Understanding post and pre increment with comma in between

Question

Consider the below program

#include <stdio.h>
void main(){
    int p = -8;
    int i = (p++, ++p);
    printf("%d\n", i);
}

I am unable to get why the output is -6.

p++ would increment after the assignment statement is executed, ++p will increment before thereby making -8 to -7.

How i is assigned -6?

The return value of `++p` is the value *after* increment, so -6 — UnholySheep, Dec 25 '18 at 11:50
I think this is undefined behavior. https://stackoverflow.com/questions/22817517/do-parentheses-force-order-of-evaluation-and-make-an-undefined-expression-define — DollarAkshay, Dec 25 '18 at 11:54
@AkshayLAradhya: again, no. `,` introduces a sequence point. — Mat, Dec 25 '18 at 11:55

Kindred · Answer 1 · 2018-12-25T11:55:02.160

6

Because for a comma operator A,B then A is done first then B, and p++ increment the p, and ++p also increment the p. Operator precedence.

Or think like this

int i = (p++, ++p);

is

p++;
int i = ++p;

edited Dec 25 '18 at 11:55

answered Dec 25 '18 at 11:50

Kindred

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So, that means even if it was p++, it is executed and the value is changed? – user3767495 Dec 25 '18 at 11:52
@user3767495 Think like this: after all, when `p++` is done, the `p` is incremented. – Kindred Dec 25 '18 at 11:52

score 4 · Answer 2 · answered Dec 25 '18 at 11:51

4

The expression (p++, ++p) has a comma operator. The comma operator evaluates its operand left-to-right and yields the result of the right most operand. Thus i gets the value -6 (after p++ and ++p operations).

answered Dec 25 '18 at 11:51

P.P

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score 2 · Answer 3 · answered Dec 25 '18 at 11:57

as ptr_user7813604 said, you are using the comma operator which is a binary operator in c. It evaluates it's first operand (in your case p++ increments p) and then discards the result (meaning p was incremented but not assigned to any variable) and then it evaluates the second operand (in your case ++p increments p) and returns this value and type. because the second operand is ++p so you receive the value after it was incremented, if the second operand was another p++ you would have assigned to i the value before it was incremented.

for additional information about the comma operator you can look up https://en.wikipedia.org/wiki/Comma_operator.

Understanding post and pre increment with comma in between

3 Answers3