How to initialize a hash with keys from an array?

Here we go using Enumerable#each_with_object and Hash::[].

 keys = [ 'a' , 'b' , 'c' ]
 Hash[keys.each_with_object(nil).to_a]
 # => {"a"=>nil, "b"=>nil, "c"=>nil}

or Using Array#product

keys = [ 'a' , 'b' , 'c' ]
Hash[keys.product([nil])]
# => {"a"=>nil, "b"=>nil, "c"=>nil}

Using the new (Ruby 2.1) to_h:

keys.each_with_object(nil).to_h

=> keys = [ 'a' , 'b' , 'c' ]
=> Hash[keys.map { |x, z| [x, z] }]
# {"a"=>nil, "b"=>nil, "c"=>nil}

=> Hash[keys.map { |x| [x, nil] }]
# {"a"=>nil, "b"=>nil, "c"=>nil}

=> Hash[keys.map { |x, _| [x, _] }]
# {"a"=>nil, "b"=>nil, "c"=>nil}

Another alternative using Array#zip:

Hash[keys.zip([])]
# => {"a"=>nil, "b"=>nil, "c"=>nil}

Update: As suggested by Arup Rakshit here's a performance comparison between the proposed solutions:

require 'fruity'

ary = *(1..10_000)

compare do
  each_with_object {  ary.each_with_object(nil).to_a  }
  product          {  ary.product([nil])  }
  zip              {  ary.zip([])  }
  map              {  ary.map { |k| [k, nil] }  }
end

The result:

Running each test once. Test will take about 1 second.
zip is faster than product by 19.999999999999996% ± 1.0%
product is faster than each_with_object by 30.000000000000004% ± 1.0%
each_with_object is similar to map