Declaring a Hash as a Constant with a default value in Ruby in one line

Question

My goal is to use a Hash with a default value as a class constant. To do this it seems to me that I must initialise a Hash in one line as a constant with pre-defined value and a default value.

According to the Ruby documentation, I can set a default value in two way :

• In the constructor then by adding value as follow :

  MY_HASH = Hash.new(-1)
  MY_HASH[1] = 0
  MY_HASH[2] = 42
  

• By adding values first then setting the default value later :

  MY_HASH = {
      1=>0,
      2=>42,
  }
  MY_HASH.default = -1
  

I tried to do it in one line like this, but it does not work :

    MY_HASH = {
        1=>0,
        2=>42,
    }.default = -1

   #It's the same as :
   MY_HASH = -1

Is there a way to declare a Hash with a default value in one line ?

Answer 1

You can use update:

MY_HASH = Hash.new(-1).update(1 => 0, 2 => 42)
MY_HASH[1]
#=> 0
MY_HASH[52]
#=> -1

Or you can use Hash#merge.

Answer 2

Here are two other solutions.

MY_HASH = { 1=>0, 2=>42 }.tap { |h| h.default = -1 }

MY_HASH[1]      #=> 0
MY_HASH[529326] #=> -1

---

MY_HASH = ->(key) { { 1=>0, 2=>42 }.fetch(key, -1) }

MY_HASH[1]      #=>  0
MY_HASH[529326] #=> -1