Undefined function 'matlabFunction' for input arguments of type 'double'.

I can't seem to figure out why I am getting the error for the code

function [r,n]= Problem25b(f,x1,tol,N)
if nargin<4 || isempty(N), N=20; end
    if nargin<3 || isempty(tol), tol=1e-6; end
    fp=matlabFunction(diff(f));
    f2p=MatLabFunction(diff(f,2));
    x=zeros(1,N+1);
    x(1)=x1;
    for n=1:N
        x(n+1)=x(n)-(f(x(n))*fp(x(n)))/(fp(x(n)))^(2)-f(x(n))*f2p(x(n));
        if abs(x(n+1)-x(n))<tol
            r=x(n+1);
            return
        end
    end

when I run it I get the error:

    [r,n]=Problem25b('x^(3)+2x^(2)+x+2',-3,1e-6,20)
Undefined function 'matlabFunction' for input arguments of type 'double'.

Error in Problem25b (line 5)
    fp=matlabFunction(diff(f));

Answer 1

[Walter Roberson]

'x^(3)+2x^(2)+x+2'

is a character vector. diff() applied to a character vector gives numeric values:

ans =

   -26   -54    11   -10     2     7    70   -26   -54    10    -9     2    77   -77     7

There is no matlabFunction() for numeric values.

You need to construct a symbolic expression out of the character vector that is passed in.

In older MATLAB releases, you could use sym() to convert character vectors into symbolic expressions. In newer MATLAB releases, you need to use str2sym()

[Brock Eubanks]

so I would do:

sym f
fp= diff(f);
f2p=diff(f,2);

[Walter Roberson]

syms x
fs = str2sym(f);
fp = diff(fs, x);
f2p = diff(fp, x);