Odds are that your computer doesn't export any language where it will do exactly as you say (amd64 machine code certainly won't execute exactly as written). And how much difference it makes varies from one language to another.

But the specific example from the OP, of uninitialized variables, is one of those cases where the C spec famously goes completely out of line and says your code can do whatever, run with a random value, fail, initialize it, format your hard drive, make a transaction on your bank account... whatever.