def number_to_ordinal(num)
  num = num.to_i
  if (10...20)===num
    "#{num}th"
  else
    g = %w{ th st nd rd th th th th th th }
    a = num.to_s
    c=a[-1..-1].to_i
    a + g[c]
  end
end

number_to_ordinal(12) => "12th"
number_to_ordinal(7) => "7th"


Note that floats will be converted to ints (without rounding) before processing.