How to write a symbolic variable funtion?

I followed the instruction for symbolic variables in this link: http://www.mathworks.com.au/help/symbolic/syms.html

But it gives me error instead a function with symbolic variable. Someone please help me! Thanks!

>> clear all
>> syms f(x, y)
Error using syms (line 61)
Not a valid variable name.

>> syms x
>> f(x) = [x x^2; x^3 x^4];
Error using mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

Error in sym/double (line 936)
            Xstr = mupadmex('symobj::double', S.s, 0);

Error in sym>privformatscalar (line 2707)
    x = double(x);

Error in sym>privformat (line 2692)
    s = privformatscalar(x);

Error in sym/subsasgn (line 1462)
                [inds{k},refs{k}] = privformat(inds{k});

>>

Answer 1

[Youssef Khmou]

hi,

No the notation f(x) is used in other programming languages ,try :

 syms x 
 f=[x x^2;x^3 x^4]
 % automatically f is f(x)

here is second example :

 syms x y
 F=[cos(x) sin(y); -sin(y) cos(x)];

[Aggie W]

Im glad to hear your response But I need two variables in f which make it f(x y) and later on I want to substitute values of x and y into f function

[Youssef Khmou]

ok, i edited the answer

[Aggie W]

What do I then obtain a matrix of f if x=complex(1,5) and y=complex(8,4)?

[Youssef Khmou]

i think the variables must have the same size :

 syms x y
 F=[cos(x) sin(y); -sin(y) cos(x)];
 X=exp(j*(0:5));
 Y=exp(5*j*(0:5));
 H=subs(F,{x,y},{X,Y})

I can not tell if they dont have the same size

Answer 2

[Walter Roberson]

The notation syms f(x, y) is relatively new; R2011b I think it was (might be R2012a). Try this:

f = evalin(symengine, '(x,y) -> [x, x^2, x^3, x^4]')

Note: this will not produce the 2 x 2 array you were looking for, but if it produces a "proc" output then you could work on forming the proper array form at that time (once proof of concept has been worked out)