Accessing elements of nested hashes in ruby [duplicate]

Answer 1

Ruby 2.3.0 introduced a new method called dig on both Hash and Array that solves this problem entirely.

value = structure.dig(:a, :b)

It returns nil if the key is missing at any level.

If you are using a version of Ruby older than 2.3, you can use the ruby_dig gem.

Answer 2

In my case, I needed a two-dimensional matrix where each cell is a list of items.

I found this technique which seems to work. It might work for the OP:

$all = Hash.new()

def $all.[](k)
  v = fetch(k, nil)
  return v if v

  h = Hash.new()
  def h.[](k2)
    v = fetch(k2, nil)
    return v if v
    list = Array.new()
    store(k2, list)
    return list
  end

  store(k, h)
  return h
end

$all['g1-a']['g2-a'] << '1'
$all['g1-a']['g2-a'] << '2'

$all['g1-a']['g2-a'] << '3'
$all['g1-a']['g2-b'] << '4'

$all['g1-b']['g2-a'] << '5'
$all['g1-b']['g2-c'] << '6'

$all.keys.each do |group1|
  $all[group1].keys.each do |group2|
    $all[group1][group2].each do |item|
      puts "#{group1} #{group2} #{item}"
    end
  end
end

The output is:

$ ruby -v && ruby t.rb
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]
g1-a g2-a 1
g1-a g2-a 2
g1-a g2-a 3
g1-a g2-b 4
g1-b g2-a 5
g1-b g2-c 6

Answer 3

Traditionally, you really had to do something like this:

structure[:a] && structure[:a][:b]

However, Ruby 2.3 added a method Hash#dig that makes this way more graceful:

structure.dig :a, :b # nil if it misses anywhere along the way

There is a gem called ruby_dig that will back-patch this for you.

Answer 4

You could just build a Hash subclass with an extra variadic method for digging all the way down with appropriate checks along the way. Something like this (with a better name of course):

class Thing < Hash
  def find(*path)
    path.inject(self) { |h, x| return nil if(!h.is_a?(Thing) || h[x].nil?); h[x] }
  end
end

Then just use Things instead of hashes:

>> x = Thing.new
=> {}
>> x[:a] = Thing.new
=> {}
>> x[:a][:b] = 'k'
=> "k"
>> x.find(:a)
=> {:b=>"k"}
>> x.find(:a, :b)
=> "k"
>> x.find(:a, :b, :c)
=> nil
>> x.find(:a, :c, :d)
=> nil

Answer 5

There is the cute but wrong way to do this. Which is to monkey-patch NilClass to add a [] method that returns nil. I say it is the wrong approach because you have no idea what other software may have made a different version, or what behavior change in a future version of Ruby can be broken by this.

A better approach is to create a new object that works a lot like nil but supports this behavior. Make this new object the default return of your hashes. And then it will just work.

Alternately you can create a simple "nested lookup" function that you pass the hash and the keys to, which traverses the hashes in order, breaking out when it can.

I would personally prefer one of the latter two approaches. Though I think it would be cute if the first was integrated into the Ruby language. (But monkey-patching is a bad idea. Don't do that. Particularly not to demonstrate what a cool hacker you are.)

Answer 6

I made rubygem for this. Try vine.

Install:

gem install vine

Usage:

hash.access("a.b.c")