Ruby Array group_by key with multiple values

Question

I have a Ruby Array:

[
    {tags: [1,2]},
    {tags: [1,3]},
    {tags: [1,4]},
    {tags: [2,4]},
    {tags: [2,5]}
]

I need to group the objects by their tags:

{
    1: [{tags: [1,2]}, {tags: [1,3]}, {tags: [1,4]}],
    2: [{tags: [1,2]}, {tags: [2,4]}, {tags: [2,5]}],
    3: [{tags: [1,3]}],
    4: [{tags: [1,4]}, {tags: [2,4]}],
    5: [{tags: [2,5]}]
}

Is there a magic Ruby Way of doing this?

Is there a question in there somewhere? What specifically are you having trouble with? — Mark Thomas
– Mark Thomas, Commented Nov 14, 2014 at 3:28
@MarkThomas The question is in the title. Given a Ruby Array of Hash elements, I want to index those elements by the values within an Array property of each Hash. It's harder to describe in words than it is to show you the code. Hence my terse question with a focus on code. — colllin
– colllin, Commented Nov 14, 2014 at 4:38
@MarkThomas, This question is akin to: "what's the next number in the sequence 3, 9, 18, 30, 45?" I couldn't figure out the question until I read the answers. It appears that if i satisfies h[:tags].include?[i] => true for at least one of the five hashes h, then i is a key in the returned hash, and its value is an array of all the hashes h for which h[:tags].include?[i] => true. (The next element in the sequence is give by 3(n+n^2)/2.) — Cary Swoveland
– Cary Swoveland, Commented Nov 14, 2014 at 4:43
Instead of only putting the question in the title, also restate it in the body of the question. Also, I see no code, only definition for a hash, but nothing that could possibly manipulate it or create it. We need to know what you've written to try to solve this yourself. Stack Overflow isn't a "write code for me" service, it's a "I need help with code I wrote" service. — the Tin Man
– the Tin Man, Commented Nov 14, 2014 at 7:17
Stack Overflow is a community maintained/moderated site, with a goal of becoming a Wikipedia-like compendium of programming problems and solutions. Accordingly, there are strongly encouraged standards for questions that, when followed, help us answer questions, and others who are searching for similar answers find those questions. As someone who's been around SO a little while I am aware of those things and trying to help you accomplish that. — the Tin Man
– the Tin Man, Commented Nov 14, 2014 at 19:41

falsetru · Accepted Answer · 2014-11-14 02:32:25Z

4

Using Enumerable#each_with_object:

tags = [
  {tags: [1,2]},
  {tags: [1,3]},
  {tags: [1,4]},
  {tags: [2,4]},
  {tags: [2,5]}
]
tags.each_with_object(Hash.new { |h,k| h[k] = [] }) { |h, result|
  h[:tags].each { |n| result[n] << h }
}
# => {1=>[{:tags=>[1, 2]}, {:tags=>[1, 3]}, {:tags=>[1, 4]}],
#     2=>[{:tags=>[1, 2]}, {:tags=>[2, 4]}, {:tags=>[2, 5]}],
#     3=>[{:tags=>[1, 3]}],
#     4=>[{:tags=>[1, 4]}, {:tags=>[2, 4]}],
#     5=>[{:tags=>[2, 5]}]}

answered Nov 14, 2014 at 2:32

falsetru

371k69 gold badges769 silver badges659 bronze badges

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

1 Comment

Cary Swoveland Over a year ago

Good answer and kudos for figuring out what was being asked.

spickermann · Accepted Answer · 2014-11-14 02:36:21Z

2

I would do something like this:

array = [{tags: [1,2]}, {tags: [1,3]}, {tags: [1,4]}, {tags: [2,4]}, {tags: [2,5]}]

result = Hash.new { |hash, key| hash[key] = [] }

array.each do |tags|
  tags[:tags].each { |tag| result[tag] << tags }
end

puts result
#=> {1=>[{:tags=>[1, 2]}, {:tags=>[1, 3]}, {:tags=>[1, 4]}], 
#    2=>[{:tags=>[1, 2]}, {:tags=>[2, 4]}, {:tags=>[2, 5]}], 
#    3=>[{:tags=>[1, 3]}], 
#    4=>[{:tags=>[1, 4]}, {:tags=>[2, 4]}], 
#    5=>[{:tags=>[2, 5]}]}

answered Nov 14, 2014 at 2:36

spickermann

108k9 gold badges115 silver badges147 bronze badges

2 Comments

colllin Over a year ago

Thanks @spickermann. This is similar to what I came up with on my own, but your passing the block to Hash.new is an elegant touch.

Cary Swoveland Over a year ago

You must have also figured out the question too, considering that your answer followed so closely on the heels of falsetru's. +1 for that alone, but good answer too.

Cary Swoveland · Accepted Answer · 2014-11-14 03:35:00Z

1

I suggest the following:

tags.each_with_object({}) {|g,h| g.values.flatten.each {|i| (h[i]||=[]) << g}}
  #=> {1=>[{:tags=>[1, 2]}, {:tags=>[1, 3]}, {:tags=>[1, 4]}], 
  #    2=>[{:tags=>[1, 2]}, {:tags=>[2, 4]}, {:tags=>[2, 5]}],
  #    3=>[{:tags=>[1, 3]}],
  #    4=>[{:tags=>[1, 4]}, {:tags=>[2, 4]}],
  #    5=>[{:tags=>[2, 5]}]}

answered Nov 14, 2014 at 3:35

Cary Swoveland

111k6 gold badges69 silver badges105 bronze badges

Collectives™ on Stack Overflow

Ruby Array group_by key with multiple values

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related