A very confusing question. I'll take a wild guess. If this does not answer your question, then you need to be FAR more lear.
Now take the elements of y with odd indexes. That is, start at index 1, then 3, then 5, then 7, etc.
But maybe you want to know the indexes of the elements in the arry that are odd numbers. How can I possibly know from your question?
As you can see, all primes in that list except for the first are odd numbers.