list_sentence.rb

# Turn a list of elements into a sentence listing them
#
# Expected outcomes:
#   []                  => ""
#   %w(one)             => "one"
#   %(one two)          => "one and two"
#   %w(one two three)   => "one, two and three"
#
def list_sentence(elements)
  case elements.size
  when 0
    ""
  when 1
    elements.first
  else
    [elements[0..-2].join(", "), "and", elements.last].join(" ")
  end
end

list_sentence [] # => ""
list_sentence %w(one) # => "one"
list_sentence %w(one two) # => "one and two"
list_sentence %w(one two three) # => "one, two and three"
list_sentence %w(one two three four five six seven) # => "one, two, three, four, five, six and seven"