From: Stephen Duncan <stephen.duncan@...>
Date: 2007-04-16T08:28:42+09:00
Subject: Re: Ruby Truth Table Generator

Thanks for the tips.  I did originally have all the string stuff in
to_s, but had decided to do it in the constructor to avoid
recalculating everything, since the formula wasn't modifiable.  I've
implemented a compromise for now.  Especially thanks for the mask tip,
I was troubling coming up with a clean way to generate the values.

-Stephen

On 4/15/07, james.d.masters@gmail.com <james.d.masters@gmail.com> wrote:
> On Apr 15, 9:21 am, "Stephen Duncan" <stephen.dun...@gmail.com> wrote:
> > I'd like to say I just whipped this up, but I'm still a Ruby-newbie,
> > so this took me a few days.  I'd love to get some feedback on the
> > code...
>
> After looking at the code, it looks like the @string instance variable
> and the methods generate_values and get_next are used only once and
> are used for building a string representation of the truth table.  I'd
> recommend doing away with these and do the calculation in to_s (this
> could open the possibility of changing the formula after object
> creation if you wanted to enhance this class to allow dynamic
> formulas).  In any case, I would recommend counting through
> possibilities and using a bitwise mask to figure out the permutations
> instead of using the generate_values method:
>
> class TruthTable
>   def initialize(formula, &expression)
>     @formula = formula
>     @expression = expression
>     @variables = @formula.gsub(/[^[:alpha:]\s]/, " ").split.uniq
>     @expression ||= eval("Proc.new {|#{@variables.join(',')}|
> #{formula} }")
>   end
>
>   def to_s
>     # create a mask for each of the variables
>     var_masks = (0..@variables.length-1).to_a.reverse.map {|i| 2**i}
>
>     # create header
>     str = @variables.join("\t") + "\t#{@formula}\n"
>
>     # Go through all permutations of variables
>     0.upto(2**@variables.length-1) do |cnt_mask|
>       # compare the count mask versus each variable mask - map as
> values
>       values = var_masks.map {|var_mask| (var_mask & cnt_mask) ==
> var_mask}
>       str += values.join("\t") + "\t#{@expression.call(*values)}" +
> "\n"
>     end
>
>     str
>   end
> end
>
> As for the overall concept of this class, I did something similar a
> while back and used the same technique that you did - accept a string
> to allow a visual representation and then eval the formula within a
> Proc.new.
>
>
>


-- 
Stephen Duncan Jr
www.stephenduncanjr.com