How to create and add elements to an array of array of hashes

Question

What is the proper way of creating and adding elements to an array of array of hashes?

Here's something similar to what I want, initializing the result variable:

col1.each_with_index do |ob1,index1|
  col2.each_with_index do |ob2,index2|
    col3.each do |ob3|
      result[index1][index2][ob3.id] = Obj.new(ob1.att, ob2.att, ob3.att)
    end
  end
end

I should be able to access result like:

result[1][2][1031]

where 1031 is an id, and the others are indexes from 0..n. It should return an instance of Obj.

please, write the instances both your array and expecting result — Ilya
– Ilya, Commented Apr 21, 2016 at 16:09
col1, col2 and col3 are of dynamic sizes, so result should be the same. — Alberto Ashimas
– Alberto Ashimas, Commented Apr 21, 2016 at 16:14
also, I may be wrong with how I'm assigning elements to result, this is just to explain what I want. — Alberto Ashimas
– Alberto Ashimas, Commented Apr 21, 2016 at 16:17
An empty array won't have any depth to it, so you need to pick at least a minimal size for this structure. — tadman
– tadman, Commented Apr 21, 2016 at 16:21

Aleksei Matiushkin · Accepted Answer · 2016-07-26 11:33:30Z

3

Just out of curiosity, the lazy instantiated object, built on top of Hash#default_proc:

result = Hash.new { |h1, k1|
  (0...col1.size) === k1 ? h1[k1] = Hash.new { |h2, k2|
    (0...col2.size) === k2 ? h2[k2] = Hash.new { |h3, k3|
      o3 = col3.detect { |o| o.id == k3 }
      o3 ? h3[k3] = Obj.new(col1[k1].att, col2[k2].att, o3.att) : nil
    } : nil
  } : nil
}

Disclamer: please do not use this coding style in production. The example is given in demonstration purposes only.

edited Jul 26, 2016 at 11:33

answered Apr 21, 2016 at 16:22

Aleksei Matiushkin

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8 Comments

tadman Over a year ago

There's a reason I recommend people avoid chaining ternary operators and this is a sterling example of why. This is one crazy intense nugget of code.

Aleksei Matiushkin Over a year ago

@tadman Have you noticed I put “just out of curiosity” disclamer at the very top of the answer?

tadman Over a year ago

I'm not sure that's a license to get crazy. This is a complicated thing to throw at someone just getting started in Ruby.

tadman Over a year ago

@CarySwoveland If you're ignoring the experience level of the person asking the question, why would anyone even bother asking a question here? I don't think it's unreasonable to keep it simple for those just getting started. I doubt most people, looking at this code, would be able to tell you exactly what it does without five minutes of serious consideration.

Cary Swoveland Over a year ago

@tadman, questions will keep coming because, as here, others will answer the question in a way that can be understood by the OP. I doubt that mudsie would have given his answer had it been the first. Suppose the OP and 19 others read this answer. It might not be understood by the OP and (say) 7 others, and of the remaining 12, 7 may not learn anything, but if 5 readers learn something new, is that not a good thing?

|

nonopolarity · Accepted Answer · 2016-04-21 16:29:55Z

2

You can do something like this:

result = []
[1,3,5].each_with_index do |ob1,index1|
  [2,4,6].each_with_index do |ob2,index2|
    [{ id: 1000 }, { id: 10000 }, { id: 10000 }].each do |ob3|
      result[index1] = result[index1] || []
      result[index1][index2] = result[index1][index2] || {}
      result[index1][index2][ob3[:id]] = { a: ob1, b: ob2, c: ob3 }
    end
  end
end

I changed your .id to [:id] here for simplicity. The idea is to set it to itself if what you want already exists, or else initialize it to what you want.

answered Apr 21, 2016 at 16:29

nonopolarity

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Comments

Uzbekjon · Accepted Answer · 2016-04-21 16:15:12Z

1

If you want to create an array of arrays of arrays, you can use the following syntax:

2.3.0 :001 > Array.new(2) { Array.new(3) }
 => [[nil, nil, nil], [nil, nil, nil]]

# Or, 3 level deep array
2.3.0 :002 > Array.new(2) { Array.new(3) { Array.new(4) } }
 => [[[nil, nil, nil, nil], [nil, nil, nil, nil], [nil, nil, nil, nil]], [[nil, nil, nil, nil], [nil, nil, nil, nil], [nil, nil, nil, nil]]]

Consider also using Hash'es.

answered Apr 21, 2016 at 16:15

Uzbekjon

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3 Comments

tadman Over a year ago

Changing the third level to { } or Hash.new for clarity would be what they were asking for.

Alberto Ashimas Over a year ago

So, translating to my example it would be: result = Array.new(col1.size) { Array.new(col2.size) { Hash.new(col3.size) } } ? That correct? @tadman

tadman Over a year ago

That creates a Hash with a default value of col3.size, which makes no sense. Omit that argument.

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