Creating a new hash with default keys

Question

I want to create a hash with an index that comes from an array.

ary = ["a", "b", "c"]
h = Hash.new(ary.each{|a| h[a] = 0})

My goal is to start with a hash like this:

h = {"a"=>0, "b"=>0, "c"=>0}

so that later when the hash has changed I can reset it with h.default

Unfortunately the way I'm setting up the hash is not working... any ideas?

Answer 1

Use the default value feature for the hash

h = Hash.new(0)

h["a"]      # => 0

In this approach, the key is not set.

h.key?("a") # => false

Other approach is to set the missing key when accessed.

h = Hash.new {|h, k| h[k] = 0}
h["a"]      # => 0
h.key?("a") # => true

Even in this approach, the operations like key? will fail if you haven't accessed the key before.

h.key?("b") # => false
h["b"]      # => 0
h.key?("b") # => true

You can always resort to brute force, which has the least boundary conditions.

h = Hash.new.tap {|h| ["a", "b", "c"].each{|k| h[k] = 0}}
h.key?("b") # => true
h["b"]      # => 0

Answer 2

You should instantiate your hash h first, and then fill it with the contents of the array:

h = {}    
ary = ["a", "b", "c"]
ary.each{|a| h[a] = 0}

Answer 3

You can do it like this where you expand a list into zero-initialized values:

list = %w[ a b c ]

hash = Hash[list.collect { |i| [ i, 0 ] }]

You can also make a Hash that simply has a default value of 0 for any given key:

hash = Hash.new { |h, k| h[k] = 0 }

Any new key referenced will be pre-initialized to the default value and this will avoid having to initialize the whole hash.

Answer 4

This may not be the most efficient way, but I always appreciate one-liners that reveal a little more about Ruby's versatility:

h =  Hash[['a', 'b', 'c'].collect { |v|  [v, 0] }]

Or another one-liner that does the same thing:

h = ['a', 'b', 'c'].inject({}) {|h, v| h[v] = 0; h }

By the way, from a performance standpoint, the one-liners run about 80% of the speed of:

h = {}
ary = ['a','b','c']
ary.each { |a| h[a]=0 }

Answer 5

Rails 6 added index_with on Enumerable module. This will help in creating a hash from an enumerator with default or fetched values.

ary = %w[a b c]
hash = ary.index_with(0) # => {"a"=>0, "b"=>0, "c"=>0}