Question

I know there are other questions similar such as:

1. Ruby: how to check if variable exists within a hash definition
2. Checking if a variable is defined?

But the answers aren’t fully satisfactory.

I have:

ruby-1.9.2-p290 :001 > a=Hash.new
 => {} 
ruby-1.9.2-p290 :002 > a['one']="hello"
 => "hello"
ruby-1.9.2-p290 :006 > defined?(a['one']['some']).nil?
 => false 
ruby-1.9.2-p290 :007 > a['one']['some'].nil?
 => true

It seems like:

if a['one']['some'].nil?
  a['one']['some']=Array.new
end 

would be sufficient. Is this correct? Would this be correct for any data type? Is defined? needed in this case?

thx

Answer 1

You seem to be confusing two concepts. One is if a variable is defined, and another is if a Hash key is defined. Since a hash is, at some point, a variable, then it must be defined.

defined?(a)
# => nil

a = { }
# => {}

defined?(a)
# => "local-variable"

a.key?('one')
# => false

a['one'] = 'hello'
# => 'hello'

a.key?('one')
# => true

Something can be a key and nil at the same time, this is valid. There is no concept of defined or undefined for a Hash. It is all about if the key exists or not.

The only reason to test with .nil? is to distinguish between the two possible non-true values: nil and false. If you will never be using false in that context, then calling .nil? is unnecessarily verbose. In other words, if (x.nil?) is equivalent to if (x) provided x will never be literally false.

What you probably want to employ is the ||= pattern that will assign something if the existing value is nil or false:

# Assign an array to this Hash key if nothing is stored there
a['one']['hello'] ||= [ ]

Update: Edited according to remarks by Bruce.