In expression (i, ++i, 1), comma used is comma operator
the comma operator (represented by the token
,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).
Leftmost i will be evaluated and its value will be discarded. Then ++i will be evaluated and will increment i by 1 and again value of the expression ++i will be discarded, but side effect is permanent . Then 1 will be evaluated and the value of the expression will be 1.
It is equivalent to
i;
++i;
i = 1 + 1;