mutations.rb

def my_swap(a,b)
  b,a = a,b
end

class Foo
attr_accessor :a, :b
  def my_swap_2()
    @b,@a = @a,@b
  end
end

def my_string_replace_1(s)
  s.gsub( /Hi/, 'Hello')
end

def my_string_replace_2(s)
  s.gsub!( /Hi/, 'Hello')
end