How to merge hashes if a specified key's values are same in a array

Question

I have a array of hashes like this:

[ {:foo=>2, :date=>Sat, 01 Sep 2014},
 {:foo2=>2, :date=>Sat, 02 Sep 2014},
 {:foo3=>3, :date=>Sat, 01 Sep 2014},
 {:foo4=>4, :date=>Sat, 03 Sep 2014},
  {:foo5=>5, :date=>Sat, 02 Sep 2014}]

And I want to merge hashes if the :date are same. What I expect from the array above is:

[ {:foo=>2, :foo3=>3, :date=>Sat, 01 Sep 2014},
 {:foo2=>2, :foo5=>5 :date=>Sat, 02 Sep 2014},
 {:foo4=>4, :date=>Sat, 03 Sep 2014}]

How can I do it?

Maybe should I reconsider data structure itself? For example should I use date value as a key of hash?

Your hash definitions aren't valid. It's important that they be reusable when we try to help you, so make sure that Ruby will accept them. — the Tin Man
– the Tin Man, Commented Dec 2, 2014 at 20:02

Cristian Lupascu · Accepted Answer · 2014-12-02 19:53:30Z

8

Here's how you could do it in a line (demo):

hashes.group_by{|h| h[:date] }.map{|_, hs| hs.reduce(:merge)}

This code does the following:

groups all hashes by their :date value
for each :date group, takes all hashes in it and merges them all into one hash

EDIT: Applied modifications suggested by tokland and Cary Swoveland. Thanks!

edited Dec 2, 2014 at 19:53

answered Dec 2, 2014 at 12:24

Cristian Lupascu

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3 Comments

tokland Over a year ago

nit-picking: hashes.group_by { |h| h[:date] }.map { |k, hs| hs.reduce(:merge) }

Cary Swoveland Over a year ago

Nice solution. You might consider |_,hs| to tell the reader you're not using the key in the block. It's a pretty small block, so that may be obvious, but still...

Conor Over a year ago

Little gotcha - If :date isn't a symbol use 'date'

Cary Swoveland · Accepted Answer · 2014-12-02 18:20:48Z

You could make use of the form of Hash#update (a.k.a. merge!) that uses a block to resolve the values of keys that are contained in both of the hashes being merged.

arr = [ {:foo=>2, :date=>'Sat, 01 Sep 2014'},
        {:foo2=>2, :date=>'Sat, 02 Sep 2014'},
        {:foo3=>3, :date=>'Sat, 01 Sep 2014'},
        {:foo4=>4, :date=>'Sat, 03 Sep 2014'},
        {:foo5=>5, :date=>'Sat, 02 Sep 2014'}]

arr.each_with_object({}) do |g,h|
  h.update({ g[:date]=>g }) { |_,o,n| o.merge(n) }
end.values
  #=> [{:foo=>2,  :date=>"Sat, 01 Sep 2014", :foo3=>3},
  #    {:foo2=>2, :date=>"Sat, 02 Sep 2014", :foo5=>5},
  #    {:foo4=>4, :date=>"Sat, 03 Sep 2014"}]

I've used a placeholder _ for the block variable containing the value of the key since it it not used in the block.

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