How to index tables in a struct? (to call each field from App Designer)

Hello,

I'm facing a problem while coding on App Designer. I have saved a workspace made of diferent tables.

The tables are created this way:

a{1}=
a{2}=
a{3}=

etc...

I created it this way to be able to call them in the code on App Designer (in which I have imported the workspace with the command "load") in a loop, for example:

app.data = load('workspace.mat'); % command used to load the workspace in App Designer

for i = 1 : numel(fieldnames(app.data)) 
    disp(app.data.a(i))  %here is the problem
end

Matlab send me an error when executing this or any combination of app.data.a with (i) or {i} ...

A "a" field of cell type is beeing created when running the workspace but this same "a" clas isn't loaded with the rest of the fields when importing in App Designer.

Thanking you in advance for your help :)

[Stephen23]

Please show the complete output of this command:

whos -file workspace.mat

Note that your for loop and use of fieldnames does not make any sense, as you only access one field of app.data within the loop (named a).

[yvesvincent abgrall]

Thank you for answering

whos -file workspace.mat

This command shows me me the field names of the tables only, no cell class indexed.

I have as many fields as of a{} command I did.

"Note that your for loop and use of fieldnames does not make any sense, as you only access one field of app.data within the loop (named a)."

Since there are as many fields as a{} command, doesn't it make sense to have a for loop?

[Stephen23]

"Since there are as many fields as a{} command..."

You seem to be confusing fields with elements of an array: a does not have any fields, as it appears to be a cell array or a table (there is no way to refer to a structure (which has fields) using {} indexing). That is why I asked you to show us the complete output of this command:

whos -file workspace.mat

Not providing this information just makes it slower for you to get an answer.

"...doesn't it make sense to have a for loop?"

Sure a loop is fine, but most likely you should be iterating over a's elements.

[yvesvincent abgrall]

In my code the "a" is in fact a "GT"

>> whos -file workspace.mat
  Name         Size            Bytes  Class    Attributes

  GT14          -              17966  table              
  GT15          -              30288  table              
  GT16          -              15826  table              
  GT17          -              15826  table              
  GT18          -              15826  table              
  GT19          -              15826  table

I started indexing at GT{14}

In the workspace, a GT variable appears, as a cell type.

As I want an automated process to extract the GT tables from the struct, and as there will be many more tables, I need a for loop or something like that, what would you advise me then?

[Nicolas B.]

Hi,

for me, the problem is that you are creating a cell array and then, accessing it as an array.

Could you try to modify your code with:

app.data = load('workspace.mat'); % command used to load the workspace in App Designer
for i = 1 : numel(fieldnames(app.data)) 
    disp(app.data.a{i})  %here is the problem
end

I hope it will resolve your problem.

[yvesvincent abgrall]

Hello Nicolas,

this is the message i get when doing so:

Reference to non-existent field 'GT'.

Error in app1/SelectionListBoxValueChanged (line 28)

disp(app.data.GT{15});

Error using matlab.ui.control.internal.controller.ComponentController/executeUserCallback (line 378)

Error while evaluating ListBox PrivateValueChangedFcn.

[Stephen23]

@yvesvincent abgrall your description and your actual data are very different. What you showed in your question uses indexing into a cell array or table, e.g.:

"The tables are created this way:"

a{1}=
a{2}=
a{3}=
etc...

But your response to my request for information shows that in fact you have a whole lot of fields, and indexing is not involved at all. You should revise basic MATLAB concepts, e.g. what is indexing, what are structure fields, etc. as you have mixed them up and provided conflicting information on this thread.

"I started indexing at GT{14}"

In fact you don't, because indices are totally unrelated to fieldnames. Do not confuse them.

Answer 1

[Stephen23]

You need to loop over the fieldnames:

app.data = load('workspace.mat');
C = fieldnames(app.data);
for k = 1:numel(C)
    T = app.data.(C{k});
    ... do whatever with table T
end

Read this:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

[yvesvincent abgrall]

Perfect it's working!

Thank you all for your help :)

Answer 2

[Steven Lord]

app.data = load('workspace.mat'); % command used to load the workspace in App Designer
for i = 1 : numel(fieldnames(app.data)) 
    disp(app.data.a(i))  %here is the problem
end

This looks suspicious. You're iterating from 1 to the number of fields in app.data, but then you're not using the fieldnames to access the data. I would expect you to use the list of fieldnames in conjunction with dynamic field names.

app.data = load('workspace.mat'); % command used to load the workspace in App Designer
FN = fieldnames(app.data);
for i = 1 : numel(FN) 
    disp(app.data.(FN{i}))  %here is the problem
end

If you need to further index into a table that is one of the fields in app.data, use one of the types of indexing shown on this documentation page. Note in particular that if you're using parentheses or curly braces, you need to specify two indices. Asking for T(1) of a table won't work, but asking for T(1, 1) or T(1, :) of that table would.