Macro Confusion

[Waleed Mujeeb]

The output is 9 49 can any1 explain how we get this output?

#define P(x) (x*x)
main( )
{

int i=3,j,k;
j=P(i++);
k=P(++i);
printf("\n%d %d",j,k);
}

[Subsonics]

i is 3, so the first one is simple: 3 * 3 is 9. x is replaced with i++, so after j=P(i++) i is 5.

The second one i is incremented twice, so k=P(++i) is 7 * 7 which is 49.

But I'm pretty sure this is undefined behaviour, so it would only explain what may have happened in this case.

[Waleed Mujeeb]

Thansk i got it. The difference lies in post and pre increment operators.

[Subsonics]

But you can make no assumptions about it in this case. For example, I tried this:

printf("%d %d\n", (i++ * i++), (++i * ++i) );

And clang gave me: 12, 42
but gcc: 25, 25

[grumpy]

The difference is not only in pre and post increment operators. The issue is that i is being modified multiple times without intervening sequence points. That is undefined behaviour.

If both lines computed a value of 42, the behaviour would still be correct. If the code reformatted your hard drive, the behaviour would still be correct. That is what "undefined" means in the C standard: any behaviour is allowed, and every possible behaviour is equally correct.

[Salem]

> The output is 9 49 can any1 explain how we get this output?
Sure -> Comma Operator - C And C++ | Dream.In.Code

You're just not going to get a consistent answer out of code like that.