MATLAB Answers

Why the pushbutton does not work?

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Sergey Lopatnikov
Sergey Lopatnikov on 26 Sep 2021 at 14:40
Answered: Sergey Lopatnikov on 26 Sep 2021 at 19:25
I wrote a simple function which incorporates creation of a figure and pusbutton.
I expected that after pressing the button, the function will provide the answer based on what callcack function of pushbutton is performing:
function [B,success] = TGA_MAIN(X,Y,L,W,Xb,Yb,Lb,Wb, STRING)
global B
B=0,
success=0
Xa=X;
Ya=Y;
La=L;
Wa=W;
p=Xb;
r=Xb;
s=Lb;
t=Wb;
STR=STRING;
handles.load_fig=figure('MenuBar','none','Position',[Xa,Ya,La,Wa])
handles.load_button=uicontrol(handles.load_fig,'style','pushbutton','string',STR,'position',[p,r,s,t])
set(handles.load_button,'callback',{@load_callback,handles})
function load_callback(gcf,event_data,handles)
global B
x=5;
B=x;
success=1;
My expectation was that after I press button, the answer must be q=5,s=1.
However, I see q=0,s=0, which I defined in very beginning of soft.
The pressing of the button changes nothing...
Example of call:
[q,s]=TGA_MAIN(500,400,400,300,120,120,120,50, 'STRING')
result is:
q =
0
s =
0
  1 Comment
Rik
Rik on 26 Sep 2021 at 18:26
You shouldn't use globals to share data to a callback. Use the guidata struct instead. For general advice and examples for how to create a GUI, have look at this thread.

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Accepted Answer

Sergey Lopatnikov
Sergey Lopatnikov on 26 Sep 2021 at 19:25
Thank you. It works

More Answers (4)

Image Analyst
Image Analyst on 26 Sep 2021 at 15:21
Set a breakpoint in the callback function. Does it stop there? What happens if you step through it line by line?
  4 Comments
Image Analyst
Image Analyst on 26 Sep 2021 at 17:14
OK you're doing it the harder way. But the question is the same. Set a breakpoint there and see if you step into that function. Do you?

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Sergey Lopatnikov
Sergey Lopatnikov on 26 Sep 2021 at 17:27
:). To my view it is way easier and more flexible way.
Yes, it nornaly stops at preack poin.
The proble is how to transfer data from inside of callback function to main function?
I thought that B and "success" wich I defines ad global must be visible in main function. Howvever by some reason they are not.
  1 Comment
Image Analyst
Image Analyst on 26 Sep 2021 at 17:42
Global does not automatically make a variable global everywhere. Any place that you want a globa variable to be seen, you must have
global B
global success
otherwise they'll be local variables, not the global variables you expected.

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Sergey Lopatnikov
Sergey Lopatnikov on 26 Sep 2021 at 18:02
It does not work like this.
If I try to define global in main AND within call back, it gives error:
[q,s] = TGA_MAIN(500,400,400,300,120,120,120,50, 'LOAD')
Error: File: TGA_MAIN.m Line: 28 Column: 12
Global or persistent variables must be declared in the same scope where they are first used. Move the
declaration for variable 'B' to the outermost function before its first use.
Again: I see corect data within callback, however, I do not know how make those data available outside of call back for main?
  1 Comment
Image Analyst
Image Analyst on 26 Sep 2021 at 18:20
You posted this as an "Answer", (rather than as a comment to my Answer), so have you answered it?
Is the code in your original post your entire program?

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Steven Lord
Steven Lord on 26 Sep 2021 at 18:22
MATLAB is behaving correctly. You create the pushbutton and give it a callback and then immediately exit from the function in which it was created, returning the values of the outputs that they have at that time. By default MATLAB will not wait for you to push the button (and so update the global variable) in the workspace of that function. You could try to make MATLAB wait using something like uiwait so the TGA_MAIN function doesn't return until the figure with that button gets closed or some property updated.

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