Access to a structure value with the field path in a String form

Hi,

I don't know how to access to a specific array stored in a structure by using the field path stored in a String form. The following code shows my issue:

clear all;
clc;

Struct.a.b.c.d  = [1 2 3; 4 5 6];   % Structure containing the Matrix
structString  = 'a.b.c.d(:,1)'; % String with the desired path

Ideally, I would like to access to the matrix with something like:

value = Struct.(structString)

But it doesn't work...

Thanks in advance for your help!

Answer 1

[Walter Roberson]

The function below only handles plain fields, no indexing. There could be some justification for extending it to handle indexing of the final element, but indexing anywhere else gets problematic, requiring more advanced checking

Sample:

Struct.a.b.c.d  = [1 2 3; 4 5 6];   % Structure containing the Matrix
structString  = 'a.b.c.d'
t = struct_stridx(Struct, structString);
t = t(:,1);

function v = struct_stridx(S, fieldlist)
  fn = regexp(fieldlist, '\.', 'split');
  bad_fields = fn(cellfun(@isempty,regexp(fn, '^[A-Za-z][A-Za-z0-9_]*$')));
  if ~isempty(bad_fields)
     error('Only plain fieldnames are allowed. First invalid one is "%s"', bad_fields{1});
  end
  v = S;
  for K = 1 : length(fn)
     thisfn = fn{K};
     if isfield(v, thisfn)
       if length(v) == 1
         v = v.(thisfn);
       else
         error('MATLAB:dotRefOnNonScalar', 'Dot name reference on non-scalar structure. Field "%s"', strjoin(fn(1:K), '.'));
       end
     else
       error('Field "%s" does not exist in structure', strjoin(fn(1:K), '.'));
     end
   end
 end

[Rodolfo_H]

Hi, Thanks for your answer and the code you provided. I actually solved my problem by converting all the structure field into a string and evaluating that. The following lines show the application of the created function.

Struct.a.b.c.d  = [1 2 3; 4 5 6];   % Structure containing the Matrix
structString  = 'a.b.c.d(:,1)';     % String with the desired path

t = eval_struct_str(Struct, structString)

with:

function v = eval_struct_str( S , string_field_list )

v = eval(['S.' string_field_list]);

end

With that solution, the indexing is possible. Thanks again for your answer, my problem is solved.

[Stephen23]

You might like to read this:

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat

Answer 2

[Image Analyst]

Why are you trying to get a string involved in this at all? Why not just get the values directly from the structure:

value = Struct.a.b.c.d(:,1)

I just don't see any need to involve a string at all.

[Rodolfo_H]

Thanks for your answer.

The few lines of code that i written here are just supposed to represent the problem. In my "real" function, the structString variable comes from an Excel File:

[num structString] = xlsread(ExcelFile,'Sheet1','A1');

I actually solved my problem with the following line:

value = eval(['Struct.' structString{1}]);

Thanks again for your answer.