On Mon, 2011-12-05 at 09:00 -0700, Ryan Culpepper wrote:
> On 12/05/2011 08:40 AM, Sam Tobin-Hochstadt wrote:
> > On Mon, Dec 5, 2011 at 10:00 AM, Geoffrey S. Knauth<ge...@knauth.org>
> > wrote:
> >> I'm wondering if there is something in the now very rich set of Racket
> >> libraries that already does this. Let's say I have 5 points {A,B,C,D,E}.
> >> I want to interconnect all of them:
> >>
> >> {AB,AC,AD,AE,AF,BC,BD,BE,BF,CD,CE,CF,DE,DF,EF}
> >>
> >> That's 15 edges rather than the 5x5=25 that a dumb interconnect
> >> would do. To start, I just need to track and sort the edge
> >> weights, and AB is the same as BA.
> >
> > Here's the start of an answer (sadly quadratic, but for N=121 I don't
> > think that will matter much):
> >
> > #lang racket
> > (require unstable/sequence)
> > (define (subsets-of-size-2 l)
> > (for*/lists (r) ([i l] [j l]
> > #:unless (eq? i j)
> > #:unless (member (cons j i) r))
> > (cons i j)))
> >
> > (for ([(p q) (in-pairs (subsets-of-size-2 '(A B C D E F)))])
> > (printf "~a<-> ~a\n" p q))
> >
>
> That looks quartic in the length of l, because of the member check.
>
> Here's a quadratic version:
>
> (require srfi/1)
> (define (subsets-of-size-2 l)
> (for*/list ([ne-sublist (pair-fold-right cons null l)]
> [b (in-list (cdr ne-sublist))])
> (cons (car ne-sublist) b)))
>
> Note: (pair-fold-right cons null l) produces a list of the non-empty
> sublists of l.
Hi Ryan,
some time ago I was looking for an efficient algorithm in Racket to
extract k-sized combinations of elements from a list l... Could you
provide a generalisation of your code, I mean "subset-of-size-k"
where k < (lenght l) ?
TIA,
Maurizio.
> Ryan
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