The following expression :-
int main()
{
int x=2, y=9;
cout << ( 1 ? ++x, ++y : --x, --y);
}
gives the following output:-
9
As per my understanding, it should return ++y which should be 10. What went wrong?
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schwillr
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unclear why you expect ++y as output. I could understand --y, but ++y is part of the condition not the output – 463035818_is_not_an_ai Jul 28 '21 at 13:28
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Write it like this: `(1 ? ++x, ++y : (--x, --y))`. That ought to do what you're looking for. The grouping here is a bit peculiar, as the current answer indicates. – Pete Becker Jul 28 '21 at 13:31
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@463035818_is_not_a_number: OP expects `1 ? (++x, ++y) : (--x, --y)`... – Jarod42 Jul 28 '21 at 13:31
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@Jarod42 oh well, I completely misread the conditional. Its a nice way to write unreadable code ;) – 463035818_is_not_an_ai Jul 28 '21 at 13:34
2 Answers
5
The ternary operator (? and :) has higher precedence compared to the comma operator (,). So, the expression inside the ternary conditional is evaluated first and then the statements are split up using the comma operator.
1 ? ++x, ++y : --x, --y
essentially becomes
(1 ? (++x, ++y) : (--x)), (--y)
/* ^^^^^^^^^^^^^^^^^^^^^^^^ is evaluated first by the compiler due to higher position in
the C++ operator precedence table */
You can eliminate the problem by simply wrapping the expression in parentheses:
1 ? (++x, ++y) : (--x, --y)
This forces the compiler to evaluate the expression inside the parentheses first without any care for operator precedence.
Ruks
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Jarod42
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2An explanation about [operator precedence](https://en.cppreference.com/w/cpp/language/operator_precedence) would be helpful here. – BoBTFish Jul 28 '21 at 13:27