Convert an array into an index hash in Ruby

Question

I have an array, and I want to make a hash so I can quickly ask "is X in the array?".

In perl, there is an easy (and fast) way to do this:

my @array = qw( 1 2 3 );
my %hash;
@hash{@array} = undef;

This generates a hash that looks like:

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

The best I've come up with in Ruby is:

array = [1, 2, 3]
hash = Hash[array.map {|x| [x, nil]}]

which gives:

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

Is there a better Ruby way?

EDIT 1

No, Array.include? is not a good idea. Its slow. It does a query in O(n) instead of O(1). My example array had three elements for brevity; assume the actual one has a million elements. Let's do a little benchmarking:

#!/usr/bin/ruby -w
require 'benchmark'

array = (1..1_000_000).to_a
hash = Hash[array.map {|x| [x, nil]}]

Benchmark.bm(15) do |x|
    x.report("Array.include?") { 1000.times { array.include?(500_000) } }
    x.report("Hash.include?") { 1000.times { hash.include?(500_000) } }
end

Produces:

                     user     system      total        real
Array.include?  46.190000   0.160000  46.350000 ( 46.593477)
Hash.include?    0.000000   0.000000   0.000000 (  0.000523)

Answer 1

Your way of creating the hash looks good. I had a muck around in irb and this is another way

>> [1,2,3,4].inject(Hash.new) { |h,i| {i => nil}.merge(h) }
=> {1=>nil, 2=>nil, 3=>nil, 4=>nil}

Answer 2

If you're not bothered what the hash values are

irb(main):031:0> a=(1..1_000_000).to_a ; a.length
=> 1000000
irb(main):032:0> h=Hash[a.zip a] ; h.keys.length
=> 1000000

Takes a second or so on my desktop.