It's effectively the same as x = 2;3;4; which doesn't do anything with 3 and 4.
x = (2,3,4);
x gets 4;
Try not to use comma's too often in normal statements. The only times I ever use them are:
1. Parameter lists int pow(int a,int b);
2. Multiple definitions int a, b, c;
3. for loops: for (int a = 1, b = 10; a < b; ++a, --b)
though some people frown upon usage #3 in this list as it makes things a little harder to read.