Having a slight problem with Newton-Raphson loop. No errors in command window..

Hi everybody!

First, thank you very much for answering my previous question about the anonymous functions.

The new issue is:

I'm trying to make Matlab solve equations using Newton - Raphson method. So far I've written my code trying to follow the guide on youtube. The only problem is that the user there is using symbolic math toolbox, which I don't have. So I am trying to use the " Numerical differentiation " method and use anonymous function as an input.

Here is my code:

 function [root, iterations] = newtonRaphson(funIn, guessValue)
 x = guessValue;
 for u=0:2814 % max number of iterations
    y=x; 
    derivative = @(x) (funIn(x+0.001)-funIn(x))/0.001; % Numerical diff.
    x=y-funIn(x)/derivative(x);  % Newton-Raphson formula.
    if x==y
        root = x
        iterations = u
        break 
    end
 end

I then use " newtonRaphson(@(x) sin(x) - x, 10) " as an input. The function, unfortunately does nothing, the command window is absolutely empty and I don't even know what the error is. Any ideas guys?

Thank you very much for your help guys!

Answer 1

[Matt Fig]

To elaborate on Walter's answer. You are comparing floating point numbers for absolute equality - a mistake in general.

Instead, set a tolerance in the code:

tol = 1e-10;

Then your conditional should read:

abs(x-y)<tol

Also, it is a good idea to define your outputs for a default case, for when the conditional is not met but the user wants two return arguments...

[Kokalz]

Thank you very much for your help!

Answer 2

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F