MATLAB Answers


Array within a structure array doesn't hold previous values

Asked by Abdurrehman on 12 Mar 2017
Latest activity Edited by Guillaume
on 14 Mar 2017
Hi Again, Initially a structure was created with following way
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
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 ?


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.
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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2 Answers

Answer by Guillaume
on 12 Mar 2017

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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Having looked at your new code, it doesn't look like you're indexing into the fields of the structure, but are actually replacing a whole row of a structure array, which should be fine.
Sorry, I don't get the problem. What is "Student{id}" in this line:
? 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?
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 by Guillaume
on 13 Mar 2017
Edited by Guillaume
on 13 Mar 2017

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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"the above line works good". Hum, no! At best it is confusing, at worst it will result in an error. If size(Student, 2) is 1 then the ,: is confusing as it implies that Student has more than one column when it hasn't, so:
CurrentMonthFee = Student(id).Fee(Month); %Don't use (id, :)
If size(Student, 2) is greater than 1, then Student(id, :).Fee will return a comma separated list of the Fee field of the whole row. You can't index comma separated lists, so the line will result in an error: Expected one output from a curly brace or dot expression ....
As for your question, without seeing the code you're using, it's difficult to tell what you're doing wrong.
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.
I'm utterly confused. With this line:
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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