From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2001-11-03T16:29:10+09:00
Subject: [ruby-talk:24251] Re: Too much eval evil? (tell me why I shouldn't do this)

In article <2334FC218D0FD6499D8B685EB518AED81FDD59@us-exchange.mosaicsoftware.net>,
Ryan Leavengood <mrcode@mediaone.net> wrote:
>> Let's say I've got a string which has some ruby variables in it and I
>> want each of those variables to take on a range of values and then I
>> want to evaluate that string given every combination of those values
>> (a cartesian product).  And of course I want to be able to use this
>> with a variable number of varying variables, so my cartesian product
>> function has to work given any number of variables.
>
>This was just too interesting of a challenge for me to pass up.  Here is
>my version (with no evaling):
>
># I chose the name "cascade" since we are cascading down through the
>array
>def cascade(a, list = [], &block)
>  return list if list.size == a.size
>  a[list.size].each do |item|
>	 list << item
>	 cascade(a, list, &block)
>	 if list.size == a.size 
>		block.call(list)
>	 end
>	 list.slice!(-1)
>  end
>  list
>end
>
>a = [(0..20), (43..150), ['z','yz','xyz','xz','zz']]
>cascade(a) do |list|
>  puts "width is: #{list[0]}, length is: #{list[1]}, hight is:
>#{list[2]}"
>end 
>
>This runs about twice as fast as the one Thomas originally wrote.  It
>could be probably be cleaned up, but it seems to work pretty well.
>Using an array instead of a hash guarantees the order in which the
>variables will be evaluated.  Being able to call a block gives you more
>flexibility than just evaling a string.  Also, when dealing with ranges
>you don't really need to turn them into arrays...they respond to "each"
>correctly.

I like Ryan's solution better than the one proposed by Thomas (it seems a 
lot cleaner), but I can see that Thomas' solution has an advantage:  What 
if one were using some 
kind of templating system where you have a file which contains embedded 
Ruby code  (like the one I proposed a few days back), then you actually 
need to have the names of the variables produced in the cartesian 
product code match the names of the variables in the template file - it 
looks like the original solution (with a few 
modifications) could do that... I think I'll try it.  

This thread turned out to be surprisingly timely, I was just thinking of 
some way of doing something similar to this in conjunction with my 
Template system for generating test code.

Phil