MATLAB: Doesn’t the callback I’ve written here seem to work? As far as I can tell all of the syntax is correct and I know that none of the pregenerated code has been altered. Thank’s in advance!

callbackguiguidematlab gui

% --- Exe
cutes on button press in btnCalc.
function btnCalc_Callback(hObject, eventdata, handles)
% hObject    handle to btnCalc (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
gasTank = str2num(get(handles.tankEdit, 'string'));                         % Assigns 'gasTank' to the text field containing the size of the gas tank
gasPrice = str2num(get(handles.priceEdit, 'string'));                       % Assigns 'gasPrice' to the text field containing the cost of gas per gallon
gasBought = str2num(get(handles.boughtEdit, 'string'));                     % Assigns 'gasBought' to the text field containing the amount of gas purchased
dist = str2num(get(handles.app.distEdit, 'string'));                        % Assigns 'dist' to the text field containing the distance traveled in miles

galUsed = gasTank - gasBought ;                                             % Calculates amount of gas consumed since last odometer reset
mpg = dist / galUsed ;                                                      % Calculates miles per gallon over the distance traveled
cpm = gasPrice / mpg ;                                                      % Calculates the cost per mile for the user to travel the distance covered
set(handles.mpgEdit,'string',num2str(mpg));                                 % Displays calculated miles per gallon
set(handles.cpmEdit,'string',num2str(cpm));                                 % Displays cost per mile of travel
These are the error messages that the command window throws when I input values and run the code I've written into this callback : 
Reference to non-existent field 'app'.
Error in mpgCalculator>btnCalc_Callback (line 177)
dist = str2num(get(handles.app.distEdit, 'string'));                        % Assigns 'dist' to the text field containing the distance traveled in miles
Error in gui_mainfcn (line 95)
        feval(varargin{:});
Error in mpgCalculator (line 42)
    gui_mainfcn(gui_State, varargin{:});
Error in matlab.graphics.internal.figfile.FigFile/read>@(hObject,eventdata)mpgCalculator('btnCalc_Callback',hObject,eventdata,guidata(hObject))
 
Error while evaluating UIControl Callback.

Best Answer

The error message:
Reference to non-existent field 'app'.
is telling you that you are trying to access handles.app, that is, the "app" field of the "handles" struct, but "app" is NOT a field of "handles". Set a breakpoint at line 177 and run there. In the command window or Workspace window, examine what fields of "handles" are available -- may you just got the name wrong, or you forgot to added this field earlier to the "handles" struct.
Related Question