nums_commas.rb

def separate_comma(num)
  separated_num = num.to_s.split("").reverse
  result = []
  separated_num.each_with_index.map do |num, index|
    if index % 3 == 0 && index != 0
      result << num + ","
    else
      result << num
    end
  end
  result.reverse.join("")
end

// can ommit the need to declare result if you use each_with_index.map, but I think this form is simpler.