One liner nested hash creation in Ruby? (I come from Perl)

Unfortunately, there is no simple, practical way. A Ruby equivalent would be an ugly, ugly beast like:

((((@date[month] ||= {})[day] ||= {})[hours] ||= {})[min] ||= {})[sec] = 1

There is a way to assign default values for missing keys in hashes, though:

@date = Hash.new { |hash, key| hash[key] = {} }

# @date[:month] is set to a new, empty hash because the key is missing.
@date[:month][:day] = 1

Unfortunately this does not work recursively.

...unless you create it yourself; hooray for Ruby!

class Hash
  def self.recursive
    new { |hash, key| hash[key] = recursive }
  end
end

@date = Hash.recursive
@date[month][day][hours][min][sec] = 1
# @date now equals {month=>{day=>{hours=>{min=>{sec=>1}}}}}

Keep in mind, though, that all unset values are now {} rather than nil.

->f{f[f]}[->f{Hash.new{|h,k|h[k]=f[f]}}]

Obviously.

Here's a couple of options similar to the answer given by @molf but without the monkey patch.

Using a factory method:

  def hash_tree
    Hash.new do |hash, key|
      hash[key] = hash_tree
    end
  end

  @date = hash_tree
  @date[month][day][hours][min][sec] = 1

With a custom class:

  class HashTree < Hash
    def initialize
      super do |hash, key|
        hash[key] = HashTree.new
      end
    end
  end

  @date = HashTree.new
  @date[month][day][hours][min][sec] = 1

Compared to the lambda expression given above, this is simpler and also in one line:

Hash.new {|h,k| h[k] = Hash.new(&h.default_proc) }