Understand comma operator

Question

int main()
{
    int a = (1,2,3);
    int b = (++a, ++a, ++a);
    int c= (b++, b++, b++);
    printf("%d %d %d", a,b,c);
}

I am beginner in programming. I am not getting how does this program shows me output of 6 9 8.

Answer 1

The , used in all the three declarations

int a = (1,2,3);
int b = (++a, ++a, ++a);
int c = (b++, b++, b++);  

are comma operator. It evaluates the first operand¹ and discard it, then evaluates the second operand and return its value. Therefore,

int a = ((1,2), 3);          // a is initialized with 3.
int b = ((++a, ++a), ++a);   // b is initialized with 4+1+1 = 6. 
                             // a is 6 by the end of the statement
int c = ((b++, b++), b++);   // c is initialized with 6+1+1 = 8
                             // b is 9 by the end of the statement.

---

_{1 Order of evaluation is guaranteed from left to right in case of comma operator.}

Answer 2

The code is not in any way good and nobody in their right mind would ever write it. You should not spend any time in looking at that kind of code, but I will still give an explanation.

The comma operator , means "do the left one, discard any result, do the right one and return the result. Putting the parts in parentheses doesn't have any effect on the functionality.

Written more clearly the code would be:

int a, b, c;

a = 3; // 1 and 2 have no meaning whatsoever

a++;
a++;
a++;
b = a;

b++;
b++;
c = b;
b++;

The pre- and post-increment operators have a difference in how they act and that causes the difference in values of b and c.

Answer 3

I am beginner in programming. I am not getting how does this program shows me output of

Just understand comma operators and prefix ,postfix .

according to rules mentioned in links given to you

int a = (1,2,3);          // a is initialized with 3 last argument .
int b = (++a, ++a, ++a);  // a is incremented three time so becomes 6 and b initilized with 6 . 
int c = (b++, b++, b++);  // b incremented two times becomes 8  and c initialized with  8.
                          // b incremented once more time becomes 9