How to run something only once from within a loop?

I need to run a loop many times, possibly hundreds. Sometime along that, one certain value may change. The loop keeps running. How do I make sure that value won't be changed again?

For example:
x = 90;
y = 5;
if x > 100 then y = y * 3;

(now y = 15 and should never change again although x may keep increasing)

TIA