Ternary and comma operator

Question

int a,b;
a = 1 ? 1,2 : 3,4; // a = 2
b = 0 ? 1,2 : 3,4; // b = 3

Comma operator returns always the right side of comma, but if we make an assignment to variable it returns left except the case when we use (). So how the hell the first expression gives the 2.

I see it as a = 1,2 so it should be 1 but actually a=2.

Why?

Just don't do it. It's probably the compiler punishing you for writing that kind of code. — gnasher729, Nov 01 '16 at 21:58
My proffesor at university showed us that. And i checked compilers. Msvc, gcc gives the same results — Magiczne, Nov 01 '16 at 22:00
@0x499602D2 I completely forgot about something. It's not UB. — Eli Sadoff, Nov 01 '16 at 22:01
`b = 0 ? 1,2 : 3,4;` === `(b = 0 ? (1,2) : 3),4;`.. how about that? — Sourav Ghosh, Nov 01 '16 at 22:02
http://stackoverflow.com/questions/54142/how-does-the-comma-operator-work — J Reid, Nov 01 '16 at 22:02
did u try `0?(1,2):(3,4)` seems like a parsing precedence issue — pm100, Nov 01 '16 at 22:03
@user7101876 This is C or C++? Both are very different languages. — Sourav Ghosh, Nov 01 '16 at 22:13
@Rhymoid fine, but let's learn to learn the difference, shall we? :) — Sourav Ghosh, Nov 01 '16 at 22:32

Sourav Ghosh · Accepted Answer · 2016-11-01T22:13:20.177

Due to operator precedence (comma operator having least precedence), your code actually looks like

int a,b;
(a = 1 ? (1,2) : 3),4; // a = 2
(b = 0 ? (1,2) : 3),4; // b = 3

So, as per the ternary condition rule, quoting C11, chapter §6.5.15

The first operand is evaluated; there is a sequence point between its evaluation and the evaluation of the second or third operand (whichever is evaluated). The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the result is the value of the second or third operand (whichever is evaluated), converted to the type described below. ¹¹⁰⁾

[...]

^{110) A conditional expression does not yield an lvalue.}

For first case, the second operand is evaluated and returned.
For second case, the third operand is evaluated and returned.

Ternary and comma operator

1 Answers1