How to determine the pre and post conditions of a program (Hoare-logic)
Here are two other programs that have the same postcondition given the same precondition:
y = 1;
while (y &lt; x) {
    y = 100 * y;
}

x = y = 1;

Your postcondition doesn't describe the output it enough detail. In particular:

The program doesn't change $x$.
The value of $y$ is the smallest power of 10 which is at least $x$.