Ruby array of arrays to hash with first item as key

Question

I have an array of arrays (result of "pluck" usage) in following form:

arr = [[1, 'A1', 'B1'],[2, 'A2', 'B2'],[3, 'A3', 'B3']]

How to get following hash from it?

{1=>[1, 'A1', 'B1'], 2=>[2, 'A2', 'B2'], 3=>[3, 'A3', 'B3']}

I know this form, but surely there is one line form, too

hash = Hash.new

arr.each do |x|
  hash[x[0]] = x
end

Personally I like the form you posted just fine - it is probably the most readable and nearly the most compact even if some other incantations might squeeze out a few characters. I'd be inclined to just put it on one line in the {} form arr.each { |x| hash[x[0]] = x} — Michael Berkowski
– Michael Berkowski, Commented Jun 2, 2021 at 13:53
@MichaelBerkowski I'd be inclined to say arr.group_by(&:first) is more compact and more understandable. — engineersmnky
– engineersmnky, Commented Jun 2, 2021 at 14:01
@engineersmnky Yes I agree, that's a slick implementation. I admit though if I found it in my codebase I'd have to go out to the Array#group_by docs to make sure I knew what it did. — Michael Berkowski
– Michael Berkowski, Commented Jun 2, 2021 at 14:04
@dawg that will not produce the desired result as it will not retain element sa[0] in the value portion of the Hash — engineersmnky
– engineersmnky, Commented Jun 2, 2021 at 14:37
@dawg In my question I mentioned {1=>[1, "A1", "B1"] ... but just because it was simpler for me to implement. You showed quite nice improvement. — jing
– jing, Commented Jun 2, 2021 at 14:53

Rajagopalan · Accepted Answer · 2021-06-02 13:58:52Z

2

Input

arr = [[1, 'A1', 'B1'],[2, 'A2', 'B2'],[3, 'A3', 'B3']]

Code

p arr.map{|a|[a.first,a]}.to_h

Output

{1=>[1, "A1", "B1"], 2=>[2, "A2", "B2"], 3=>[3, "A3", "B3"]}

answered Jun 2, 2021 at 13:58

Rajagopalan

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Comments

Hiren Bhalani · Accepted Answer · 2021-06-02 14:11:54Z

2

There are few ways I can think.

Use group_by(I personally choose this one), reduce and inject method.

arr = [[1, 'A1', 'B1'],[2, 'A2', 'B2'],[3, 'A3', 'B3']]

arr.reduce({}) { |result, record| result.merge!(record.first => record) }

Or 

arr.inject({}) { |result, record| result.merge!(record.first => record) }

OR

arr.group_by(&:first) # Short and sweet, I think you will pick this up.

edited Jun 2, 2021 at 14:11

answered Jun 2, 2021 at 14:05

Hiren Bhalani

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6 Comments

Rajagopalan Over a year ago

The last one is fantastic!

jing Over a year ago

Is original order inside of hash preserved if I use group_by?

Tom Lord Over a year ago

arr.group_by(&:first) does not quite get the same result. This collects an array of elements, e.g. the first key/value pair is 1=>[[1, "A1", "B1"]], not 1=>[1, "A1", "B1"] as desired.

Tom Lord Over a year ago

You could slap a .transform_values(&:flatten) on the end to get the desired format. However, despite this being "clever", I'd rather opt for a basic solution like this.

Hiren Bhalani Over a year ago

I would suggest to use reduce or inject in that case, the complexity will ne O(n), while group_by and then transform_values will be O(2n). So save time and computing power and user reduce.

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