From: Robert Klemme <bob.news@...>
Date: 2004-05-13T18:48:55+09:00
Subject: Re: How to make combinations of an array to produce all possible expressions?


"Erik Terpstra" <erik@terpnet.nl> schrieb im Newsbeitrag
news:40a333fc$0$65124$e4fe514c@news.xs4all.nl...
> I have an array 'conds', which contains some sub-expressions for an
> xpath query:
>
>    conds = ["@title='Foo'", "@edition='Bar'", "@date='20040513'"]
>
> Is there an existing library that lets me construct all possible
> combinations like this?
>
>    puts conds.<some Array extension method>.collect{|n| n.join ' and '}
>
> which produces:
>
> @title='Foo' and @edition='Bar' and @date='20040513'
> @title='Foo' and @edition='Bar'
> @title='Foo' and @date='20040513'
> @edition='Bar' and @date='20040513'
> @title='Foo'
> @edition='Bar'
> @date='20040513'

Not yet, but:

module Enumerable
  def combine
    masks = inject([[], 1]){|(ar, m), e| [ar<<m, m<<1]}[0]
    all = masks.inject(0){|al, m| al|m}

    result = []

    for i in 1..all do
      tmp = []

      each_with_index do |e, idx|
        tmp << e unless (masks[idx] & i) == 0
      end

      result << tmp
    end

    result
  end
end


irb(main):098:0> puts conds.combine.collect{|n| n.join ' and '}
@title='Foo'
@edition='Bar'
@title='Foo' and @edition='Bar'
@date='20040513'
@title='Foo' and @date='20040513'
@edition='Bar' and @date='20040513'
@title='Foo' and @edition='Bar' and @date='20040513'
=> nil

    robert