Attempt to reference field of non-structure array in GUI callback?

I am trying to make my GUI to display a calculated value after two numbers have been entered and after a push of a button. The error i get is

Attempt to reference field of non-structure array.

Error in DRAG2>pushbutton_compute_Callback (line 125) length = str2num(get(handles.length_et,'String'));

Error in gui_mainfcn (line 95) feval(varargin{:});

Error in DRAG2 (line 86) gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)DRAG2('pushbutton_compute_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating uicontrol Callback

My code for this is

    % --- Executes on button press in compute_push.
function pushbutton_compute_Callback(hObject, eventdata, handles)
% hObject    handle to compute_push (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
length = str2num(get(handles.length_et,'String'));
width = str2num(get(handles.width_et,'String'));
Cd = PS09_DragCoeff_hossaina(width,length);
set(handles.dragcoefficient_et,'String',num2str(Cd))

The error appears whenever a variable is assigned to something. How would I fix this issue?

Answer 1

[Adam]

I assume that length_et is not a field on your handles struct.

What is it supposed to be? Judging from the syntax I assume it is the tag of an edit box component. Is there an edit box component with this tag in your GUI?

If so then have you accidentally overridden the handles structure with something else in onoe of your callbacks?

[Asaad Hossain]

length_et is a edit text box, and the component with matching tag is in my GUI. I am not sure if I have overridden. How can I tell? There is a callback function and CreateFcn for length_et after the given code, but both are empty.

[Adam]

Put a breakpoint in on the line of the error (or select the 'Stop on errors' option from the breakpoints dropdown menu in the editor - there is a way to set that option in code too, I just never use that so can't remember it off-hand - something related to dbstop).

Then look at what 'handles' is. Judging from the error message it is not actually a struct at that point which it should be.

[Asaad Hossain]

After selecting stop on errors, I ran the GUI again, and in the workspace, eventdata, handles and hObject showed up. handles and eventdata are empty, where as hObject has a value.

[Adam]

That implies that somewhere in your GUI file you have accidentally overwritten the 'handles' structure.

Look for all your calls to

guidata( hObject, handles )

(or similar)

and check that they are not saving something other than the handles structure. If you want to store additional data you must add it as a field to handles but still pass handles to guidata, don't pass some other object to guidata. (You can also use setappdata to store other data in your gui, but if you use guidata it must always be handles that is passed in).

[Asaad Hossain]

There is only one call to guidata(hObject, handles). I created a fresh new GUI and .m file with guide, and i attempted to run it again, but it gives me the same error. I have no edited anything in the new one besides adding the above code. All the tags for the components match.