问题
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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
回答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.
来源：https://stackoverflow.com/questions/3812850/output-of-multiple-post-and-pre-increments-in-one-statement