convert popupmenu's selected string to number form

i've popupmenu having entries as

 2

 4

 8

 16

 32

 64

 128

i want when user selects '2' , this string value convert to 2 as number so that i can use it in variable and i can use it in my mathematical calculations.

i tried following code....

 str = get(handles.popupmenu3,'String');
 val = get(handles.popupmenu3,'Value');
 switch str{val};
    case '2'
          a = get(handles.popupmenu3,'String');
          a1 = str2num(a);

          b = get(handles.popupmenu3,'String');
          b1 = str2num(b);

    case '4'
          a = get(handles.popupmenu3,'String');
          a1 = str2num(a);

          b = get(handles.popupmenu3,'String');
          b1 = str2num(b);
   case '8'
         a = get(handles.popupmenu3,'String');
          a1 = str2num(a);

          b = get(handles.popupmenu3,'String');
          b1= str2num(b);
  case '16' 
         a = get(handles.popupmenu3,'String');
          a1 = str2num(a);

          b = get(handles.popupmenu3,'String');
          b1 = str2num(b);
  case '32'
         a = get(handles.popupmenu3,'String');
          a1 = str2num(a);

          b = get(handles.popupmenu3,'String');
          b1 = str2num(b);
  end

same for other cases like 64 and 128...

when i run this i got the error:

Error using str2num (line 33)
Requires string or character array input.

plz help me in correcting my error...

Answer 1

[Walter Roberson]

You don't need the switch and cases. Just use

a1 = str2double(str{val});
b1 = a1;