The K&R C rearrangement license applies to the example expression because addition is mathematically commutative and associative. To distinguish between regular parentheses and the actual grouping of an expression, the left and right curly braces designate grouping. The three possible groupings for the expression are:
i = { {*++p + f()} + g() };
i = { *++p + {f() + g()} };
i = { {*++p + g()} + f() };
All of these are valid given K&R C rules. Moreover, all of these groupings are valid even if the expression were written instead, for example, in either of these ways:
i = *++p + (f() + g()); i = (g() + *++p) + f();
If this expression is evaluated on an architecture for which either overflows cause an exception, or addition and subtraction are not inverses across an overflow, these three groupings behave differently if one of the additions overflows.
For such expressions on these architectures, the only recourse available in K&R C was to split the expression to force a particular grouping. The following possible rewrites respectively enforce the previous three groupings:
i = *++p; i += f(); i += g() i = f(); i += g(); i += *++p; i = *++p; i += g(); i += f();