What is the best way to convert an array to a hash in Ruby

Question

In Ruby, given an array in one of the following forms...

[apple, 1, banana, 2]
[[apple, 1], [banana, 2]]

...what is the best way to convert

Answer 1

NOTE: For a concise and efficient solution, please see Marc-André Lafortune's answer below.

This answer was originally offered as an alternative to approaches using flatten, which were the most highly upvoted at the time of writing. I should have clarified that I didn't intend to present this example as a best practice or an efficient approach. Original answer follows.

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Warning! Solutions using flatten will not preserve Array keys or values!

Building on @John Topley's popular answer, let's try:

a3 = [ ['apple', 1], ['banana', 2], [['orange','seedless'], 3] ]
h3 = Hash[*a3.flatten]

This throws an error:

ArgumentError: odd number of arguments for Hash
        from (irb):10:in `[]'
        from (irb):10

The constructor was expecting an Array of even length (e.g. ['k1','v1,'k2','v2']). What's worse is that a different Array which flattened to an even length would just silently give us a Hash with incorrect values.

If you want to use Array keys or values, you can use map:

h3 = Hash[a3.map {|key, value| [key, value]}]
puts "h3: #{h3.inspect}"

This preserves the Array key:

h3: {["orange", "seedless"]=>3, "apple"=>1, "banana"=>2}

Answer 2

Update

Ruby 2.1.0 is released today. And I comes with Array#to_h (release notes and ruby-doc), which solves the issue of converting an Array to a Hash.

Ruby docs example:

[[:foo, :bar], [1, 2]].to_h    # => {:foo => :bar, 1 => 2}

Answer 3

You can also simply convert a 2D array into hash using:

1.9.3p362 :005 > a= [[1,2],[3,4]]

 => [[1, 2], [3, 4]]

1.9.3p362 :006 > h = Hash[a]

 => {1=>2, 3=>4} 

Answer 4

Appending to the answer but using anonymous arrays and annotating:

Hash[*("a,b,c,d".split(',').zip([1,2,3,4]).flatten)]

Taking that answer apart, starting from the inside:

• "a,b,c,d" is actually a string.
• split on commas into an array.
• zip that together with the following array.
• [1,2,3,4] is an actual array.

The intermediate result is:

[[a,1],[b,2],[c,3],[d,4]]

flatten then transforms that to:

["a",1,"b",2,"c",3,"d",4]

and then:

*["a",1,"b",2,"c",3,"d",4] unrolls that into "a",1,"b",2,"c",3,"d",4

which we can use as the arguments to the Hash[] method:

Hash[*("a,b,c,d".split(',').zip([1,2,3,4]).flatten)]

which yields:

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

Answer 5

Simply use Hash[*array_variable.flatten]

For example:

a1 = ['apple', 1, 'banana', 2]
h1 = Hash[*a1.flatten(1)]
puts "h1: #{h1.inspect}"

a2 = [['apple', 1], ['banana', 2]]
h2 = Hash[*a2.flatten(1)]
puts "h2: #{h2.inspect}"

Using Array#flatten(1) limits the recursion so Array keys and values work as expected.