While Loop & Switch Case troubles

Having a bit of trouble with a while loop and switch case (code below)

what I am trying to do:

A GUI returns a updates a global variable (command), which then causes code sections to execute (in this case dummy code), I want to to be able to jump from case 0 to say case 3 and back again before exiting by selecting case 5.

Problem:

While loop, without the pause(0.0000000001) does not seem to let the GUI update the global variable, or it does (Ctrl+C) shows the variable to have changed, but does not detect it has changed and therefore remains stuck in case 0.

having this pause in there is obviously inefficient and to my mind a pointless exercise

Question:

Is there a better way of achieving this and getting rid of this pause, and can anyone suggest a reason why this might be the case that a short pause (no matter how short) is required inside the while loop?

Thank you!

if true
clear all; close all; clc
gui = UserInterface; % load UI
pause(1)
global command; % command is a global returned by UI
command = 0;
while command ~= 6
  pause(0.000000000000001) % WHY??????? If no pause in loop, does not detect change of command
  switch(command)
      case 0
          display('case 0')
      case 1 
          display('case 1')
      case 2 
          display('case 2')
      case 3
          display('case 3')
      case 4 
          display('case 4')
      case 5
          command = 6;
          display('case 5')
      otherwise
          command = 6;
          display('exit at otherwise statement')
  end
end

end

Answer 1

[Walter Roberson]

The rules for timer interrupts are different than the rules for graphics interrupts (or at least have been; I can't say for R2014b onwards.)

Timer interrupts can, as of a couple of years ago (but not earlier) interrupt between any two lines of code, with actual lines seemingly important; the discussion implied that multiple commands on the same line would be handled as a group.

Graphics interrupts, however, at least up to R2014a, happen only when there is a pause() or uiwait() or waitfor() or drawnow(), or a return to the command line.

I recommend against using global variables. You should instead have the callbacks store the information in a place the loop can find it, and periodically the loop should drawnow() to give a chance for queued interruptions to happen, and the loop should fetch the current stored value.

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.3F

Example:

guifig = gcf;

while true
  drawnow();
  handles = guidata(guifig);
  command = handles.command;
  ....
end

[Yale Brewer]

Thanks, that explains a lot! had a bit of a change and now use the 'UserData' element of the figure to store which radiobutton was selected, which I call back inside while loop... pause no longer required!

thanks!

Answer 2

[Jan]

A loop is a bad method to catch a change, because it costs processor time without solving any work.

waitfor is smarter for this job:

set(FigHandle, 'UserData', []);
waitfor(FigHandle, 'UserData');
UserData = get(FigHandle, 'UserData');

Now let the callback in the GUI change the UserData, e.g. set it to the wanted command instead of using ugly global variables, which are prone to bugs.

The pause command was necessary in your code to let Matlab process graphic events like mouse clicks in the figure, as Walter has explained already. A drawnow would be sufficient also.

pause(0.000000000000001) is the same as pause(0.01), because this is the shortest possible pause. Several Java callbacks work "more reliabale" (if this is not a contradiction) with pause(0.02), e.g. after updates of teh graphics before a getframe.