MATLAB: I’m making a Pong Game and trying to set the limit of the y direction of the paddles to y max to 9 and y min to -9

animationkeypressfcnMATLAB

 function pong
    clc
    clear all
    close all
    global x
    global y
    global y1
    global y2
    global h_rec1
    global h_rec2
    h_fig=figure;
    set(h_fig, 'KeyPressFcn', @kpfcn)
    axis([-10 10 -10 10]);
    y5=-2;
    y6=2;
    y7=-2;
    y8=2;
    x1=[-8 -7 -7 -8];
    y1=[-6 -6 y5 y7];
    h_rec1=patch(x1,y1,'k');
    x2=[7 7 8 8];
    y2=[6 y8 y6 6];
    h_rec2=patch(x2,y2,'k');
    x3=[-10 -10 10 10];
    y3=[-10 -9 -9 -10];
    patch(x3,y3,'r')
    x4=[-10 10 10 -10];
    y4=[9 9 10 10];
    patch(x4,y4,'r')
    hold on
    x0=5;
    y0=4;
    v0x=2;
    v0y=2;
    dt=0.005;
    %v0=sqrt(v0x^2+v0y^2);
    b=plot(x0,y0,'go','MarkerSize',10,'MarkerFaceColor','g');
    ylim([-10 10]);
    xlim([-10 10]);
    x=x0;
    y=y0;
    vx=v0x;
    vy=v0y;
    e = [0.9 1 1.2];
    while(1)
      x=(x+(vx*dt));
      y=(y+(vy*dt));
      disp(x);
      disp(y);
      if ( y <= -9 || y>= 9 )
          vy=-e(randi([1,3]))*vy;
      end
       if (y >= y1(1,3) && y<=y1(1,4)  && x <= x1(1,3))
          vx= -e(randi([1,3]))*vx;
          vy= -e(randi([1,3]))*vy;
          if (y >= y2(1,3) && y<=y2(1,4) && x >= x2(1,2))
              vx= -e(randi([1,3]))*vx;
              vy= -e(rand([1,3]))*vy;
          end
       end
       disp([y1(1,3),y1(1,4)]);
       disp([y2(1,3),y2(1,4)]);
       set(b,'Xdata',x)
       set(b,'Ydata',y)
       pause(0.001)
       if (y <= y2(1,3) && y>=y2(1,4) && x >= x2(1,2))
              vx= -e(randi([1,3]))*vx;
              vy= -e(rand([1,3]))*vy;
       end
    end
 end
 function kpfcn(hObject,event)
    global y1
    global y2
    global y5
    global y6
    global y7
    global y8
    global h_rec1
    global h_rec2
    switch event.Key
      case 'uparrow'
          y2= y2+[1 1 1 1];
      case 'downarrow'
          y2= y2-[1 1 1 1];
      case 'w'
          y1= y1+[1 1 1 1];       
      case 's'
          y1= y1-[1 1 1 1];
    end
    set(h_rec1, 'YData', y1)
    set(h_rec2, 'YData', y2)
    if y5>9
      y5=9;
    end
    if y5<-5
      y5=-5;
    end
    if y6>5
      y6=5;
    end
    if y6<-9
      y6=-9;
    end
 end

Best Answer

David - it looks like you are using your kpfcn to move the paddles and are using the code
    if y5>9
      y5=9;
    end
    if y5<-5
      y5=-5;
    end
    if y6>5
      y6=5;
    end
    if y6<-9
      y6=-9;
    end
to control the positions of the paddles so that they don't exceed the minimum and maximum bounds of the gaming area. But this code is affecting variables y5 and y6 which are no longer relevant. They were only used when creating the initial positions of the paddles but since then are no longer used. It is the y1 and y2 arrays that control the positions of the paddles and so it is the elements within these arrays that you should be adding your checks to. For example, you could do
    function kpfcn(hObject,event)
        switch event.Key
          case 'uparrow'
              if max(y2) < 9
                  y2 = y2 + [1 1 1 1];
              end
          case 'downarrow'
              if min(y2) > -9
                  y2 = y2 - [1 1 1 1];
              end
          case 'w'
              if max(y1) < 9
                  y1 = y1 + [1 1 1 1];
              end      
          case 's'
              if min(y1) > -9
                  y1 = y1 - [1 1 1 1];
              end  
        end
        set(h_rec1, 'YData', y1)
        set(h_rec2, 'YData', y2)
    end
where we only adjust the position of the paddle if its maximum or minimum position (along the y-axis) allows for the paddle to be moved up or down. Note how I've removed the global variables which are not necessary if you nest this function within the parent function. That way the nested function has access to all local variables within the parent (and so you can remove the global qualifier from those variables). See nested functions for more details.
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