Could you please explain this code segment

[Joel Dice]

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 18
>
> I 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
>
>
>