The important part that makes the parens in the second operation unnecessary is that addition is associative, not that it's commutative. And concatentation is a

The important part that makes the parens in the second operation unnecessary is that addition is associative, not that it's commutative. And concatentation is also associative, so even if those were strings, the parens would still be unnecessary. That is, (a+b)+c == a+(b+c). Interestingly, C integer addition is not actually associative, since (1+INT_MAX) + (-1) is UB, but 1+(INT_MAX+(-1)) should in principle be defined as INT_MAX.