pre-increment and post-increment in C

Question

First of all, I would like to apologize, if this question has been asked before in this forum. I searched, but could't find any similar problem.

I am a beginner in C. I was going through a tutorial and came across a code, the solution of which I can't understand.

Here is the code -

#include <stdio.h>
#define PRODUCT(x) (x*x)

int main()
{
    int i=3, j, k;

    j = PRODUCT(i++);
    k = PRODUCT(++i);

    return 1;
}

I tried running the code through compiler and got the solution as "j = 12" and "k = 49".

I know how #define works. It replace every occurrence of PRODUCT(x) by (x*x), but what I can't grasp is how j and k got the values 12 and 49, respectively.

Any help would be appreciated.

Thank you for your time.

"I can't grasp is how j and k got the values" - neither can your code, so you're in good company. Thus the nature of *undefined behavior*. — WhozCraig, Apr 15 '16 at 19:54
Hurry up and close before any more rep-PersonalServicesWorkers show up:( — Martin James, Apr 15 '16 at 19:55
If that code came from a tutorial then you need to find a better tutorial. — John Bollinger, Apr 15 '16 at 20:05

score 4 · Answer 1 · answered Apr 15 '16 at 19:52

4

Your code will invoke undefined behavior. Anything could happen. The macro in statements

j = PRODUCT(i++);
k = PRODUCT(++i);

will be expanded to

j = x++ * x++;
k = ++x * ++x;

In both statements x is being modified more than once between two sequence points.

answered Apr 15 '16 at 19:52

haccks

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...and that goes into my 'Rep-PersonalServicesWorker of the Month' bookmanrks. – Martin James Apr 15 '16 at 20:01
@MartinJames; Well I already got your comment on the question. No need to repeat here. I am not taking it offensive anyway. Different people different shades. – haccks Apr 15 '16 at 20:05

pre-increment and post-increment in C

1 Answers1