Create list of lists from list of json objects ruby

Question

I have a list of json objects something like this:

test = [{"a": 1, "b": 2, "c": 3}, {"a": 4, "b": 5, "c":6}]

I want to fetch the fields 'a' and 'c' from the above test list to create a list of list something like this:

[[1, 4], [3, 6]]

The idea is to make a list of all the values of a, then the values of c.

When I am using pluck:

test.pluck(:a, :c)

I am getting output like this:

[[1, 3], [4, 6]]

One approach I tried which is working fine.

res = []
res << test.pluck(:a)
res << test.pluck(:c)

But I am thinking it would be better if I get one or two liner solution,
with or without inbuilt function because the number of fields in the future may increase.

Good question, but “...”fields 'a' and 'c'” should be “fields :a and :c”. It would be better to assign a variable to an array of those keys (e.g., target = [:a, :b]), so that readers could refer to that variable in answers and comments without having to define it ( just as they can refer to test). — Cary Swoveland
– Cary Swoveland, Commented Oct 7, 2019 at 15:21

ray · Accepted Answer · 2019-10-07 06:38:19Z

8

You were looking for following,

%i(a c).map { |x| test.map { |e| e[x] } }

answered Oct 7, 2019 at 6:38

ray

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7 Comments

Bhawan Over a year ago

It is working perfect. Can you please explain it a little bit?

ray Over a year ago

first check how map works to provide data in array, so then test this -> test.map { |e| e[:a] } which will provide you [1, 4] so for :c so I have iterated this code on one more map iterated on [:a, :c] array which provided array of array.

Sebastián Palma Over a year ago

@ray, sorry, I didn't see the big difference between [[1, 4], [3, 6]] and [[1, 3], [4, 6]].

ray Over a year ago

@SebastianPalma I guess, me too

Cary Swoveland Over a year ago

My initial reaction was that this was relatively inefficient, but since hash lookups are very fast and have a computational complexity close to O(1), if nis the number of keys of interest (2 in the example), your solution has a computational complexity close to the minimum possible of O(n*test.size). Moreover, your solution reads well, but would read better, and be slightly more efficient, if you were to adopt @3limin4t0r‘s suggestion. Good answer.

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3limin4t0r · Accepted Answer · 2019-10-07 11:56:19Z

4

You could use Array#transpose on the pluck result. Which assumes the array represents rows or columns and swaps the representation around.

test.pluck(:a, :c).transpose
#=> [[1, 4], [3, 6]]

edited Oct 7, 2019 at 11:56

answered Oct 7, 2019 at 11:46

3limin4t0r

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2 Comments

Cary Swoveland Over a year ago

...the pure Ruby equivalent being test.map { |h| h.values_at(:a, :c) }.transpose. Good answer.

3limin4t0r Over a year ago

@CarySwoveland Yep, but OP already uses pluck and tagged the question with ruby-on-rails, so I went with pluck since it's a lot shorter. But providing a pure Ruby equivalent doesn't hurt, and is useful for readers who don't use Rails.

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