On Dec 5, 2011, at 10:00 AM, Geoffrey S. Knauth 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.
I presume you mean 6 points {A,B,C,D,E,F}, and 15 edges rather than the 6x6=36
that a dumb interconnect would do.
As it happens, I ran into a similar problem in helping a student last week.
He's working in Java, so he's probably out of luck :-) Anyway, I had pointed
out to him the Java "for-each" loop, he got the idea, and he wanted to loop
over all the distinct 2-element subsets of a given list. He had two nested
for-loops with integer indices, and wanted to know if there was a slicker way.
We concluded that Java for-each-loops by themselves don't do the job, but one
could write a subsetsOfSize function and then say
for (Pair p : myList.subsetsOfSize(2)) { ... }
or in Racket
(for (p (subsets-of-size myList 2)) ...)
Of course, many scripting languages would allow you to extract the elements of
p at the same time, e.g. Ruby
myList.subsetsOfSize(2).each { |x,y| ...}
which would take a bit of macro-wrangling in Racket.
Stephen Bloch
sbl...@adelphi.edu
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