precedence strange thing

[baverlyhills2]

OK guys, I got this code-lines:

int x = 1;
int z = ++x * ++x * ++x * ++x; 

According to my understanding, ++ has a higher precedence than *, thus it will first increment x 4 times and later do the multiplying.
However, what happens is that it increments the two first x's, later multiplies them, then it goes to the next x, increments it, and multiplies it with the result before and so on.

Am I missing something?
Trying to find something about it at the K&R book, but have no idea.

Thanks.

[brewbuck]

It has nothing to do with operator precedence. This is classic undefined behavior due to multiple references to a variable with an pre-increment operator. Only one such reference can occur between two sequence points.

In other words the line of code you wrote is crazy and meaningless.

[baverlyhills2]

OK, I had the feeling, you will say that.
Then, how can I know when to apply the precedence laws and when I know it is meaningless.

Another question - is this code-line also a meaningless?

int a = 1;
int b = (a=1) + (a=9); 

b can be 2 or 18, it depends on the compiler, right?

Thanks in adv.

[Salem]

Expressions
Really get to understand things like "modify once" and "sequence points" and such-like.

Nobody writes code like that in real life - it only seems to be idiot tutors who think it shows something deep and meaningful (whereas all it shows is how shallow their knowledge of C really is).

[baverlyhills2]

Thanks Salem,

So if I understood correctly, the (++x) * (++x) * (++x) is creepy code cause it doesn't have a clear sequence point, and it depends on the compiler which one is evaluated first.
It can increment x 3 times, and then multiply or it can increment 2 times and multiply and so on.
But when I have ++x + 1, then it first increments x then it add 1 to the result (due to precedence law). Am I right?

[Salem]

++x + 1 is well defined, so long as there isn't another x in the same expression.