ruby cookbook -- Creating a Hash with a Default Value

Creating a Hash with a Default Value

first_letter_hash = Hash.new { |hash, key| hash[key] = [] }
text.split(/\W+/).each { |word| first_letter_hash[word[0,1].downcase] << word }
first_letter_hash
# => {"m"=>["mainly"], "p"=>["plain"], "f"=>["falls"], "r"=>["rain"],
#        "s"=>["Spain"], "i"=>["in", "in"], "t"=>["The", "the"]}
first_letter_hash["m"]
# => ["mainly"]

When a letter can't be found in the hash, Ruby calls the block passed into the Hash constructor. That block puts a new array into the hash, using the missing letter as the key. Now the letter is bound to a unique array, and words can be added to that array normally.

Note that if you want to add the array to the hash so it can be used later, you must assign it within the block of the Hash constructor. Otherwise you'll get a new, empty array every time you access first_letter_hash["m"]. The words you want to append to the array will be lost.