hash.rb

# First: Product - Default without setting hash defaults:

x = %w(a b c)
h = Hash[x.product([true])]
p h # => {'a' => true, 'b' => true, 'c' => true}
p h['a'] # => true
p h['d'] # => nil

# e.g: sphinx takes a hash of field name -> weight pairs, so we use
def search_weights
  @search_weights ||= Hash[fields.product([25]) + sanitized_fields.product([1])]
end
# to generate the hash.

# Second: normal default

i = Hash.new(false)
p i # => {}
p i['a'] # => false
p i # => {}

# With a block
j = Hash.new { |h,k| h[k] = false }
p i # => {}
p i['a'] # => false
p i # => {'a' => false}

# Now, doing magic! - Super useful for data munging.

k = Hash.new { |h,k| h[k] = [] }
%w(a b c d).each_with_index do |key, idx|
  k[key] << idx
end
p k # => {"a"=>[0], "b"=>[1], "c"=>[2], "d"=>[3]}


# And, for the grand finale:

k = Hash.new { |h,k| h[k] = Hash.new(&h.default_proc) }
p k # => {}
p k[:a][:b][:c] # => {}
p k # => {:a=>{:b=>{:c=>{}}}}