why is this code not working?

I'm using this code that is supposed to work as is, but I instead get errors as I try to input things, normally "variable not defined" and "array indices must be positive integers or logical values". It is a function that does the Newton Raphson approximation of a function given its derivative and more parameters. Any help appreciated.

function  [root,iter,v] = newton(f,df,x0,itmax,eps)
% this program computes the approximated root given by Newton method
% for f(x)=0
% input: 
%        f,df: function f(x) and derivative of f(x)
%        x0: initial guess
%        itmax: maximum number of iterations allowed
%        eps: error tolerance (we stop when |xnew-xold|<eps*max(1,|xold|)
% output:
%        root: xnew (when convergence is achieved)
%        iter: number of given iterations
%        v: a matrix that writes on each iteration [xold f df xnew-xold] 
iter=0;
coc=1.e9;
x=x0;
while abs(coc)>eps*max(1,abs(x)) && iter<=itmax
    iter=iter+1;fx=f(x);dfx=df(x);
    coc=fx/dfx;
    v(iter,1:4)=[x fx dfx coc];
    x=x-coc;
end
if iter>itmax
    'Newton-Raphson iteration does not converge'
    return
end
    root=x;
end

[Torsten]

Please show us how you call the function.

Answer 1

[David Hill]

Function works fine.

f=@(x)x.^2.*sin(x)-x.*cos(x)+3;%sample function
df=@(x)x.^2.*cos(x)+3*x.*sin(x)-cos(x);
[root,iter,v]=newton(f,df,3,10,1e-8)

root = 3.6314

iter = 5

v = 5×4

3.0000 7.2401 -6.6499 -1.0888 4.0888 -8.1831 -19.1364 0.4276 3.6611 -0.4767 -16.2202 0.0294 3.6317 -0.0049 -15.8834 0.0003 3.6314 -0.0000 -15.8798 0.0000

x=0:.01:5;
plot(x,f(x))

function  [root,iter,v] = newton(f,df,x0,itmax,eps)
iter=0;
coc=1.e9;
x=x0;
while abs(coc)>eps*max(1,abs(x)) && iter<=itmax
    iter=iter+1;fx=f(x);dfx=df(x);
    coc=fx/dfx;
    v(iter,1:4)=[x fx dfx coc];
    x=x-coc;
end
if iter>itmax
    'Newton-Raphson iteration does not converge'
    return
end
    root=x;
end