On Thu, 17 Dec 2009, Bob Plantz wrote:
On Thu, 2009-12-17 at 12:23 +0530, W.H. Kalpa Pathum wrote:The following block of code when compiled with gcc (gcc version 4.4.2 20091027 (Red Hat 4.4.2-7) (GCC) ) resulted in the following output. Could you please explain how the output differs (when assigned to the variable b and in printf statement) and the way the compiler executes each segment of the variable? int main(){ int a = 10; int b = (++a) + (++a); printf("%d %d %d %d\n", b, a++, a, ++a); printf("%d %d %d %d\n", ++a + ++a, a++, a, ++a); return 0; } OUTPUT 24 13 14 14 36 15 18 18I spent much of my life trying to convince CS students that they should never write code like this. Not because of implementation issues, but because it's nearly impossible for the maintenance programmer to determine the intent of the algorithm here. And I always pointed out that the maintenance programmer might be them in a few weeks. I have seen many, many examples, both in industry and in academia, where programmers write tricky code claiming it is more efficient. I claim (a) efficiency is seldom an issue, and (b) looking at the generated assembly language almost always shows it is not more efficient. I believe that the best code is that which (a) correctly solves the problem, and (b) is the most simple-minded in appearance.
Perhaps, but the code in question here is not merely obscure - it's simply meaningless. I think it's a mistake to put such code in the same category as e.g. an affine transform implemented in inline assembly which has well-defined meaning. The latter may pose a challenge for a maintenance programmer, but at least it will yield to persistence. In contrast, undefined code is no better than a "todo" comment - you're only option is to replace it completely with something well-defined based on the documentation and/or context.
Thus my answer to your question is that you should rewrite the code such that it clearly shows what you are trying to do. This example would not have received a high grade in any of my classes. --Bob
