pre/Post Increment

**ganesh bala** · 02-12-2009

from below Snippet, how i value is calculated??

Code:

1)
int i = 2;

int k = i++ - i++;         

printf("%d %d",k i);  

 o/p  I m getting --->  k = 0, i = 3

My doubt is ++ operator has R - L evaluation , here am doing 
post incerement so first expression would be evaluated with original value 
nd next value will get incremented twice so i value is 4.

but am getting  i  value is 3 ???

2)

Code:

int i = 2;

int k = ++i - ++i;

printf("%d %d",k,i);

o/p i m getting ----> k = -1, i = 4

 Here am doing preincrement so first value will get incremented nd 
next expression will be evaluated .My doubt is 3 - 4 or 4 - 3 ?? first 
which variable will get incremented??(left side or right side).

Could u plz clarify it....

**matsp** · 02-12-2009

Both of your cases are undefined, as the C standard does specifically says [something like] "no variable must be updated twice within the same sequence point". Since a sequence point is roughly the same as a statement [there are places where it isn't, but we'll ignore that for now], your

Code:

k = i++ - i++;

breaks that rule.

What actually happens is that the compiler will order the increement of i and subtract independently of each other, which means that it may do:

Code:

k = i - i;
i++;
i++;

or

Code:

i++;
k = i (i++;) - i;   // Not C syntax! It does i++ after taking the first value of i.

And likewise for the ++i variant.

Finally, in theory, the compiler is perfectly allowed to come up with ANY numeric answer - the fact that you are getting some sort of reasonable answer is entirely based on the compiler doing "something sensible", but it's not guaranteed to do that.

--
Mats

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