Question

class Klass
  attr_accessor :keys
  def change_keys(opt)
    if opt == 1
      keys = [keys[0], keys[keys.length - 1]]
    else
      tmp = keys[0]
      keys[0] = keys[keys.length-1]
      keys[keys.length-1] = tmp
    end
    keys
  end
end
klass = Klass.new
klass.keys = [1,2,3,4,5]

# puts klass.change_keys(1)
# puts klass.change_keys(2)

This is not working at all, the error says: undefined method ‘[]’ method for nil:NilClass

Answer 1

Ruby interprets keys[0] and the others in line 5 as local variables because it has seen a keys = ... . There is an ambiguity in Ruby grammar when it comes to differentiate local variables from method calls without arguments that gets disambiguated by that heuristic. that is, if the parser sees an assignment to that identifier then is a local variable if not is a method call.

You can solve this by referring to self.keys instead. to make clear that you want to use the accessor method.

class Klass
  attr_accessor :keys
  def change_keys(opt)
    if opt == 1
      self.keys = [keys[0], keys[keys.length - 1]]
    else
      tmp = keys[0]
      keys[0] = keys[keys.length-1]
      keys[keys.length-1] = tmp
    end
    keys
  end
end
klass = Klass.new
klass.keys = [1,2,3,4,5]

puts klass.change_keys(1)
puts klass.change_keys(2)