A self assignment from scalar product

Hi there, i want to have a variable x which is involved in a scalar product with another vector but the result becomes an element of x itself. I 've tried this code ( I DON'T WANT TO USE FOR LOOP ):

x=ones([1,20]);
j=primes(100);
j=j(20);
a=1:20;
x(a+1)=x*j(randperm(20))'

But the result is :

x =

     1    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71    71

It seems the product operation is executed before the assignament. How should I do?