Substruct array disappear when a struc object is assigned

Hi guys,

I have started to use struct to create nested arrays which will be assigned in a second time , for example:

   S = struct( ... 
  'A',               struct( ... 
  'A_ID',            struct( ...
  'C', [], ...
  'D', [], ...
  'E',               struct( ... 
  'EF',[], ... 
  'EG',[], ...
  'EH',[], ... 
  'EI',[]))));

but when i try to assign the object values, something strange happens, the substructure information disappeares. For examples i assign to the first postion of the object A_ID the value of C = 'test1'

S.A.A_ID(1).C='test1'

and i obtain

S.A.A_ID(1) =

• C: 'test1'
• D: []
• E: [1x1 struct]

now if i assign a new value for C in a new position of A_ID for example

S.A.A_ID(2).C='test2'

i don't obtain

S.A.A_ID(2) =

• C: 'test2'
• D: []
• E: [1x1 struct]

but

S.A.A_ID(2) =

• C: 'test2'
• D: []
• E: []

and i lost all the information contained in the nested struct E !!

i don't really understand why, have someone an idea??

thank's in advance

Luca

Answer 1

[Geoff Hayes]

Luca - the substructure (for the second element) doesn't disappear as it was never there to begin with. Since you are initializing it for the first time with

 S.A.A_ID(2).C='test2';

then the D and E will be set with default values of the empty matrix for each. You would need to copy the elements over from S.A.A_ID(1).

Remember, when you initialize your S as above, then

 numel(S.A.A_ID)

is just one. There is no second element, and S.A.A_ID is identical/equivalent to S.A.A_ID(1).

[Luca Di Leta]

Hi Geoff, my idea it's to create a dynamic struct taht's grow up in dimension with my assignment, for example when I assing the element

S.A.A_ID(2).C='test2'

the struct object A_ID(i) should become a vector and should be include all the substruc as designed in the master layout, how can I attempt to obtain my objective?

many thank's in advance for your support

Luca

[Geoff Hayes]

Luca - if you want to update the kth element of the array, then you should probably check to see if this index is greater than the length of the array and update it accordingly. For example,

 if k > length(S.A.A_ID)
     for m=length(S.A.A_ID)+1:k
         S.A.A_ID(m) = S.A.A_ID(1);
     end
 end
 S.A.A_ID(k).C = 'test2';

We initialize all elements to the master copy and then update the C field for the kth element.

[Luca Di Leta]

Hi Geoff! thank you very much for your input, I make now a test and I give you a feedback

Luca

Answer 2

[Walter Roberson]

You need to assign a copy of the master layout and then change the fields you want changed. MATLAB automatically extends with 0s or empty (or NAt, Not a Time, for datetime arrays) and never automatically copies from templates

[Luca Di Leta]

Hi Walter, that's no other way to build a dynamic struct array? I could also first set the dimension of my arrays but it's not an elegant solution for struct they no have always the same dimension...