Erro in calling function using horner()

I wrote the following code to fild the roots of a given function. In program, the example function ig given by f(x) = x^3-6x^3+11x-6. When I compile the code the following messages appears:

Incorrect number or types of inputs or outputs for function 'horner'.

Error in test>newtonhorner (line 34)

[pz,b] = horner(a,x);

Error in test (line 4)

a = [1 -6 11 -6]; [x,niter]=newtonhorner(a,0,1.e-15,100);

How can I solve this? Thanks if anyone can help!

My code:

a = [1 -6 11 -6]; [x,niter]=newtonhorner(a,0,1.e-15,100);
function [roots , iter]= newtonhorner(a,x0,tol,nmax)
%NEWTONHORNER Newton - Horner method
% [roots , ITER]= NEWTONHORNER(A,X0) computes the roots of
% polynomial
% P(X) = A(1)*X^N + A(2)*X^(N-1) + ... + A(N)*X +
% A(N+1)
% using the Newton - Horner method starting from the
% initial datum X0. The method stops for each root
% after 100 iterations or after the absolute value of
% the difference between two consecutive iterates is
% smaller than 1.e-04.
% [roots , ITER]= NEWTONHORNER(A,X0 ,TOL , NMAX) allows to
% define the tolerance on the stopping criterion and
% the maximal number of iterations.
if nargin == 2
    tol = 1.e-04; nmax = 100;
elseif nargin == 3
    nmax = 100;
end
n=length(a)-1; 
roots = zeros (n,1); 
iter = zeros(n,1);
for k = 1:n
    % Newton iterations
    niter = 0; x = x0; diff = tol + 1;
    while niter <= nmax & diff >= tol
        [pz,b] = horner(a,x); 
        [dpz,b]= horner(b,x);
        xnew = x - pz/dpz; 
        diff = abs(xnew -x);
        niter = niter + 1; 
        x = xnew;
    end
    if niter >= nmax
        fprintf('Fails to converge within maximum ',...
            'number of iterations\n ');
    end
    % Deflation
    [pz ,a] = horner(a,x); roots(k) = x; iter(k) = niter;
end
return
end

Answer 1

[Torsten]

I don't understand what you intend by using the lines

[pz,b] = horner(a,x); 
[dpz,b]= horner(b,x);
[pz ,a] = horner(a,x)

They will error because of two reasons:

a and b must be symbolic polynomials.

"horner" has only one output, namely the input polynomial in horner form.