Output of multiple post and pre increments in one statement [duplicate]

Question

I'm new to C language so please sum1 help me out. A C code written

int i=3;
printf("%d",++i + ++i);

Complier gives O/P =9. How?

Fortunetelling

why do you think it doesn't give that result? – Perce

Answer 1

The results are undefined. You're modifying a variable more than once in an expression (or sequence point to be more accurate).

Modifying a variable more than once between sequence points is undefined, so don't do it.

It might be your compiler, for this particular case decides to evalate ++i + ++i as

• increment the last ++i , yielding 4, leaving i to be 4
• increment the first ++i, yielding 5, leaving i to be 5 (as the prior step left i as 4, incrementing it to 5)
• sum the two values, 4 + 5.

Another compiler, or if you alter the optimization level, or if you change the code slightly, might produce different output.

Fredrickafredrickson

agreed: I checked my gcc 4.1.2 and recv'd the value 10 (which could be just as easily explained as your 9) – Perce

@Perce could you please explain how would you account for 10? – Algo

@Algo - yes, the standard says that this results in undefined behavior. So, any result is equally (in)valid. – Perce

@Perce Yes, it is undefined behaviour and anything can happen because the standard imposes no requirement on the implementation. I was just wondering about which could be just as easily explained as your 9 part in your comment. – Algo