All of the possible combinations of letters in a string

char_combinations.rb

def char_combinations(s)
  length = s.size
  list_comp = Array.new(length) do |start_index|
    length.times.inject([]) do |combinations, end_index|
      sub_str = s[start_index..end_index]
      sub_str.size > 0 ? combinations << sub_str : combinations
    end
  end.flatten
end

char_combinations('Joe') # => ["J", "Jo", "Joe", "o", "oe", "e"]