Because = has a higher precedence than , (which has the lowest), the first is the same as
(i = 1),2,3,4,5;
which assigns 1 to i (i = 1) then evaluating the expressions 2, 3, 4, and 5 through the comma operators (the whole expression actually results in 5, which is not used). In the second one,
(1,2,3,4,5)
is parenthesized, therefore it will be first evaluated before =. It results in 5 (the right-most expression; this is the behavior of the comma operator) which is then assigned to i.
i = (1,2,3,4,5);
| |
\---------\--- results in 5 then is assigned to i
See operator precedence Wikipedia article.
