Take an array of numbers and condense it into a list of ranges using recursion.

find_ranges.rb

def find_range(a, str = '')
  return str[2..-1] if a.empty?
  start_val = end_val = a[0]

  a.each do |val|
    break if val > end_val + 1
    end_val = val
  end

  if start_val == end_val
    str << ", #{end_val}"
  else
    str << ", #{start_val}-#{end_val}"
  end

  find_range(a.drop(a.find_index(end_val) + 1), str)
end

a = [1, 2, 3, 6, 7, 8, 10, 12, 14, 16, 17, 18]

puts find_range(a)

1-3, 6-8, 10, 12, 14, 16-18