group_numbers.rb

nums = [1,2,3,5,8,9,12,13,14,15,16,19]

def numbers_grouped(nums)
  nums = nums.sort
  start, last = nums.first, nil
  sequences = []

  nums.each_with_index do |num, i|
    # Are we out of sequence?
    if last && last+1 != num
      sequences << [start, last]
      start = num
    end

    # Last iteration, still in sequence
    if (i+1) == nums.length
      sequences << [start, num]
    else
      last = num
    end
  end

  sequences
end

puts nums.inspect
puts numbers_grouped(nums).inspect
puts numbers_grouped(nums).map { |s| s.uniq.join('-') }.inspect
==>
[1, 2, 3, 5, 8, 9, 12, 13, 14, 15, 16]
[[1, 3], [5, 5], [8, 9], [12, 16]]
["1-3", "5", "8-9", "12-16"]

have to use #next not +1 so it also works for letters but yeah.

Nice Jared!

numbers.reduce([]) { |memo, n|
  if memo.last && memo.last.last.next == n
    memo.last.push(n)
  else 
    memo.push([n])
  end
  memo
}.map { |group| [group.first, group.last].uniq.join('-') }

numbers.sort.each_with_object([]) { |n, groups|
  if groups.last && groups.last.last.next == n
    groups.last.push(n)
  else 
    groups.push([n])
  end
}.map { |group| [group.first, group.last].uniq.join('-') }