Ruby: Split / Sort array based on condition

Question

I have an array of holidays:

Holiday.all                        
Holiday Load (0.7ms)  SELECT "holidays".* FROM "holidays"
=> #<ActiveRecord::Relation [#<Holiday id: 1, name: "Dia da Confraternização Universal", holidayable_id: 1, holidayable_type: "Country", date: "2014-01-01", created_at: "2014-11-05 21:38:13", updated_at: "2014-11-05 21:38:13", verified?: false>, #<Holiday id: 2, name: "Natal", holidayable_id: 1, holidayable_type: "Country", date: "2014-12-25", created_at: "2014-11-05 22:02:33", updated_at: "2014-11-05 22:02:33", verified?: false>]>

I'd like to split this big array of dates by it's dates months, so I'd end up with 12 arrays (one for each month).

Please all give an example of the array, with as few elements as needed for the question, and the desired output. — Cary Swoveland
– Cary Swoveland, Commented Nov 5, 2014 at 23:00
If you are retrieving these from a database, why not simply iterate over the months and pull them by month? — the Tin Man
– the Tin Man, Commented Nov 5, 2014 at 23:06
Sorry about posting pseudocode instead of actual code before. I thought it would be more useful. — Victor Ribeiro
– Victor Ribeiro, Commented Nov 5, 2014 at 23:07

Simone Carletti · Accepted Answer · 2014-11-05 22:34:23Z

3

What you want to use is the Enumerable#group_by method.

Groups the collection by result of the block. Returns a hash where the keys are the evaluated result from the block and the values are arrays of elements in the collection that correspond to the key.

If no block is given an enumerator is returned.
(1..6).group_by { |i| i%3 }   #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}

For instance

["12/25", "01/01", (...)].group_by { |date| date.split("/").first }

answered Nov 5, 2014 at 22:34

Simone Carletti

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2 Comments

Victor Ribeiro Over a year ago

That's what I needed. Strangely, the "ruby/array" api page doesn't mention "group_by".

Cary Swoveland Over a year ago

That's because it's here: Enumerable#group_by. If you're not familiar with the class Enumerable, have a browse. Those methods can be used with all types of collections (e.g., arrays, hashes, ranges, sets and others).

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