The StrawpigScript class for creating and evaluating random arithmetic expressions in a simple linear GP language, and two examples from the book I'm working on.

example_1.rb

require './strawpig'

puts "sample, x-20, x-10, x-1, x0, x1, x10, x20"
100.times do |i|
  straw = StrawpigScript.new(10)
  puts "#{i}" +
    [-20,-10,-1,0,1,10,20].inject("") {|line, x| line + ", #{straw.evaluate(x).round(5)}"}
end

example_2.rb

require './strawpig'

class StrawpigScript
  
  def wtf
    stack = []
    @script.split.each do |token|
      begin
        case token
        when /([-+]?[0-9]*\.?[0-9]+)/, /[-+]?[0-9]+/, "x"
          stack.push(token)
        when "+", "-", "*", "/"
          raise ArgumentError, "short stack" if stack.length < 2
          arg1, arg2 = stack.pop(2)
          result = "(#{arg1} #{token} #{arg2})"
          stack.push(result)
        else
          # ignore unrecognized tokens
        end
      rescue ArgumentError => e
        # don't pop items from the stack
      end
    end
    return stack
  end
end


straw = StrawpigScript.new(50)
puts "#{straw.script}\n\n[top]"
straw.wtf.reverse.each {|item| puts ">> #{item}\n"}

20.times {puts StrawpigScript.new(20).wtf.length}

strawpig.rb

class StrawpigScript
  attr_accessor :script
  
  TOKENS = ["+", "-", "*", "/", "x", "integer", "float"]
  
  
  def initialize(length)
    intermediate = length.times.inject("") {|program, i| program + " #{TOKENS.sample}"}
    with_ints = intermediate.gsub(/integer/) {rand(21)-10}
    @script = with_ints.gsub(/float/) {(rand*20.0-10.0).round(2)}
  end
  
  
  def evaluate(x)
      stack = []
      @script.split.each do |token|
        begin
          case token
          when /([-+]?[0-9]*\.?[0-9]+)/ # float
            stack.push($1.to_f)
          when /[-+]?[0-9]+/ # integer
            stack.push($1.to_i)
          when "+", "-", "*"
            raise ArgumentError, "short stack" if stack.length < 2
            arg1, arg2 = stack.pop(2)
            result = arg1.send(token.intern, arg2)
            stack.push(result)
          when "/"
            raise ArgumentError, "short stack" if stack.length < 2
            arg1, arg2 = stack.pop(2)
            result = (arg2 == 0) ? 0 : arg1.send(token.intern, arg2)
            stack.push(result)
          when "x"
            stack.push x
          else
            # ignore unrecognized tokens
          end
        rescue ArgumentError => e
          # don't pop items from the stack
        end
      end
      return stack[-1] || 1000000 # the top stack item, or a huge value if it's missing
  end
end