Ruby: Array to hash, without any local variables

Question

I have an array of strings.

array = ["foo","bar","baz"]

What I'm trying to transform this into is the following:

{"foo"=>nil, "bar"=>nil, "baz" => nil}

I've been doing this with the following:

new_hash = {}
array.each { |k| new_hash[k] = nil }
new_hash

I was wondering if there's any way to accomplish this in a one-liner / without any instance variables.

This question (or very similar) may have appeared like a hundred times in SO. Why on earth was this rejected? bugs.ruby-lang.org/issues/666. A to_hash/mash method is sorely needed. — tokland
– tokland, Commented Jul 23, 2012 at 21:54

Stefan · Accepted Answer · 2012-07-23 21:55:01Z

11

This would work:

new_hash = Hash[array.zip]
# => {"foo"=>nil, "bar"=>nil, "baz"=>nil}

array.zip returns [["foo"], ["bar"], ["baz"]]
Hash::[] creates a Hash from these keys

edited Jul 23, 2012 at 21:55

answered Jul 23, 2012 at 21:36

Stefan

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Kristján · Accepted Answer · 2012-07-23 21:35:25Z

2

You can use Hash[]:

1.9.3p194 :004 > Hash[%w[foo bar baz].map{|k| [k, nil]}]
 => {"foo"=>nil, "bar"=>nil, "baz"=>nil}

or tap

1.9.3p194 :006 > {}.tap {|h| %w[foo bar baz].each{|k| h[k] = nil}}
 => {"foo"=>nil, "bar"=>nil, "baz"=>nil}

answered Jul 23, 2012 at 21:35

Kristján

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1 Comment

Kristján Over a year ago

I withdraw this response in favor of @Stefan's

Henry · Accepted Answer · 2012-07-23 21:52:43Z

1

Hash[array.zip([nil].cycle)]

This answer is too short.

answered Jul 23, 2012 at 21:52

Henry

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user1284966 · Accepted Answer · 2012-07-23 21:33:51Z

0

In one line:

array.inject({}) { |new_hash, k| new_hash[k] = nil ; new_hash }

It's not exactly elegant, but it gets the job done.

Is there a reason you need the hash to be already initialized, though? If you just want a hash with a default value of nil, Hash.new can do that.

Hash.new {|h, k| h[k] = nil}

answered Jul 23, 2012 at 21:33

user1284966

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megas · Accepted Answer · 2012-07-23 21:51:06Z

0

array.each_with_object({}) { |i,h| h[i] = nil }

answered Jul 23, 2012 at 21:51

megas

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