Operator Precedence in C

Question

Cannot explain the output of the following program. According to my knowledge the output should be 19, but running it gives me output of 20. I used gcc to compile this program.

int main()
{ 

    int x, y = 5;

    x = ++y + ++y + --y;
    printf("%d", x);

    return 0;
}

Your program exploits undefined behavior. undefined as in: You can't know what will happen. — fuz, Feb 23 '13 at 16:50

score 2 · Accepted Answer · answered Feb 23 '13 at 16:52

2

Your program exploits undefined behavior as you modify y multiple times between two sequence points (in your case, the end of the statement). If you turn on warnings with -Wall, your compiler is probably even going to warn you about that.

answered Feb 23 '13 at 16:52

fuz

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score -6 · Answer 2 · answered Feb 23 '13 at 16:53

-6

6+7+6 = 19 so 19 will be your output

answered Feb 23 '13 at 16:53

Faisal Amjad

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No. This is undefined behavior. Your compiler might return whatever it wants. – fuz Feb 23 '13 at 16:54
I suggest you test it out. My machine also gives 20 as stated in the question, but it's undefined behavior so the compiler would be within its rights to print a million or delete your hard drive. – Kevin Feb 23 '13 at 16:56
@Kevin: What happened to nasal daemons? – thejh Feb 23 '13 at 17:01
@thejh Sure, those too, but I can't list all the calamities that could befall a user of undefined behavior :) – Kevin Feb 23 '13 at 18:08

Operator Precedence in C

2 Answers2