Sort hash by key, return hash in Ruby

Question

Would this be the best way to sort a hash and return Hash object (instead of Array):

h = {"a"=>1, "c"=>3, "b"=>2, "d"=>4}
# => {"a"=>1, "c"=>3, "b"=>2, "d"=>4}

Hash[h.sort]
# => {"a"=>1, "b"=>2, "c"=>3, "d"=>4}

Solution 1

In Ruby 2.1 it is simple:

h.sort.to_h

Solution 2

Note: Ruby >= 1.9.2 has an order-preserving hash: the order keys are inserted will be the order they are enumerated. The below applies to older versions or to backward-compatible code.

There is no concept of a sorted hash. So no, what you're doing isn't right.

If you want it sorted for display, return a string:

"{" + h.sort.map{|k,v| "#{k.inspect}=>#{v.inspect}"}.join(", ") + "}"

or, if you want the keys in order:

h.keys.sort

or, if you want to access the elements in order:

h.sort.map do |key,value|
  # keys will arrive in order to this block, with their associated value.
end

but in summary, it makes no sense to talk about a sorted hash. From the docs, "The order in which you traverse a hash by either key or value may seem arbitrary, and will generally not be in the insertion order." So inserting keys in a specific order into the hash won't help.

Solution 3

I've always used sort_by. You need to wrap the #sort_by output with Hash[] to make it output a hash, otherwise it outputs an array of arrays. Alternatively, to accomplish this you can run the #to_h method on the array of tuples to convert them to a k=>v structure (hash).

hsh ={"a" => 1000, "b" => 10, "c" => 200000}
Hash[hsh.sort_by{|k,v| v}] #or hsh.sort_by{|k,v| v}.to_h

There is a similar question in "How to sort a Ruby Hash by number value?".

Solution 4

Sort hash by key, return hash in Ruby

With destructuring and Hash#sort

hash.sort { |(ak, _), (bk, _)| ak <=> bk }.to_h

Enumerable#sort_by

hash.sort_by { |k, v| k }.to_h

Hash#sort with default behaviour

h = { "b" => 2, "c" => 1, "a" => 3  }
h.sort         # e.g. ["a", 20] <=> ["b", 30]
hash.sort.to_h #=> { "a" => 3, "b" => 2, "c" => 1 }

Note: < Ruby 2.1

array = [["key", "value"]] 
hash  = Hash[array]
hash #=> {"key"=>"value"}

Note: > Ruby 2.1

[["key", "value"]].to_h #=> {"key"=>"value"}

Solution 5

You gave the best answer to yourself in the OP: Hash[h.sort] If you crave for more possibilities, here is in-place modification of the original hash to make it sorted:

h.keys.sort.each { |k| h[k] = h.delete k }

View more solutions

Comments

[PJP]

I'm not sure there's much advantage to sorting a hash, unless you are using each or each_pair to iterate over it. Even then, I'd probably still grab the keys, sort those, then iterate over them grabbing the values as needed. That ensures the code will behave correctly on older Rubies.

[Adit Saxena]

Makes sense in ruby 1.9 too. I had a collection of appointments grouped by dates (as keys) coming from db and i manually sorted through ruby. Eg. { "2012-09-22": [...], "2012-09-30": [...], "2012-10-12": [...] }

[Douglas]

Yes, I find your Hash[h.sort] process more effectiv than sorting keys then accessing again the hash trough the sorted keys.

[PJP]

"What is the fastest way to sort a Hash? will be useful.

[Mark Thomas]

You've had a few years to think about your solution, are you ready to accept an answer? ;-)

[mlabarca]

It's been 11 years, let's accept a solution now :P

[Aaron Scruggs]

This is correct. I would suggest using the RBtree gem to gain ordered set functionality in ruby.

[David]

Starting 1.9.2 hash insert order will be preserved. See redmine.ruby-lang.org/issues/show/994

[Peter]

@David: very interesting, thanks!! Didn't realize this had changed.

[tokland]

on the last example "h.sort" should be enough.

[PJP]

"Starting 1.9.2 hash insert order will be preserved.", and it's sweet.

[Jo Liss]

I agree that it's a sweet feature (thanks David for pointing it out), but I still feel a little funny about using it. I'd perhaps just use the array that comes out of h.sort when I care about the element ordering.

[Jo Liss]

@Peter: By the way, instead of h.sort_by(&:first) in your examples, you can just equivalently write h.sort. (Docs at ruby-doc.org/core/classes/Hash.html#M002865 .)

[Jo Liss]

Re my first comment (feeling funny): For example, relying on hash ordering would silently and unpredictably break for Ruby versions older than 1.9.2.

[Peter]

@Jo, @tokland: have amended my answer to remove the &:first part.

[Redoman]

How this answer got about 20 +1's without answering not even one of the two parts of the OP question? "1) Would that (OP example) be the best way to sort a hash, 2) and return Hash object" ? I don't envy +1's :) it's just after that reading the answer I am still left with the original questions. Also if the point is that there is not such thing as a sorted hash look at the comments for choosen answer to this question stackoverflow.com/questions/489139/…

[boulder_ruby]

This is how you would do this if ruby didn't have methods like Hash#sort_by

[stevenspiel]

using sort_by on a hash will return an array. You would need to map it out as a hash again. Hash[hsh.sort_by{|k,v| v}]

[boulder_ruby]

yes, the enumerator class interprets hashes as arrays I think

[Cary Swoveland]

Note that hsh.sort_by passes tupples [k,v] into the block, so you can write Hash[hsh.sort_by(&:first)].

[boulder_ruby]

@CarySwoveland wouldn't that be :last then because the first of each tuple would be the key, and we want to sort by value (?? not sure)

[Cary Swoveland]

Right, sort is on values: hsh.sort_by(&:last).to_h => {"b"=>10, "a"=>1000, "c"=>200000}.

[Phrogz]

Note that calling to_h is only supported in Ruby 2.1.0+

[nitrogen]

For the curious, this runs slightly faster than the "keys sort" in stackoverflow.com/a/17331221/737303 on Ruby 1.9.3.

[jitter]

there is a typo in the comment, correction: sort_by{|k,v| v}.to_h)

[Linju]

You can get keys and values like this too. Key of the smallest value: *hsh[0][0] First key value pair: Hash[*hsh[0]]

[whitehat101]

@zachaysan but it does work: h.sort{|a,z|a<=>z}.to_h (tested 2.1.10, 2.3.3)

[zachaysan]

@whitehat101 You are right. I had a bug (a is an array, not just the key). I've deleted my comment.

[Snake Sanders]

The link is broken

[eremite]

I fixed the link to point to Google in case some historian wants to research how this could have been done in the past. But anyone coming on this now should be using a more recent version of Ruby.

[lcjury]

@jj_ This question adds a lot of valuable information around the question. Yeah, it doesn't answer the question itself, but StackOverflow is not a Q&A site. This answer goes with the spirit of SO about collectively building answers.

[silva96]

if not using v you should prefix an underscore hash.sort_by { |k, _v| k }.to_h

[J.M. Janzen]

Just in case someone else is looking for a way to sort an array of hashes, this will do the trick (where h is the array): h.map(&:sort).map(&:to_h).

[Kaplan Ilya]

I wouldn't use this solution as it's not the fastest. For small hash it's fine but for couple of thousands of keys it's 10 times slower than h.keys.sort.map { |k| [k, h[k]] }.to_h. Not sure where is the difference on C level, but it's very clear on benchmark. Even sort_by(&:first) is much faster.

[Mark Thomas]

@KaplanIlya Likely the difference is sorting keys vs. sorting the whole hash. Still, I prefer simplicity and brevity and would reach for this solution over something more complicated unless the hash was big enough where it was a noticeable difference. And if that's the case, sorting a hash is probably not the best architectural decision anyway :)