Undefined function or variable 'x'

I'm no matlab expert but i'm writing a function for Simpson's rule when I run the function with a function f in terms of x as one of my inputs it returns an error message "Undefined function or variable 'x'."; I wrote this yesterday, I came back to it today and now it won't work. any help would be massively appreciated function simpsons_rule_function( a, b, fun, n )

    if (mod(n,2)~=0)
      error('n is odd, n must be even');
    end
h=(b-a)/(n);
xp=a:h:b;
syms x;
f(x)= (fun);
Y=f(xp);
Y_0=f(a);
Y_n=f(b);
Y_odds=Y(2:2:end);
Y_evens=Y(3:2:end-1);
Y_evens1=sum(Y_evens);
Y_odds1=sum(Y_odds);
I=(h/3)*((Y_0+Y_n)+4*Y_odds1+2*Y_evens1)
end

[Rena Berman]

(Answers Dev) Restored Question.

Answer 1

[Jan]

What do you expect the line "f(x)= (fun);" to do? How should indexing with the symbolic expression "x" work?