Problem on hash with default value

Ruby
1

Hi,

I face the following issue with a hash having an empty array as default
value.

I don’t how to describe it in English, here is an illustrative code:

hash = Hash.new Array.new

hash[:a] = [1]

hash[:b] << 2

puts hash[:b]

=> returns [2]

puts hash.keys

=> returns [:a], not [:a, :b] ?

hash[:b] is set to [2] but I don’t understand why hash.keys does not
include :b.

Thanks!,
Florent

2

On Thu, Nov 17, 2011 at 6:45 PM, Florent G. [email protected] wrote:

hash[:b] << 2

puts hash[:b]

=> returns [2]

puts hash.keys

=> returns [:a], not [:a, :b] ?

hash[:b] is set to [2] but I don’t understand why hash.keys does not
include :b.

The reason is that the Hash constructor you are using uses the object
you pass, in your case, Array.new, to return when you access a
non-existing key. But, it doesn’t assign the array to the key. You
have to do it yourself. One way is to use the default proc:

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

This will create a new array and assign it to a key, whenever you
access a non-existing key:

ruby-1.8.7-p334 :001 > h = Hash.new {|hash, k| hash[k] = Array.new}
=> {}
ruby-1.8.7-p334 :002 > h[:a]
=> []
ruby-1.8.7-p334 :003 > h
=> {:a=>[]}

Also, keep in mind that the other constructor you were using will
return the same Array instance to all non-existing keys:

ruby-1.8.7-p334 :004 > h = Hash.new Array.new
=> {}
ruby-1.8.7-p334 :005 > h[:a]
=> []
ruby-1.8.7-p334 :006 > h[:a] << 2
=> [2]
ruby-1.8.7-p334 :007 > h[:a]
=> [2]
ruby-1.8.7-p334 :008 > h[:b]
=> [2]

Jesus.

3

Go ahead and check the doc:

4

maybe its interessing for you:
an Hash wich used an default_proc
is not mashalble anymore, so you can not dump it

5

Make sense, thanks!