On 12 Feb 2009, at 19:33, Brent Yorgey wrote:
On Thu, Feb 12, 2009 at 11:58:21AM -0500, Andrew Wagner wrote:
<rant> It seems everyone has just been reading the first few words of Jan's email and not the actual content. Jan is clearly trying to write a *random list shuffling* function, not a function to generate permutations. Let's try to be helpful, people... </rant>
Agreed, I've been quite confused by this thread. In the spirit of laziness, though, wouldn't it seem like the "right" method is to generate all the permutations lazily, and then choose a random element of that list?
Well, it sounds nice, but it's pretty inefficient. And by "pretty inefficient" I mean "horrendously, terribly inefficient" -- there are n! permutations of a list of length n, so this would take time O(n!) as opposed to O(n); O(n!) is even worse than O(2^n).
Would it? We're talking about lazyness here... it's not gonna compute one it doesn't need, and if you're somewhat cleverer with your permute function than I was, I'm sure you can do as little computation as the imperative version. Bob