How can I initialize an Array inside a Hash in Ruby

Question

I am trying to initialize a Hash of Arrays such as

@my_hash = Hash.new(Array.new)

so that I can:

@my_hash["hello"].push("in the street")
=> ["in the street"]
@my_hash["hello"].push("at home")
=> ["in the street", "at home"]
@my_hash["hello"]
=>["in the street", "at home"]

The problem is that any new hash key also return ["in the street", "at home"]

@my_hash["bye"]
=> ["in the street", "at home"]
@my_hash["xxx"]
=> ["in the street", "at home"]

!!!???

What am I doing wrong what would be the correct way to initialize a Hash of Arrays?

related: http://stackoverflow.com/questions/190740/setting-ruby-hash-default-to-a-list — tokland, Dec 10 '10 at 15:38

score 101 · Accepted Answer · answered Dec 10 '10 at 15:09

@my_hash = Hash.new(Array.new)

This creates exactly one array object, which is returned every time a key is not found. Since you only ever mutate that array and never create a new one, all your keys map to the same array.

What you want to do is:

@my_hash = Hash.new {|h,k| h[k] = Array.new }

or simply

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

Passing a block to Hash.new differs from simply passing an argument in 2 ways:

The block is executed every time a key is not found. Thus you'll get a new array each time. In the version with an argument, that argument is evaluated once (before new is called) and the result of that is returned every time.
By doing h[k] = you actually insert the key into the hash. If you don't do this just accessing @my_hash[some_key] won't actually cause some_key to be inserted in the hash.

The one gotcha to be aware of here is that now even *checking* a key will create it, which may not be what you want. For this reason I sometimes create the Hash normally (with just {}), and then do the array-creation in the call, like `(@myhash["hello"] ||= []).push("in the street")`. — glenn mcdonald, Dec 10 '10 at 16:32

score 8 · Answer 2 · edited Mar 04 '15 at 06:53

8

Try this:

@my_hash = Hash.new { |h, k| h[k] = Array.new }

edited Mar 04 '15 at 06:53

Rajdeep Singh

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answered Dec 10 '10 at 15:08

Incidently

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score 2 · Answer 3 · answered Dec 10 '10 at 15:08

The argument for Hash.new is for the default value for new hash keys, so when you pass it a reference that reference will be used for new hash keys. You're updating that reference when you call...

hash["key"].push "value"

You need to pass a new reference into the hash key before pushing values to it...

hash["key1"] = Array.new
hash["key1"].push "value1"
hash["key2"] = Array.new
hash["key2"].push "value2

You could try encapsulating this into a helper method as well.

How can I initialize an Array inside a Hash in Ruby

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