From: Robert Klemme <bob.news@...>
Date: 2004-05-13T20:18:51+09:00
Subject: Re: How to make combinations of an array to produce all possible expressions?


"Harry Ohlsen" <harryo@qiqsolutions.com> schrieb im Newsbeitrag
news:40A349EB.8040002@qiqsolutions.com...
> Erik Terpstra wrote:
>
> > I have an array 'conds', which contains some sub-expressions for an
> > xpath query:
> >
> >   conds = ["@title='Foo'", "@edition='Bar'", "@date='20040513'"]
> <snip>
>
> There's no method I know of, but this seems to work (note that I've
explicitly avoided generating an empty set, because you didn't have one in
your example, but it should probably be included if we wanted to call this
method "subsets" as I have) ...
>
> module Enumerable
>   def subsets
>     values = []
>
>     (2 << length - 1).times do |n|
>       items = []
>
>       length.times do |i|
>         if n[i] == 1
>           items << self[i]
>         end
>       end
>
>       if items.length > 0  # I'd omit this test for a real "subsets"
>         values << items
>       end
>     end
>
>     return values
>   end
> end

Ah, similar idea but nicer coding.  I like especially your calculation of
the counting range and int[idx] as bit test.  I didn't know that one.

Btw, you don't need the test for length 0 if you do

for n in 1 ... (2<<length) do
....

Regards

    robert