[Why?] Error using diffex Not enough input arguments.

I'm really a fresh user about Matlab, I type the code which is in our textbook as follwing:

function diffex(func,dfunc,x,n)
format long
dftrue=dfunc(x);
h=1;
H(1)=h;
D(1)=(func(x+h)-func(x-h))/(2*h);
E(1)=abs(dftrue-D(1));
for i=2:n
h=h/10;
H(i)=h;
D(i)=(func(x+h)-func(x-h))/(2*h);
E(i)=abs(dftrue-D(i));
end
L=[H' D' E']';
fprintf(' step size finite difference true error\n');
fprintf('%14.10f %16.14f %16.13f\n',L);
loglog(H,E),xlabel('Step Size'),ylabel('Error')
title('Plot of Error Versus Step Size')
format short

BUT! there was an error and I don't konw why:

Error using diffex (line 3)
Not enough input arguments.

I hope someone can teach me how to deal with this .

Answer 1

[Image Analyst]

Type "doc diffex" on the command line.