Ternary doesn't work?

The ternary operator's purpose is to set a value, not evaluate an equation such as an if-else statement is designed to do. You could rig it up to get the desired results, but I don't think it would be as productive. This is how it would work:

int someTempVariable = 0;
for(int i=0;i<15;i++)
   someTempVariable = i % 2 == 0 ? (even += ar[i]) : (odd += ar[i]);

Here it is setting the value of the "someTempVariable" to either "even" after it is increased by the current index, or odd after it is increased by the current index. The parentheses are required around each separate expression as only the whole can be considered to properly set the value of "someTempVariable".