check if every cell of a cell array contains an empty array, then you can use cellfun :

cellfun('isempty',cell_array)

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Marshall 2014-9-30

Ah, thanks! I'm coming from embedded C so vector operations and so on are all very new territory for me.

Stephen23 2014-9-30

It is a good thing that you keep this in mind. Basically in MATLAB think of the arrays/matrices more as the mathematical concept, and not as a programming abstraction of some lower-level data (like in C).

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Answer 3

David Young 2014-9-29

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The two earlier answers are both correct. Another suggestion which might help is to use ismember() instead of ~isempty(strfind()), with the opposite argument ordering:

if ismember('1', measurement.TempSequence) && ismember('5', measurement.TempSequence)
  ...

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Marshall 2014-9-30

Great suggestion. I think it's cleaner and reads better too. I'm coming from embedded C to OO MatLab so it's a steep learning curve!

Thanks for the help.

Marshall 2014-9-30

编辑：Marshall 2014-9-30

ismember is the sort of function I was looking for in the first place, but there's such a huge number of functions to learn (but so it is with all languages). Your suggestion worked perfectly. So glad for this rad community. M_

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