From: Kristof Bastiaensen <kristof@...>
Date: 2004-05-13T21:34:00+09:00
Subject: Re: How to make combinations of an array to produce all possible expressions?

On Thu, 13 May 2004 10:38:19 +0200, Erik Terpstra wrote:

> I have an array 'conds', which contains some sub-expressions for an 
> xpath query:
> 
>    conds = ["@title='Foo'", "@edition='Bar'", "@date='20040513'"]

Hi,
I have found at least to ways.
Using recursion (maybe not so efficient):

class Array
  def combine
    comb = Proc.new do |a, *t|
      tail = (t.empty? ? [] : comb[t])
      [[a]] + tail.collect{ |e| [a] + e} + tail
    end
    comb[self]
  end
end

The following should perform better:

class Array
  def combine2
    (1..(2**(size) - 1)).collect do |i|
      i <<= 1
      self.select { (i >>= 1) & 1 == 1}
    end
  end
end

irb(main):018:0> puts conds.combine.collect{|n| n.join ' and '}
@title='Foo'
@title='Foo' and @edition='Bar'
@title='Foo' and @edition='Bar' and @date='20040513'
@title='Foo' and @date='20040513'
@edition='Bar'
@edition='Bar' and @date='20040513'
@date='20040513'
=> nil
irb(main):019:0> puts conds.combine2.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


Kristof