Quite an interesting technique, and as a disclaimer I really do like it. That being said, this technique will not always get you the fastest iterator. The holy grail of iterators for combinatorial structures is a constant amortized time (CAT) algorithm, meaning that any N successive calls to the iterator takes O(N) time. This is as good as it gets.

Your technique is great as the iterator algorithm mirrors the easy-to-understand recursive algorithm. And kudos for noting (and fixing) the double-recursion. However, your algorithm's running time for iterating through all (N choose K) combinations is proportional to (N+2 choose K+1), so it is not CAT. For a detailed analysis of this and many other interesting iteration algorithms, see section 4.3 of the book Combinatorial Generation by Frank Ruskey. The book does give a CAT iterator for generating combinations, and I believe your general technique of mirroring the recursive algorithm inherently cannot achieve CAT running time (unless you memoize, which defeats the whole purpose of iterating to save memory).

I guess it's a matter of what you want to trade -- (sometimes small) factors of running time, or readability of the algorithm. Although for iteration problems like this, I usually just refer to Ruskey's book and find a pre-packaged CAT algorithm ;)

blokhead

In reply to Re: Recursively-generated Iterators by blokhead
in thread Recursively-generated Iterators by Roy Johnson

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