MATLAB: Accessing Fields in Structs

struct

I have a more theoretical question to struct arrays in Matlab. Given I have two structures a and b, defined as follows:

a = struct('b', 0, 'c', 'test')
a(2).b = 5
b = struct('b', {0,5}, 'c', {'test'})

WHY is it now that if I use a(2) the answer is simply

b: 5
c: []

i.e. the second element of c is shown as empty, whereas when I use b(2) the answer is

b: 5
c: 'test'

i.e. the first element of c is repeated as its second element, too?

Best Answer

Look at the documentation for struct():

If any of the value inputs are nonscalar cell arrays, then s has the same dimensions as the nonscalar cell arrays. For any value that is a scalar cell array or an array of any other data type, struct inserts the contents of value in the relevant field for all elements of s.

Notice the second sentence there. {'test'} is a scalar cell array so it gets copied to all elements of the output.

Related Solutions

MATLAB: Question about the ‘stuct’ of matlab

That's the way the function struct is designed to work. See struct, Create structure array "[...]If value is an empty cell array {}, then s is an empty (0-by-0) structure."

MATLAB: Partial overwrite of data in a structure

"when I am restructuring the data it always a hassle to put every data holder variable to a proper place" If it's a hassle, then your storage structure is not optimal. You may be better off revising the way you store your data rather than constantly finding workarounds.

To answer your question, this should do what you want:

   function into = mergestruct(into, from)
      %MERGESTRUCT merge all the fields of scalar structure from into scalar structure into
      validateattributes(from, {'struct'}, {'scalar'});
      validateattributes(into, {'struct'}, {'scalar'});
      fns = fieldnames(from);
      for fn = fns.'
         if isstruct(from.(fn{1})) && isfield(into, fn{1})
              %nested structure where the field already exist, merge again
              into.(fn{1}) = mergestruct(into.(fn{1}), from.(fn{1}));
         else
             %non structure field, or nested structure field that does not already exist, simply copy
             into.(fn{1}) = from.(fn{1});
         end
      end
   end

Test:

>> a = struct('a', 1, 'b', 2, 'c', 3, 'd', struct('a', 11, 'b', 12, 'c', struct('a', 111))) 
a =   
  struct with fields:  
    a: 1
    b: 2
    c: 3
    d: [1×1 struct]
>> a.d  
ans =   
  struct with fields:  
    a: 11
    b: 12
    c: [1×1 struct]  
>> a.d.c 
ans =   
  struct with fields:  
    a: 111
>> b = struct('a', -1, 'b', -2, 'd', struct('b', -12, 'c', struct('b', -112)))  
b =   
  struct with fields:  
    a: -1
    b: -2
    d: [1×1 struct]
>> aa = mergestruct(a, b)  
aa =   
  struct with fields:  
    a: -1
    b: -2
    c: 3
    d: [1×1 struct]
>> aa.d  
ans =   
  struct with fields:  
    a: 11
    b: -12
    c: [1×1 struct]
>> aa.d.c  
ans =   
  struct with fields:  
    a: 111
    b: -112

Seems to do what you want.

Best Answer

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MATLAB: Partial overwrite of data in a structure

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