gistfile1.rb

letters = ['a', 'b', 'c', 'd']

letters.each do |letter|
  puts letter
end

=> prints each letter. easy.

# below, '' is an empty string.
# result_string is initially '', the argument passed to inject.
# that's how inject works.
letters.inject('') do |result_string, letter| 
  result_string << letter # concatenation, tack a letter onto the result_string
  return result_string    # after returning this here, the next iteration will receive _this_ value, not the initial
end