Reverse Polish notation (ruby)

polish.rb

require "test/unit"

OPERATIONS = {
    "+" => lambda { |a, b| a + b },
    "-" => lambda { |a, b| a - b },
    "*" => lambda { |a, b| a * b },
    "/" => lambda { |a, b| a / b }
}

def eval_polish(expression)
  elements = expression.split
  stack = []
  elements.each { |element|
    if OPERATIONS.keys.include? element
      b = stack.pop
      a = stack.pop
      stack.push OPERATIONS[element].call a, b
    else
      stack.push element.to_i
    end
  }
  stack.pop
end


class PolishTest < Test::Unit::TestCase

  def test_soustraction
    assert_equal 1, eval_polish('3 2 -')
    assert_equal 3, eval_polish('5 2 -')
  end

  def test_addition
    assert_equal 5, eval_polish('3 2 +')
  end

  def test_addition_2_etages
    assert_equal 6, eval_polish('3 2 1 + +')
  end

  def test_multiplication
    assert_equal 6, eval_polish('3 2 *')
  end

  def test_division
    assert_equal 2, eval_polish('4 2 /')
  end

  def test_acceptance
    assert_equal 2, eval_polish('4 2 5 * + 1 3 2 * + /')
    assert_equal 14, eval_polish('5 1 2 + 4 * 3 - +')
  end

end

Same, using functional style :
https://gist.github.com/3976975#file_postfix.rb