Anonymous Function Differentiation Problem?

Hello Experts,

I have the following function:

>>f = @(x) x^2-1.

I want to find a function that will do differentiation: g = f' = 2*x so that g will be anonymous too and it will be possible to evaluate for example: g(5) = 10.

Please help me,

Steve

Answer 1

[Richard Edmonds]

Yes you can do it. Here is the code to acheive exactly what you want. (too late for you perhaps but this was the first link in my search so I came beack to drop in the best answer I found).

fdash=@(fun,x,epsilon)imag(fun(x(:)+1i*epsilon))/epsilon;
g=@(x)fdash(f,x,10^-20);

Source: http://blogs.mathworks.com/cleve/2013/10/14/complex-step-differentiation/#487a1d22-7fe2-446e-a2a3-69ec283c2d97

Answer 2

[Matt Tearle]

If you know the independent variable is called 'x':

fprime = str2func(['@(x) ',char(diff(sym(regexprep(char(f),'^@\(x\)',''))))])
fprime(pi)

If not... well, you can do it, but it will be a bit messier.

Answer 3

[Walter Roberson]

You cannot do that, not without converting the anonymous function in to a character string and doing symbolic differentiation on that, and constructing the appropriate anonymous function from the result.

If you attempt to do this numerically, then because you can only sample at a finite number of numeric points, there will be an infinite number of functions that match the numeric results, leaving you unable to select the correct function to differentiate. You will, for example, be unable to find removable discontinuities numerically (except by luck). And good luck doing a numeric reconstruction of tan(Pi*exp(x)) .

Answer 4

[Doug Hull]

If you know you will be dealing only with polynomials, can you make something like this work?

PolyDer

Answer 5

[Konstantin Ninidze]

you can try my function: handle_to_sym:

function Fs=handle_to_sym(F)

% input F function_handle

% output Fs symbolic expression

S=char(F);

S=S(5:end);

Fs = evalin(symengine,S);