Reverse Polish Notation

rpn.rb

def rpn expression
  stack = []

  expression.split(' ').each do |token|
    stack << if token =~ /\d/
      token
    else
      [stack.pop, stack.pop].map { |t| t.to_i }.inject do |sum, x|
        eval "#{x} #{token} #{sum}"
      end
    end
  end

  stack.last
end

rpn_spec.rb

require './rpn'

describe 'rpn' do
  it 'should support 1 + 1' do
    rpn('1 1 +').should eql 2
  end

  it 'should support minus' do
    rpn('1 2 3 + -').should eql -4
  end

  it 'should support varying orders' do
    rpn("1 2 + 3 -").should eql 0
  end

  it "should support zeros" do
    rpn('1 0 +').should eql 1
  end
end