Array within a structure array doesn't hold previous values

Question

Abdurrehman 2017-3-12

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编辑： Guillaume 2017-3-14

Hi Again, Initially a structure was created with following way

Student=struct('Class','Junior','ID',1631,'Name','XYZ','Fees',0);
  save('file.mat','Student','-v7.3');

It was saved in D:\, then In my GUI I am accessing and updating this structure as

% Respective edit boxex are read as
  id  =  str2double(get(handles.IDBox,'String'));
  name  =  get(handles.NameBox,'String');
  .
  .
  .
  Month  =  str2double(DoA(4:5));%DoA is date of admission
  fees  =  str2double(get(handles.FeesBox,'String'));% Fee edit box read
  FeesArray(Month)  = fees;% Current month fee is added in FeeArray
%above values are saved in structure as
      filename=fullfile('D:\file.mat');
      h=matfile(filename,'Writable',true);
      h.Student(id,:)=Student{id};

All fields are updated correctly but Fees array retains only last updated value and previous values are replaced with 0.

Please help how can I program to retain all values in Fee Array ?

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Jan 2017-3-12

编辑：Jan 2017-3-12

If "Student" is a "struct" (not "Structure"), accessing it by curly braces would cause an error. Therefore I assume, that "Student" is a cell. What is "id"? Please edit the original question and add the additional details. Thanks.

Muhammad Abdurrehman 2017-3-13

编辑：Muhammad Abdurrehman 2017-3-13

Hi Jan,

Thanks a lot for answering, Same effect if we use parenthesis instead of curly brackets. I have updated the question please have a review. Thanks

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Answer 1

Guillaume 2017-3-12

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At the bottom of the documentation for matfile, it says:

matfile does not support indexing into:
  [...]
  Fields of structure arrays
  [...]

You have to replace the whole field.

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Jan 2017-3-13

在 MATLAB Online 中打开

Sorry, I don't get the problem. What is "Student{id}" in this line:

h.Student(id,:)=Student{id};

? In the test you speak about the "Fee array". Is this "fees" or "FeesArray(Month)"? Is "FeesArray(Month)" a vector of zeros except for the last index at position Month?

Muhammad Abdurrehman 2017-3-14

Hi, thanks for the reply. Yes this is the issue

FeesArray(Month)" a vector of zeros except for the last index at position Month. That's why it saves only last value in Structure Field i.e. Fees. Can you please suggest how to save only current value while unchanging previously entered ?

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Answer 2

Guillaume 2017-3-13

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https://ww2.mathworks.cn/matlabcentral/answers/329380-array-within-a-structure-array-doesn-t-hold-previous-values#answer_258421

编辑：Guillaume 2017-3-13

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Starting a new answer, since it looks like my initial answer was a misunderstanding...

h.Student(id, :) = ...

would indicate that the Student variable in the mat file is a 2D array. I would assume it is a 2D structure array although your code shows it being saved as a scalar structure.

= ... Student{id}

would indicate that the Student variable in the current workspace is a cell array. Presumably, the cell array contains structures but why isn't it structure array like in the mat file?

So, it looks like you're replacing a whole row of a structure array. I'm unclear what problem you're having. Aren't the values in the whole row identical after your update?

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Muhammad Abdurrehman 2017-3-14

actually when you are accessing your structure via a handler (created using matfile) then you have to use (id,:) because it doesn't support linear indexing.

Guillaume 2017-3-14

编辑：Guillaume 2017-3-14

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I'm utterly confused. With this line:

CurrentMonthFee=Student(id,:).Fee(Month);

you are not accessing anything using a matfile. And if you were with e.g.:

CurrentMonthFee = h.Student(id,:).Fee(Month); %I would use 1 instead of :

the .Fee part would throw an error.

With no valid code to go with, I still have no idea what you're trying to do. If you are trying to modify the monthly fee for a particular student:

h = matfile(filename, 'Writable', true);
%id: student id
%Month: Month to replace
%fee: value to assign
student = h.Student(id, 1);
student.Fee(Month) = fee;
h.Student(id, 1) = student;

You have to use a temporary variable to modify the fields of a structure through matfile. As per my 1st answer, it is explictly stated in the doc that you cannot index into the fields of a structure.

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