Error in using diff for derivative

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Anantha Krishna B
Anantha Krishna B on 18 Jan 2022
Commented: Torsten on 19 Jan 2022
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
  3 Comments
Torsten
Torsten on 19 Jan 2022
Numerical difference approximations exist also for derivatives of order n>1:
Or use symbolic maths and MATLAB's diff-command.

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Answers (1)

VBBV
VBBV on 19 Jan 2022
t = f(a);
while (true)
if (gradient(t) ~= 0)
b = a - t./gradient(t);
if (abs(b-a) <= error)
root = b;
break
end
else
root = a;
end
end
check with this
  1 Comment
Torsten
Torsten on 19 Jan 2022
f(a) is a scalar. "gradient" makes no sense in this case.

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