Flattening nested hash to an array

Question

I'd like to flatten nested hash to an array. For example:

a = {'1'=>{'2'=>{'5'=>{},'6'=>{'8'=>{}}}},'3'=>{},'4'=>{'7'=>{}}}

and result of flatten_nested_hash(a) would be:

["1", "2", "5", "6", "8", "3", "4", "7"]

Finally I wrote some recursive function, but I feel that there must be some easier, non-recursive way of doing it.

My function looks like this:

  def flatten_nested_hash(categories)
    categories.map do |k,v|
      if v == {} 
        k
      else
        [k,flatten_nested_hash(v)]
      end
    end.flatten
  end

@raam86 PHP and Ruby are completely different languages. There may be a duplicate somewhere, but definitely not that one. — Mischa
– Mischa, Commented Jul 12, 2013 at 11:28
@Mischa I completely agree. Must say I was too lazy to generate the comment myself. Fixed. — raam86
– raam86, Commented Jul 12, 2013 at 11:34
stackoverflow.com/questions/16047106/… Can give another perspective on this question — raam86
– raam86, Commented Jul 12, 2013 at 11:34

sawa · Accepted Answer · 2013-07-12 11:24:05Z

15

Recursive.

def flatten_nested_hash(categories)
  categories.flat_map{|k, v| [k, *flatten_nested_hash(v)]}
end

Defining it on the Hash class.

class Hash
  def flatten_nested; flat_map{|k, v| [k, *v.flatten_nested]} end
end

edited Jul 12, 2013 at 11:24

answered Jul 12, 2013 at 11:13

sawa

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Comments

patkoperwas · Accepted Answer · 2016-05-03 14:56:53Z

5

With Ruby 2.1 and later you can use refinements to add a method to Hash and expose it only to the modules / classes you need.

module HashRefinements
  refine Hash do
    def flatten_nested
      flat_map { |k, v| [k, *v.flatten_nested] }
    end
  end
end

In your class / module

class MyThing
  using HashRefinements

  ...

  def flatten_categories
    categories.flatten_nested
  end

  ...
end

edited May 3, 2016 at 14:56

answered Jun 15, 2015 at 23:27

patkoperwas

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Comments

Chris Heald · Accepted Answer · 2013-07-12 11:13:43Z

3

This is a nested data structure - you're going to have to use some kind of recursive or iterative method to extract all the keys. That's a bit easier than what you have, though:

def deep_extract_keys(hash)
  hash.keys + hash.values.flat_map {|value| deep_extract_keys value }
end

p deep_extract_keys({"1"=>{"2"=>{"5"=>{}, "6"=>{"8"=>{}}}}, "3"=>{}, "4"=>{"7"=>{}}})

This is a breadth-first search, rather than a depth-first search, so the output is:

["1", "3", "4", "2", "5", "6", "8", "7"]

answered Jul 12, 2013 at 11:13

Chris Heald

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1 Comment

sawa Over a year ago

That changes the order of elements.

michael_wu · Accepted Answer · 2013-07-12 13:34:08Z

0

This is not (explicitly) recursive and works only for non-negative integer keys:

a.to_s.scan(/\d+/).map(&:to_i)   # [1, 2, 5, 6, 8, 3, 4, 7]

:-)

edited Jul 12, 2013 at 13:34

answered Jul 12, 2013 at 11:39

michael_wu

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Comments

gabeodess · Accepted Answer · 2016-03-12 20:39:21Z

0

module Flattener
  def deep_flatten
    flatten.map do |item|
      case item
      when Hash, Array
        item.deep_flatten
      else
        item
      end
    end.flatten
  end
end

class Hash

  include Flattener

end

class Array

  include Flattener

end

answered Mar 12, 2016 at 20:39

gabeodess

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