If key does not exist create default value

If you already have a hash, you can do this:

h.fetch(key, "default value")

Or you exploit the fact that a non-existing key will return nil:

h[key] || "default value"

To create hashes with default values it depends on what you want to do exactly.

• Independent of key and will not be stored:

  h = Hash.new("foo")
  h[1] #=> "foo"
  h #=> {}
  

• Dependent on the key and will be stored:

  h = Hash.new { |h, k| h[k] = k * k } 
  h[2] #=> 4
  h #=> {2=>4}