Preprocessors Directives

Question

I am not able to understand the output of the following code can someone please help me out?

#define PRODUCT(x) (x*x)
main()
{
     int i=3,j,k;
     j=PRODUCT(i++);
     k=PRODUCT(++i);
     printf("\n %d%d",j,k);
}

For the above I got the output as:

9 and 49

I am not able to understand how 49 is coming and when I commented out

k=PRODUCT(++i);

I got the output as 25. Don't know what is going on in the program

When you don't understand code which contains macros, the first step is to expand them manually. (If you use macros at all, of course - which you shouldn't.) — Daniel Daranas, Dec 03 '14 at 10:53
possible duplicate of http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points — Natasha Dutta, Dec 03 '14 at 10:54

score 1 · Answer 1 · answered Dec 03 '14 at 10:54

1

Macros do text substitution. PRODUCT(i++) expands to (i++*i++). Aside from being extremely surprising that there are now two modifications of i, this is also undefined behavior, because the two modifications are not sequenced.

answered Dec 03 '14 at 10:54

Sebastian Redl

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Preprocessors Directives

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