Specifying default value for Hash of hashes in Ruby [duplicate] - ruby

Question

This question already has answers here:
Strange, unexpected behavior (disappearing/changing values) when using Hash default value, e.g. Hash.new([])
(4 answers)
Closed 5 years ago.
Lets say I have a Hash called person whose key is name and value is a hash having keys 'age' and 'hobby'. An entry in the hash person would look like
=> person["some_guy"] = {:hobby => "biking", :age => 30}
How would I go about specifying the default for the hash 'person'? I tried the following
=> person.default = {:hobby => "none", :age => 20}
but it doesn't work.
EDIT:
I was setting one attribute and expecting others to be auto-populated. For eg.
=> person["dude"][:age] += 5
and this is what I was seeing
=> person["dude"]
=> {:hobby => "none", :age => 25}
which is fine. However, typing person at the prompt, I get an empty hash.
=> person
=> {}
However, what I was expecting was
=> person
=> {"dude" => {:hobby => "none", :age => 25}}

Answer 1

Why It May Not Work for You
You can't call Hash#default if an object doesn't have the method. You need to make sure that person.is_a? Hash before it will work. Are you sure you don't have an array instead?
It's also worth noting that a hash default doesn't populate anything; it's just the value that's returned when a key is missing. If you create a key, then you still need to populate the value.
Why It Works for Me
# Pass a default to the constructor method.
person = Hash.new({hobby: "", age: 0})
person[:foo]
=> {:hobby=>"", :age=>0}
# Assign a hash literal, and then call the #default method on it.
person = {}
person.default = {hobby: "", age: 0}
person[:foo]
=> {:hobby=>"", :age=>0}
What You Probably Want
Since hash defaults only apply to missing keys, not missing values, you need to take a different approach if you want to populate a hash of hashes. One approach is to pass a block to the hash constructor. For example:
person = Hash.new {|hash,key| hash[key]={hobby: nil, age:0}}
=> {}
person[:foo]
=> {:hobby=>nil, :age=>0}
person[:bar]
=> {:hobby=>nil, :age=>0}
person
=> {:foo=>{:hobby=>nil, :age=>0}, :bar=>{:hobby=>nil, :age=>0}}

Answer 2

You can specify a hash's default value on instantiation by passing the default value to the .new method:
person = Hash.new({ :hobby => '', :age => 0 })

Answer 3

You can use Hash#default_proc to initialise a hash with default values dynamically.
h = Hash.new
h.default_proc = proc do |hash, key|
hash[key] = Hash.new(0) unless hash.include? key
end
h[0][0] # => 0
h[1][0] # => 0
h[0][0] = 1
h[0][0] # => 1
h[1][0] # => 0