Differentiate an inline function

Hi!

I have an assignment in Matlab and I need to find a function's 8th and 9th derivate. This function is an inline function, as it is an input from the user.

I have been trying to use the function diff, but it tells me: «Function 'diff' is not supported for class 'inline'.»

Any ideas?

[Azzi Abdelmalek]

Post your code

[Jose]

f = vectorize(inline(string_from_input)); d8f = diff(f, 8); ...

[Azzi Abdelmalek]

It's not clear,what do you want to do?

[Walter Roberson]

Is it required that you use an inline function? Could you use a symbolic function instead? Considering that you are using the symbolic diff() routine?

[Jose]

I wish, but the function is an input from the user, and I don't know any way to convert it to a symbolic one.

But if there is an alternative to the diff function, it may work.

[Walter Roberson]

syms x
f = sym(string_from_input);
diff(f,x,x,x,x,x,x,x,x)

Answer 1

[Matt Fig]

f = inline('x^8');  % Our inline function.
D8 = diff(sym(f),8) % Find the 8th derivative
D8 == prod(1:8)     % Check.  ans = 1 as expected.

[Jose]

Thanks you so much!

Answer 2

[Matt J]

You cannot do symbolic differentiation on inline functions. If you don't have the Symbolic Toolbox, you will have to either consider a specific family of functions and pre-analyze their derivatives. Or, if you want to do numeric differentiation,

t=linspace(0,T,N);
f = vectorize(inline(string_from_input));
result = diff(f(t),8)./( t(2)-t(1) )^8;