对于序数的Ruby格式:'1'为'1st','2'为'2nd'等

def ordinalize(number)
  if (11..13).include?(number % 100)
    "#{number}th"
  else
    case number % 10
    when 1
      "#{number}st"
    when 2
      "#{number}nd"
    when 3
      "#{number}rd"
    else
      "#{number}th"
    end
  end
end

puts ordinalize(1)   # 输出：1st
puts ordinalize(2)   # 输出：2nd
puts ordinalize(3)   # 输出：3rd
puts ordinalize(4)   # 输出：4th
puts ordinalize(11)  # 输出：11th
puts ordinalize(21)  # 输出：21st