From: Ruben <Ruben.Vandeginste@...>
Date: 2004-05-13T21:47:20+09:00
Subject: Re: How to make combinations of an array to produce all possible expressions?


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

Something i made some weeks ago seems related to this. In my case, i
wanted to generate a number of combinations by assigning a set of
possible values to a number of 'fields' (you also seem to have fields
and values you want to assign to them). I don't know whether this is
relevant to you, but it might give you some ideas...

So, the first thing i had, was a class 'Profile' which is merely a
placeholder for the 'fields'. The constructor of 'Profile' takes a
Hash as parameter. This Hash is used to assign a certain value to
certain fields (the fields are just instance variables), fields not
included as key in the Hash recieve a default value.

I needed to build a lot of these Profiles and my idea was to define a
number of 'constraints' on certain fields, like this:

constraints = {"size" => [22,23,24,25],
               "tidy" => [true,false],
               "new" => [true],
               "freq" => [[2,2],[2,4],[2,8]] }

With those constraints, i want to generate Profiles initialized with
the following Hashes:

{"size" => 22, "tidy" => true, "new" => true, "freq" => [2,2]}
{"size" => 22, "tidy" => true, "new" => true, "freq" => [2,4]}
{"size" => 22, "tidy" => true, "new" => true, "freq" => [2,8]}
{"size" => 22, "tidy" => false, "new" => true, "freq" => [2,2]}
{"size" => 22, "tidy" => false, "new" => true, "freq" => [2,4]}
{"size" => 22, "tidy" => false, "new" => true, "freq" => [2,8]}
{"size" => 23, "tidy" => true, "new" => true, "freq" => [2,2]}
{"size" => 23, "tidy" => true, "new" => true, "freq" => [2,4]}
{"size" => 23, "tidy" => true, "new" => true, "freq" => [2,8]}
{"size" => 23, "tidy" => false, "new" => true, "freq" => [2,2]}
{"size" => 23, "tidy" => false, "new" => true, "freq" => [2,4]}
{"size" => 23, "tidy" => false, "new" => true, "freq" => [2,8]}
...

To do this, i made the following 'GeneratingHash'. You construct it
with a constraints-Hash, and then you can use it as an iterator with
the 'each' method.  You could use the same idea to generate
combinations of Arrays. A good thing about this way of working is that
you can use it to generate one Hash at a time, do stuff, forget about
the first Hash, generate the next one, do stuff,... might be important
for memory usage in case you have *lots* of combinations. (and yeah, i
know, it is recursive and will already create x Hashes with 'x' the
number of keys you use...)

class GeneratingHash

  attr_accessor :constraints

  def initialize(constraints)
    @constraints = constraints
  end

  def recursive_each(keys,hash,&block)
    if keys.empty?
      yield hash
    else
      key = keys.pop
      @constraints[key].each { |val|
        my_hash = hash.clone
        my_hash[key]=val
        my_keys = keys.clone
        recursive_each(my_keys,my_hash,&block)
      }
    end
  end

  def each(&block)
    mykeys = @constraints.keys
    recursive_each(mykeys,Hash.new(),&block)
  end

end


For those who are interested, i also included the rest of the
(relevant) code. (this is used in a program to perform a number of
benchmarks based on a lot of different 'setups'(Profiles))
Comments on the code are welcome :)

Ruben


==============================

class Profile

  attr_accessor :size, :tidy, :new, :lo_freq, :hi_freq

  def initialize(values)
    @size  = 22     unless @size  = values["size"] 
    @tidy  = false  unless @tidy  = values["tidy"]
    @new   = false  unless @new   = values["new"]
    if values["freq"]
      @lo_freq = values["freq"][0]
      @hi_freq = values["freq"][1]
    else
      @lo_freq = @hi_freq = 0
    end
  end

  ...

end


class ProfileCollection

  attr_accessor :constraints

  def initialize(constraints)
    @constraints = constraints
  end

  def each(&block)
    gh = GeneratingHash.new(@constraints)
    gh.each { |hash|
      prof = Profile.new(hash)
      yield prof
    }
  end

end

=========================================

and those classes are used like this:

constraints = {"size" => [22,23,24,25],
               "tidy" => [true,false],
               "new" => [true],
               "freq" => [[2,2],[2,4],[2,8]] }
prof_collection = ProfileCollection.new(constraints)
prof_collection.each { |profile|
   # do stuff with the profile
}