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

Question

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• Why are these constructs using pre and post-increment undefined behavior? 14 answers
• int b=0,a=1;b= ++a + ++a; what is the value of b? what is the calculation for it? [duplicate] 2 answers

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

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

Complier gvs O/P =9. How?
Thanx in advance

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.