The StickpigScript class for creating and evaluating random arithmetic expressions in a simple S-expression tree GP language, and two examples from the book I'm working on.

example_1.rb

require './stickpig'

100.times do |i|
  stick = StickpigScript.new(0.4)
  puts "#{i}, #{stick.script.length}" +
    [-20,-10,-1,0,1,10,20].inject("") {|line, x| line + ", #{stick.evaluate(x).round(5)}"}
end

stickpig.rb

class StickpigScript
  attr_accessor :script
  
  LEAVES = ["x", "constant"]
  BRANCHES = ["negative( # )", "add( #, # )", "subtract( #, # )", "multiply( #, # )", "divide( #, # )"]
  
  def initialize(terminal_chance=0.44)
    tree = "#"
    while tree.include?("#") && tree.length < 2000
      tree = tree.gsub(/#/) {|point| (Random.rand<terminal_chance) ? LEAVES.sample : BRANCHES.sample}
    end
    tree = tree.gsub(/#/) {|point| LEAVES.sample} # cleanup
    @script=tree.gsub(/constant/) {|k| (Random.rand*100-50).round(3)}
  end
  
  def add(arg1, arg2)
    arg1 + arg2
  end
  
  def subtract(arg1, arg2)
    arg1 - arg2
  end
  
  def multiply(arg1, arg2)
    arg1 * arg2
  end
  
  def divide(arg1, arg2)
    arg2==0 ? 0 : arg1 / arg2
  end
  
  def negative(arg)
    -arg
  end
  
  def evaluate(x)
    self.instance_eval(@script.gsub("x",x.to_s))
  end
end