C question

Norman Diamond ndiamond at watdaisy.UUCP
Tue Apr 9 04:05:17 AEST 1985

> An inexperienced C programmer wrote a program containing the following:
> 	x = x++;
> Assuming that x originally had the value 5, what should the value be after
> execution of the above expression ?
> In favor of 5:
> 	According to the operator precedence table in K&R on page 49, the ++
> operator has higher precedence than the = operator.  Hence the sequence of
> operations should be:
> 	evaluate x (value == 5)
> 	increment x (x now == 6)
> 	assign the computed value to x (x now == 5 again)
> In favor of 6:
> 	In K&R on page 42, the text says: "But the expression ++n increments n
> *before* using its value, while n++ increments n *after* its value has been
> used."  Hence, the sequence of operations should be:
> 	evaluate x (value == 5)
> 	assign the computed value to x (x still == 5)
> 	increment x (x now == 6)

The language definition doesn't say, and the construct is ambiguous
(semantically ambiguous, not syntactically).  Therefore all four compilers
are correct (for this statement anyway) and the program is incorrect.

Actually I am usually quick to catch ambiguities (roughly the same as
finding counter-examples to false theorems), but never would have thought
of that second interpretation.  Yes the increment occurs after binding
the old value, but in this particular case, even I never thought of other
operations sliding in between.  It's rather astounding.


   Norman Diamond

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