Error using pause Interrupt while evaluating figure KeyPressFcn ?

I have downloaded and modified a snake game script from here. both in the original version and in my version you can only press direction keys exactly 30 times before the game runs into a bug and freezes or ends.

in my version, it seems for every key press there is an error after I push ctrl+c to terminate the program.

the error is: "Operation terminated by user during snark/keyPress (line 47)

Error using pause Interrupt while evaluating figure KeyPressFcn."

so what is wrong?

this is the part of the code that I think is relevant to the problem (pause in line 47 is the one below):

h_fig = figure;
set(h_fig,'menubar','none');
set(h_fig,'CurrentObject',imagesc(grid));
set(h_fig,'KeyPressFcn',@keyPress);

    %Called for any key press.
    function keyPress (~,evt)
        youGottaMove(evt.Key);

        % execute command only once
        % then go straight
        evt.Key = 'q';

        while(~gameover)
            pause(0.4);
            youGottaMove(evt.Key);

        end
    end

the game shows a red box with an arrow. the user must either press right or left arrow keys to move the box (depending on the direction of the arrow right means move towards the right of the arrow and left to the left of the arrow)

this is the whole code :

%snark game;
function snark ()

%Size of the 'playing field'.
max_x = 4;
max_y = 4;
grid = zeros(max_x,max_y);

%Starting position.
x = 1;
y = 1;
grid(x,y) = 1;

length = 1;     %length of snake
positionX = zeros(length);  %array holds all the coordinates
positionY = zeros(length);  %  of the snake's body elements
positionX(1) = x;
positionY(1) = y;

%Position of the food.
x_f = x;
y_f = y;
getNewFood()

justGotFoodFlag = 0;
gameover = 0;

h_fig = figure;

set(h_fig,'menubar','none');

% ADJUST THE SIZE AND POSITION HERE IF DESIRED.
%set(h_fig,'position',[724 46 200 200]);

set(h_fig,'CurrentObject',imagesc(grid));
set(h_fig,'KeyPressFcn',@keyPress);

      %Called for any key press.
      function keyPress (~,evt)
          youGottaMove(evt.Key);

          % execute command only once
          % then go straight
          evt.Key = 'q';

          while(~gameover)
              pause(0.4);
              youGottaMove(evt.Key);

          end
      end

      %Called after a keypress or after timedelay
      function youGottaMove(mov)

          makeMovement(mov);

          if ~gameover    
              %checkBody();
              if ~gameover
                  justGotFoodFlag = 0;

                  grid(positionX(1),positionY(1)) = 0;

                  if (length~=1)
                      for i = 1:length-1
                          positionX(i) = positionX(i+1);
                          positionY(i) = positionY(i+1);
                      end
                  end

                  positionX(length) = x;
                  positionY(length) = y;

                  checkPosEqFood()

                  grid(x,y) = 1;
                  set(h_fig,'CurrentObject',imagesc(grid));

                  % update point
                  xlabel(['you have earned : ' num2str(points) ' points !' ]);

                  % draw snakes head
                  hold on ;

                  % set xl and yl to the right lower corner

                  xl = x+unit/2;

                  yl = y+unit/2;

                  % draw the arrow indocatin the orientation of head
                  switch(orient)

                      case 0
                          plot( [yl-1 , yl],[xl-1 , xl-unit/2] ,'g');
                          plot( [yl-1 , yl] , [xl , xl-unit/2] ,'g');

                      case 1
                          plot( [yl-1 , yl-unit/2], [xl-1 , xl] ,'g');
                          plot( [yl , yl-unit/2 ], [xl-1 , xl] ,'g');

                      case 2
                          plot ( [yl , yl-1], [xl-1 ,xl-unit/2 ] , 'g');
                          plot( [yl , yl-1], [xl , xl-unit/2] , 'g');

                      case 3
                          plot( [yl-1,yl-unit/2], [xl,xl-1] , 'g');
                          plot( [yl,yl-unit/2], [xl,xl-1] , 'g');

                  end

                  hold off;
              end
          end
      end

% orientation of the head

orient = 0;

% unit of the square width
unit = 1;

      %Moves the position of the snake in direction of 'mov' (key)
      function makeMovement(mov)

              switch(mov)

                  case 'q' % go straight

                      switch(orient)

                          case 0
                              y = y+1;

                          case 1
                              x = x+1;

                          case 2
                              y = y-1;

                          case 3
                              x = x-1;

                      end

                  case 'rightarrow'

                      switch(orient)

                          case 0
                              orient = 1;
                              x = x+1;

                          case 1
                              orient = 2;
                              y = y-1;

                          case 2
                              orient = 3;
                              x = x-1;

                          case 3
                              orient = 0;
                              y = y+1;

                      end

                  case 'leftarrow'

                        switch(orient)

                            case 0
                                orient = 3;
                                x =x-1;
                            case 1
                                orient = 0;
                                y = y+1;
                            case 2
                                orient = 1;
                                x = x+1;
                            case 3
                                orient = 2;
                                y = y-1;

                        end

              end

          % check for max and min

          if (y > max_y)
              y = 1;
          end

          if (y < 1)
              y = max_y;
          end

          if (x > max_x)
              x = 1;
          end

          if (x < 1)
              x = max_x;
          end

      end

% points got (foods eaten)
points = 0;
    %Check if you've reached the food
    function checkPosEqFood() 
        if (x==x_f)&&(y==y_f)
            %length = length +1 ;

              % add point
              points = points + 1;

              % update point
              xlabel(['you have earned : ' num2str(points) ' points !' ]);

              positionX(length) = x;
              positionY(length) = y;
              getNewFood();
              justGotFoodFlag = 1;
          end
      end

      %Create new food element
      function getNewFood()
          flag = 1;
          while (flag)
                  x_f = randi(max_x);
                  y_f = randi(max_y);
                  flag = 0;
                  for i = 1:length
                      if (x_f==positionX(i))&&(y_f==positionY(i))
                          flag=1;
                      end
                  end
          end
          grid(x_f,y_f) = 0.5;
      end

end

Answer 1

[Jan]

Nothing is wrong.

Operation terminated by user during snark/keyPress

is the expected message for a ctrl+c. It is likely that the keypress is caught in pause(0.4) and exaggerated that this is an "Error" message: it is wanted by the user obviously.

Please post the error message you get without pressing Ctrl-C. What does this mean in detail: "the game runs into a bug and freezes or ends"? A bug, and freeze or ends? It is unlikely that Matlab has 3 actions at the same time.

[EDITED]

The program design is flawed: Each keypress starts a new instance of the loop over the youGottaMove function. If you stop the game by closing the window, you get as many error messages about the "Invalid or deleted object" as you had key presses during the game. This is a bad idea.

Perhaps this causes the troubles on your computer: The RAM might be exhausted due to the recursive code. But this is a guess only, because I cannot reproduce this.

A better idea is using a timer for the loop and letting the keypress function set the UserData of the timer only, which contains the key code.

[Panonian]

I mean the red box does not move when I press right or left (after 30 key presses of right or left ) . And in this state there is no message or any response in the matlab command line . The whole code is up there you can copy paste it and run it to see what I mean .

[Jan]

@Panonian: If I copy&paste the code to my computer, I can infact run it to debug it. And you can use the debugger also.

When I try to run the code in Matlab R2016b, I get this error at the first keypress already:

You cannot set the read-only property 'Key' of KeyData.
Error in snark/keyPress (line 44)
      evt.Key = 'q'; 
Error while evaluating Figure KeyPressFcn

When I modify the code:

function keyPress (~,evt)
  key = evt.Key;
  youGottaMove(key);
  key = 'q';
  while(~gameover)
     pause(0.4);
     youGottaMove(key);
  end
end

I do not have any problems: neither a crash, a freeze or an error.

See [EDITED] in my answer above for another problem.