Error in using diff for derivative

Hello

I am writing a function for calculating a root of a function using Newton's Method. In this i need to calculate value of derivative of the function. So I used diff but I am getting an error. I am thinking that the error is because diff only acts on symbolic functions and this somehow causes error. Is there any alternative way?

Below is my code

function [root] = Newton_rap_fun(a, error, f)
% function for newton's method
%   a is the initial approximation, error is the error tolerance given by
%   user
%   f is the given function


while (true)
    if (diff(f(a)) ~= 0)
         b = a - f(a)/diff(f(a));
         if (abs(b-a) <= error)
             root = b;
             break
         end

    else
         root = a;
    end    
end

[Torsten]

Use

df = (f(a+1e-6)-f(a))*1e6

instead of

diff(f(a))

[Anantha Krishna B]

@Torsten Thank you, this is a clever way. But is there any function that evaluates the derivative at point? I am asking because there will be some situations in which I need to evaluate higher order derivatives.

[Torsten]

Numerical difference approximations exist also for derivatives of order n>1:

https://en.wikipedia.org/wiki/Finite_difference_coefficient

Or use symbolic maths and MATLAB's diff-command.