Getting ruby hash values by an array of keys

Question

What I'm aiming to do is to create an object which is initialized with a hash and then query this object in order to get values from that hash. To make things clearer here's a rough example of what I mean:

class HashHolder
  def initialize(hash)
    @hash = hash
  end

  def get_value(*args)
    # What are my possibilities here?
  end
end

holder = HashHolder.new({:a => { :b => { :c => "value" } } } )
holder.get_value(:a, :b, :c) # should return "value"

I know I can perform iteration on the arguments list as in:

def get_value(*args)
  value = @hash
  args.each do |k|
    value = value[k]
  end
  return value
end

But if I plan to use this method a lot this is going to degrade my performance dramatically when all I want to do is to access a hash value.

Any suggestions on that?

Ekkstein · Accepted Answer · 2017-05-03 01:17:55Z

19

To update the answer since it's been a while since it was asked.

(tested in ruby 2.3.1)

You have a hash like this:

my_hash = {:a => { :b => { :c => "value" } } }

The question asked:

my_hash.get_value(:a, :b, :c) # should return "value"

Answer: Use 'dig' instead of get_value, like so:

my_hash.dig(:a,:b,:c) # returns "value"

Since the title of the question is misleading (it should be something like: how to get a value inside a nested hash with an array of keys), here is an answer to the question actually asked:

Getting ruby hash values by an array of keys

Preparation:

my_hash = {:a => 1, :b => 3, :d => 6}
my_array = [:a,:d]

Answer:

my_hash.values_at(*my_array) #returns [1,6]

answered May 3, 2017 at 1:17

Ekkstein

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1 Comment

knut Over a year ago

Thanks for the "misleading" answer - that was exaclty what I searched :)

sawa · Accepted Answer · 2012-11-27 08:37:43Z

4

def get_value(*args)
  args.inject(@hash, &:fetch)
end

In case you want to avoid iteration at lookup (which I do not feel necessary), then you need to flatten the hash to be stored:

class HashHolder
  def initialize(hash)
    while hash.values.any?{|v| v.kind_of?(Hash)}
      hash.to_a.each{|k, v| if v.kind_of?(Hash); hash.delete(k).each{|kk, vv| hash[[*k, kk]] = vv} end}
    end
    @hash = hash
  end
  def get_value(*args)
    @hash[args]
  end
end

edited Nov 27, 2012 at 8:37

answered Nov 27, 2012 at 8:15

sawa

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3 Comments

Erez Rabih Over a year ago

Very elegant but it still iterates over the args array. Is there any way it can be done without iteration?

244an Over a year ago

@sawa, I have implemented appr. this in our system some time ago, I have a question. Your solution with inject is it quicker than using each and iterate through sub hashes? At the end I assume it's almost doing the same thing?

sawa Over a year ago

Alomost the same. You cannot get faster than this unless you do my second solution.

Chris Lewis · Accepted Answer · 2012-11-27 09:02:17Z

-3

If you know the structure of the hash is always in that format you could just do:

holder[:a][:b][:c]

... returns "value".

answered Nov 27, 2012 at 9:02

Chris Lewis

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