[Haskell-beginners] permuting a list
tom.davie at gmail.com
Thu Feb 12 14:19:46 EST 2009
On 12 Feb 2009, at 19:33, Brent Yorgey wrote:
> On Thu, Feb 12, 2009 at 11:58:21AM -0500, Andrew Wagner wrote:
>>> 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...
>> Agreed, I've been quite confused by this thread. In the spirit of
>> though, wouldn't it seem like the "right" method is to generate all
>> 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
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