Accessing the last key–value pair in a Ruby (1.9) hash

Question

As of Ruby 1.9, hashes retain insertion order which is very cool. I want to know the best way to access the last key–value pair.

I've written some code which does this:

hash.values.last

This works and is very easy to comprehend, but perhaps it's possible to access the last value directly, rather that via an intermediary (the array of values). Is it?

score 30 · Answer 1 · answered Feb 05 '14 at 01:28

30

Hash have a "first" method, but that return the first pair in array mode, for last, you can try:

my_hash.to_a.last

this return last pair in array mode like "first method"

answered Feb 05 '14 at 01:28

Daniel Antonio Nuñez Carhuayo

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sjagr · Answer 2 · 2018-08-02T21:59:31.717

29

One more alternative that I'm using myself:

hash[hash.keys.last]

which works out better when you want to directly assign a value onto the last element of the hash:

2.4.1 :001 > hash = {foo: 'bar'}
 => {:foo=>"bar"} 
2.4.1 :002 > hash[hash.keys.last] = 'baz'
 => "baz" 
2.4.1 :003 > hash.values.last = 'bar'
NoMethodError: undefined method `last=' for ["baz"]:Array
Did you mean?  last
    from (irb):3
    from /home/schuylr/.rvm/rubies/ruby-2.4.1/bin/irb:11:in `<main>'

edited Aug 02 '18 at 21:59

answered Jul 21 '15 at 17:47

sjagr

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3

This is an awesome solution! – Constant Meiring Sep 04 '15 at 08:36
In new Ruby versions you could also do `hash.values.last` – Edgar Ortega Aug 02 '18 at 05:27
@EdgarOrtega that's exactly what davidchambers was using and led them to asking this question. – sjagr Aug 02 '18 at 21:20
@sjagr , I was pointing to this specific question, but yeah, you're right! – Edgar Ortega Aug 02 '18 at 21:28
@EdgarOrtega the power of my answer is that you can easily do this `hash[hash.keys.last] = 'bar'` whereas you cannot do this `hash.values.last = 'bar'` – sjagr Aug 02 '18 at 21:52

score 14 · Accepted Answer · answered Oct 13 '11 at 03:28

14

Nothing built in, no. But you could monkey-patch one if you were so inclined (not usually recommended, of course):

class Hash
  def last_value
    values.last
  end
end

And then:

hash.last_value

answered Oct 13 '11 at 03:28

Ben Lee

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SHS · Answer 4 · 2015-04-27T11:37:00.057

3

I just did this for a very large hash:

hash.reverse_each.with_index do |(_, value), index|
  break value if (index == 0)
end

edited Apr 27 '15 at 11:37

answered Apr 27 '15 at 11:12

SHS

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This is about 2-3 times faster: `v = hash.each_value.reverse_each.next rescue StopIteration`. Still 2-3 times slower than `hash.values.last` for 100M entries, though. – jaynetics Jun 08 '18 at 21:19
1

Looking up why this is so slow, I found that `#reverse_each` only makes sense on Arrays. Hash and other Enumerables first [create an intermediate Array](https://github.com/ruby/ruby/blob/trunk/enum.c#L2307) when `#reverse_each` is called, in the case of Hash one with all keys and values. – jaynetics Jun 09 '18 at 17:57

Accessing the last key–value pair in a Ruby (1.9) hash

4 Answers4