How to assign different value to selection in listbox.

function listbox1_Callback(hObject, eventdata, handles)
% hObject    handle to listbox1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: contents = cellstr(get(hObject,'String')) returns listbox1 contents as cell array
%        contents{get(hObject,'Value')} returns selected item from listbox1
str1 = cellstr(get(handles.listbox1, 'string'));
val1 = get(handles.listbox1, 'value');
if isempty(val1)
  selection1 = '';
else
  selection1 = str1{val1};
end
a = selection1;
b = str2double(handles.edit1.String);
c = a + b;
handles.edit2.String = num2str(c);
b = str2double(handles.edit1.String);
if isnan(b)
    warningMessage = sprintf('b is empty.\nPlease put something in the edit field for b.');
    uiwait(errordlg(warningMessage));
    handles.edit2.String = 'Please enter b';
    return;
end

i want to assign some other values to val1 using if else ...how can it bo done