I am trying to plot a function and its derivative using fplot.
fplot('3*x*sin(x) - 2*x', [-2*pi 2*pi], 'r')
This is working, But the command below is showing error.
fplot('diff(3*x*sin(x) - 2*x)', [-2*pi 2*pi], 'g')
Error Shown
??? Subscripted assignment dimension mismatch.
Error in ==> fplot at 105
x = xmin+minstep; y(2,:) = feval(fun,x,args{4:end});
Please give me some suggestions?
The problem is including diff in the function string. Your function needs to produce N outputs for N inputs, but having diff give N-1 outputs. From the documentation on fplot (emphasis added):
The function must be of the form y = f(x), where x is a vector whose range specifies the limits, and y is a vector the same size as x and contains the function's value at the points in x (see the first example).
zplesivcak's answer shows one way to work around this limitation. You could also evaluate the function non-symbolically:
x = linspace(-2*pi, 2*pi, 1000);
y = diff(3*x.*sin(x) - 2*x);
figure
plot(y)
@shoelzer Thanks, But I was exploring the ``fplot` function.You could first calculate derivative, transform result in a string, and then plot obtained function:
syms x;
di = char( diff(3*x*sin(x)-2*x) );
fplot(di, [-2*pi 2*pi], 'g');
Edit: @shoelzer provides correct explanation for the error you've received.
@zplesivcak, Thank you for your fast reply
d = diff(3*x*sin(x)-2*x)gives the derivative of that function.fplot('d', [-2*pi 2*pi], 'g')gives a wrong plot