How can I initiate a while loop with a GUI button (using UI control) and then end that loop with a different GUI button?

I am working with a tracking system and I have been using a while loop to continuously scan for data. The whole time I have been terminating the loop with control-c, but now I am manipulating an old GUI and trying to initiate and terminate the loop with buttons in the GUI. I thought this might work but I am not having any luck:

%pivot shift button %collects data while subject performs pivot shift PS_data = uicontrol('Position',[1125 250 100 25],'String','Pivot Shift', 'FontSize',12,'Callback','GUI_data_collection'); %calls data collection

%This ends the pivot shift PivotShiftEndButton = uicontrol('Position',[1125 200 100 25],'String','End Shift',..., 'FontSize',12,'Callback','delete(PS_data)');

fprintf(Ftrak,'C'); shift_data = [];

while ishandle(PS_data)

    scan = fscanf(Ftrak,'%f');
    scan= scan';

    if length(scan) == 7 
    shift_data = [shift_data;scan];

        if shift_data(end,1) == shift_data(end - 1,1)
                shift_data(end,:) = [];

        end
    end
end

[Jan]

Please use the "{} Code" button to format you code, such that it becomes readable. Then explain "not having any luck" with details. It is easier to fix a problem than to guess, what the problem is.

Answer 1

[Jan]

Do not use a string to define a callback. This is supported for backward compatibility with the ancient Matlab 5.3 only. Use a function handle instead:

...'Callback', @GUI_data_collection

'delete(PS_data)' is a bad callback also: Remember that callbacks, which are defined as strings, are evaluated in the base workspace. There PS_data is unknown. Better use a flag stored in the GUI:

PivotShiftEndButton = uicontrol('Position',[1125 200 100 25],'String','End Shift',...,
                                'Callback', @StopDataCollection, ...
                                'UserData' 1);   % <-- The flag

fprintf(Ftrak,'C');
shift_data = [];
while get(PivotShiftEndButton, 'UserData')
  scan = fscanf(Ftrak,'%f').';
  if length(scan) == 7 
  shift_data = [shift_data;scan];    
      if shift_data(end,1) == shift_data(end - 1,1)
              shift_data(end,:) = [];
      end
  end
  drawnow;  % Give the GUI a chance to update
end

function StopDataCollection(hObject, EventData)
set(hObject, 'UserData', 0);
end

Note that the iterative growing of the array uses a huge amount of resources. In each iteration 7 elements a 8 bytes are appended. Therefore a new array is created an the old values are copied. for 10'000 iterations, this does not allocate 10'000*56 bytes=560kB, but sum(1:10000) * 56 bytes: 2.8 GB!

Better pre-allocate the output with a maximum number of recorded data. Although such a limit is a drawback, the current method is limited by exhausting the computer until it crashs.

[Joseph]

Thanks, sorry about the {} I did not know about it. Your method works. I will definitely pre-allocate I did not realize it was that computationally expensive. Thanks a lot!

[Joseph]

Is there a way to re-initialize? I want to be able to collect data for multiple trials.

[Jan]

What does "re-initialize" mean? In my example, allocating 560kB is not such expensive, so doing this repeatedly is okay.

[Joseph]

My Matlab has been down for a while in my lab so I am just now coming back to the problem sorry for the late reply. When I go to collect data again after hitting "End Shift" the while loop does not initiate again and data is not collected. I am going to assume that this is because "UserData" is now 0 instead of 1. How should I fix this?

Thank you so much

[Jan]

You can fix the value of the UserData by setting them:

set(PivotShiftEndButton, 'UsersData', 1);

I do not know where this must be inserted, because I do not know the complete code for this GUI. Perhaps before the loop or after it, maybe in a callback.