Ruby - Merge an Array into a Hash

Question

I have an array which looks like this:

array = [[:foo, :bar], [:foo, :baz], [:baz, {a: 1, b: 2}], [:baz, {c: 1, d:2}]]

and I need to turn it into a hash which looks like this:

{:foo =>[:bar, :baz], :baz => {a: 1, b: 2, c: 1, d: 2}}

This is the code I have so far:

def flatten(array)
  h = {}
  array.each_with_object({}) do |(k, v), memo|
    if v.is_a?(Hash)
      memo[k] = h.merge!(v)
    else
      # What goes here?
    end
  end
end

When used like so:

flatten(array)

outputs:

{baz => {:a => 1, :b => 2, :c => 1, :d => 2}}

May someone please point me in the right direction? Help appreciated.

Wow that's such an inconsistent transformation. You perform different unification operations depending on the types. What if array = [[:foo, {:a => 1}], [:foo, :bar]]? — Niklas B.
– Niklas B., Commented Jul 23, 2015 at 19:19
Your sample code does not compile: for example, list and memo are undefined. Please paste the code you actually tested. — Mario Zannone
– Mario Zannone, Commented Jul 23, 2015 at 19:48

Cary Swoveland · Accepted Answer · 2015-07-23 19:54:23Z

1

def convert(arr)
  arr.each_with_object({}) do |a,h|
    h[a.first] =        
      case a.last
      when Hash
        (h[a.first] || {}).update(a.last)
      else
        (h[a.first] || []) << a.last
      end
  end
end

convert array
  #=> {:foo=>[:bar, :baz], :baz=>{:a=>1, :b=>2, :c=>1, :d=>2}}

answered Jul 23, 2015 at 19:54

Cary Swoveland

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Kirill · Accepted Answer · 2015-07-23 19:53:43Z

0

Hash[ array.group_by(&:first).map{ |k,a| [k,a.map(&:last)] } ]

answered Jul 23, 2015 at 19:53

Kirill

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3 Comments

Athar Over a year ago

you were right @kirill. i removed my answer actually. bcoz my approach was changed and becomes similar to Cary. but thanks.. i didnt notice that difference... Thanks.

Wand Maker Over a year ago

Output of this answer is not same as the one expected by OP

Kirill Over a year ago

@WandMaker, I figured out after posting. And that was mentioned already (comment is removed).

Wand Maker · Accepted Answer · 2015-07-23 20:09:22Z

0

Here is my attempt at solving this problem. I have to make assumption that in the input array, entries like the ones similar to :baz will always be paired with Hash objects. The solution will not work if you have one :baz with a symbol and another with hash.

array = [[:foo, :bar], [:foo, :baz], [:baz, {a: 1, b: 2}], [:baz, {c: 1, d:2}]]

h = Hash.new
array.each do |n1, n2|
    if n2.class == Hash
      h[n1] = (h[n1] || {}).merge(n2) 
    else
      h[n1] = (h[n1] || []) << n2
    end
end
p h

Output

{:foo=>[:bar, :baz], :baz=>{:a=>1, :b=>2, :c=>1, :d=>2}}
[Finished in 0.1s]

answered Jul 23, 2015 at 20:09

Wand Maker

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