How to keep the value of certain variables as it is while switching PopUp menu option and updating other variable value?

Hello Everyone,

I have designed a Matlab Gui using guide, with a popupmenu, one update push button and two edit box. Let say, I want to update four variables a, b, c, d. For case 1 of the pop up menu, a and b will be updated with my two edit box value. For case 2, c and d will be u[dated with same edit tool box. I can do that. but, I also want to keep the previous values of a and b, while updating c and d also. But, i am failing to do so, since with case 1, matlab deals with only a and b, and with case 2, it deals with c and d.. But, After updating I want to keep all four variables,

My code for push button is given below:

    function pushbutton1_Callback(hObject, eventdata, handles)
 contents = get(handles.popupmenu1,'String'); 
  popupmenu1value = contents{get(handles.popupmenu1,'Value')};
  switch popupmenu1value
      case 'ML'
        a = str2num(get(handles.edit1, 'String'));
        b = str2num(get(handles.edit2, 'String'));
        a
        b
      case 'CL'
        c = str2num(get(handles.edit1, 'String'));
        d = str2num(get(handles.edit2, 'String'));
        c
        d
        a
        b
  end

So, even during case two, I would like to have a and b with the values of case 1, when they were updated with push button earlier.

Thanks in Advance.