问题
I'm trying to calculate the derivative of a an edit text box (edit1) and display the answer in a static text box (text1). But it is just displaying numbers. What am I doing wrong?
% --- Executes on button press in pushbutton1.
function pushbutton1_Callback(hObject, eventdata, handles)
% hObject handle to pushbutton1 (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
x=-10:.1:10;
equation = get(handles.edit1, 'String');
y = eval(equation);
derive_func = diff(y);
set(handles.text1, 'String', derive_func);
plot(y);
GUI Image - As you can see, it plots the function, but returns with 3 lines of numbers when it tries to differentiate:
回答1:
You are seeing a conflict between two different uses of the overloaded function diff. The default builtin use is numeric differentiation, and you are applying the function to numeric variable y so you are getting numeric output.
What you seem to want to do is use diff from the symbolic math toolbox to diplay 5*x^4 and that requires that you tell matlab that you want to use the symbolic math toolbox by providing diff with the right input, usually a string.
I am using matlab R14 and a lot has changed in the sym toolbox with newer versions, but the following should work for you.
str = 'x^5';
diff(str,'x')
where str is the expression you want to differentiate symbolically. Note that in my version the sym toolbox is unhappy with the notation x.^5 and prefers x^5, I don't know how it might work on MuPad, but you may have to find a workaround to make sure that you feed MuPad (or whichever sym engine you are using) with a string it can handle.
edit
Earlier suggestions on the use of cd or addpath to control which version of overloaded function diff is used have been removed.
来源：https://stackoverflow.com/questions/18291121/derivative-in-matlab-gui