Array to Hash : words count

With inject:

str = 'I have array of words and I want to get a hash, where keys are words'
result = str.split.inject(Hash.new(0)) { |h,v| h[v] += 1; h }

=> {"I"=>2, "have"=>1, "array"=>1, "of"=>1, "words"=>2, "and"=>1, "want"=>1, "to"=>1, "get"=>1, "a"=>1, "hash,"=>1, "where"=>1, "keys"=>1, "are"=>1}

I don't know about the efficiency.

The imperative approach you used is probably the fastest implementation in Ruby. With a bit of refactoring, you can write a one-liner:

wf = Hash.new(0).tap { |h| words.each { |word| h[word] += 1 } }

Another imperative approach using Enumerable#each_with_object:

wf = words.each_with_object(Hash.new(0)) { |word, acc| acc[word] += 1 }

A functional/immutable approach using existing abstractions:

wf = words.group_by(&:itself).map { |w, ws| [w, ws.length] }.to_h

Note that this is still O(n) in time, but it traverses the collection three times and creates two intermediate objects along the way.

Finally: a frequency counter/histogram is a common abstraction that you'll find in some libraries like Facets: Enumerable#frequency.

require 'facets'
wf = words.frequency

Posted on a related question, but posting here for visibility as well:

Ruby 2.7 onwards will have the Enumerable#tally method that will solve this.

From the trunk documentation:

Tallys the collection. Returns a hash where the keys are the elements and the values are numbers of elements in the collection that correspond to the key.

["a", "b", "c", "b"].tally #=> {"a"=>1, "b"=>2, "c"=>1}