Output arguments not getting assigned?

I keep getting the error Output argument "prime2" (and maybe others) not assigned during call to "sum_of_primes". Can someone explain why?

 % This function takes an even integer greater than 2
    % and expresses it as the sum of two prime integers

 function [prime1, prime2] = sum_of_primes (num)

 %%If input is an even integer greater than 2
if (mod(num,2)  == 0) && (num > 2)
    %%Initializing vector x to numbers from 1 to num
        % any non-prime number is set to 0
    for n = 1:num
        if (isprime(n) == true)
            x(n) = n;
        end
    end
    %%Initializing vector z as array x without the zeros
        % z will then be a vector of prime numbers
    z = (x(x~=0))';
    z = z<num;
    %%Doing things and stuff

    for n = 1:size(z)
        prime1 = z(n);
        if (isprime(num-prime1) == true)
            prime2 = num-prime1;
            break;
        end
    end
else
    disp('Please enter an integer greater than 2.')
end

end

Answer 1

[MJE]

I fixed it. z = z<num; was messing up the initialization of vector z. I didn't need this line anyways.

Answer 2

[Jan]

for n = 1:size(z)

This will not do, what you expect, because size replies a vector. Better:

for n = 1:numel(z)

The line z = z<num let z be a logical vector. Most likely you mean:

z = z(z < num);

A simplification for the first loop:

   v = 1:num;
   x = v(isprime(v));

[MJE]

I'd found a workaround for that by making the vector z a column vector so that size(z) technically returned the number of elements...but I'll revise it just so that's a little clearer. But, the same error occurs.