But, other than sparse arrays1, what would be practical benefit of this data structure be?
Worst-case constant time operations, regardless of how many elements there are in the data structure.
If you add in a hashing function to map strings to your set of i, you still have the collision issue.
Well, yes. Don't do that then. ;-) As I said, it only 'works' if you know certain things about your set - in this case, you know you are only going to store non-negative integers not exceeding U.
Are you considering this as a possible implementation of the arrays Perl's hashes use?
No.
I say sparse arrays because in the 2-array method, U doesn't have to be the upper limit of all i. It simply has to be the upper limit of how many i you can have at one time. So, if you know you'll only ever have a max of 10 things, they could be in the set of 2**32, but you would only need 2 10-element arrays and a short as your C.
In the 2-array method I described, it's essential U is the upper limit of all i, because i is used as an index in the array A.

Abigail

In reply to Re: Data structure challenge by Abigail-II
in thread Data structure challenge by Abigail-II

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