Multiplication of a numeric array and a symbolic array in Matlab

Question

I have the following code:

a=zeros(1,3);
syms x
y=zeros(1,3);
for j = 1:3
    a = [zeros(1,j-1) 1 zeros(1,3-j)];
    y(1,j)=x*a(1,j);
    display(y(1,j));
end;

which I want to give me an array like [0 x 0] for each iteration. But, on contrary, it gives me the following error:

The following error occurred converting from sym to double:
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 
Untitled123 (line 6)
y(1,j)=x*a(1,j);

Any suggestions?

Answer 1

The problem is that you allocated y as a numeric (floating-point) array, not a symbolic one. a is numeric, but when you multiply it by x the result becomes symbolic. Because x is a symbolic variable rather than a symbolic value, Matlab cannot perform the automatic cast back to double precision. You can allocate y like this:

y = sym(zeros(1,3));