Ruby hash of arrays from array

Question

I have the following array

a=[["kvm16", "one-415"], ["kvm16", "one-416"], ["kvm5", "one-417"]]

I would like to convert this to a hash that looks like this

{"kvm5"=>["one-417"], "kvm16"=>["one-417", "one-416"]}

everything I have tried squashes the value.

v=Hash[ a.collect do |p| [p[0], [ p[1] ] ] end ]
=> {"kvm5"=>["one-417"], "kvm16"=>["one-416"] }

I was thinking I could check to see if v[p[0]] is an array, but the variable isn't defined inside this block.

Is there a clean way to accomplish what I am looking for?

you mean {"kvm16"=>["one-415", "one-416"], "kvm5"=>["one-417"]}? — tokland
– tokland, Commented Jan 24, 2013 at 20:56

Sergio Tulentsev · Accepted Answer · 2013-01-24 20:26:34Z

5

Yeah, you have to do it yourself, I'm afraid.

a = [["kvm16", "one-415"], ["kvm16", "one-416"], ["kvm5", "one-417"]]

h = a.each_with_object({}) do |(k, v), memo|
  (memo[k] ||= []) << v
end

h # => {"kvm16"=>["one-415", "one-416"], "kvm5"=>["one-417"]}

Or, if you're on older ruby (1.8.x):

h = {}
a.each do |k, v|
  (h[k] ||= []) << v
end

h # => {"kvm16"=>["one-415", "one-416"], "kvm5"=>["one-417"]}

answered Jan 24, 2013 at 20:26

Sergio Tulentsev

231k43 gold badges381 silver badges373 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

J.T. Over a year ago

undefined method `each_with_object'

J.T. Over a year ago

unfortunately I am unable to move to 1.9.3 in this instance. I am at ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux] Pasting the above code results in [["kvm16", "one-415"], ["kvm16", "one-416"], ["kvm5", "one-417"]]

J.T. Over a year ago

a.each { |k,v| if h[k].is_a?(Array) ; h[k].push(v);else;h[k] = [v];end } => [["kvm16", "one-415"], ["kvm16", "one-416"], ["kvm5", "one-417"]] I must be missing something fundamental

Sergio Tulentsev Over a year ago

Just copy/paste exactly, don't retype.

tokland · Accepted Answer · 2013-01-25 09:25:15Z

4

Let's see some functional approaches. This is a common abstraction, you can find it as Enumerable#map_by in Facets:

require 'facets'
hs.map_by { |k, v| [k, v] }
#=> {"kvm16"=>["one-415", "one-416"], "kvm5"=>["one-417"]}

This pattern replaces the cumbersome group_by + map + Hash:

Hash[hs.group_by(&:first).map { |k, gs| [k, gs.map(&:last)] }]
#=> {"kvm16"=>["one-415", "one-416"], "kvm5"=>["one-417"]}

edited Jan 25, 2013 at 9:25

answered Jan 24, 2013 at 20:49

tokland

68.2k13 gold badges151 silver badges174 bronze badges

Comments

J.T. · Accepted Answer · 2013-01-24 20:53:00Z

0

this has it. in the face of not having each_with_object in my version of ruby and some googleze I arrived at

{}.tap{ |h| items.each{ |item| h[item.id] = item } }
=> {"kvm5"=>["one-417"], "kvm16"=>["one-415", "one-416"]}

mucho thanks to Sergio for getting me thinking along that line

answered Jan 24, 2013 at 20:53

J.T.

1632 silver badges8 bronze badges

Collectives™ on Stack Overflow

Ruby hash of arrays from array

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related