急！MATLAB的GUI界面中怎样通过edit text把值赋给变量a？

我原程序是这样的，试过不行
function length_input_Callback(hObject, eventdata, handles)
% hObject handle to length_input (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
% Hints: get(hObject,'String') returns contents of length_input as text
% str2double(get(hObject,'String')) returns contents of length_input as a double
length=str2num(get(handles.length_input,'string'));
%变量名为length，edit text标签为length_input

相关了解……