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comp.lang.ruby

hash of hashes by default

Belorion

6/16/2005 4:07:00 PM

I want a Hash of Hashes. Furthermore, I want it so that if a key for
the first has does not exist, the default value is a new hash. It's
simple enough to do a has_key? on a hash to see if it already exists,
but it seems like there might be a trick to it that I'm not aware of.

So, instead of

foo = Hash.new

mylist.each do |ii|
anotherlist.each do |jj|
if foo.has_key?( ii )
foo[ii][jj] = "hello world"
else
foo[ii] = { jj => "hello world" }
end
end
end

I can do:

mylist.each do |ii|
anotherlist.each do |jj|
foo[ii][jj] = "hello world"
end
end

Any suggestions?
3 Answers

James Gray

6/16/2005 4:12:00 PM

0

On Jun 16, 2005, at 11:07 AM, Belorion wrote:

> I want a Hash of Hashes. Furthermore, I want it so that if a key for
> the first has does not exist, the default value is a new hash.

irb(main):001:0> foo = Hash.new { |hash, key| hash[key] = Hash.new }
=> {}
irb(main):002:0> foo["ii"]["jj"] = "Hello World."
=> "Hello World."
irb(main):003:0> foo["ii"]
=> {"jj"=>"Hello World."}
irb(main):004:0> foo["ii"]["jj"]
=> "Hello World."

Hope that helps.

James Edward Gray II

Austin Ziegler

6/16/2005 4:17:00 PM

0

On 6/16/05, James Edward Gray II <james@grayproductions.net> wrote:
> On Jun 16, 2005, at 11:07 AM, Belorion wrote:
>> I want a Hash of Hashes. Furthermore, I want it so that if a key
>> for the first has does not exist, the default value is a new
>> hash.
> irb(main):001:0> foo = Hash.new { |hash, key| hash[key] = Hash.new }
> => {}
> irb(main):002:0> foo["ii"]["jj"] = "Hello World."
> => "Hello World."
> irb(main):003:0> foo["ii"]
> => {"jj"=> "Hello World."}
> irb(main):004:0> foo["ii"]["jj"]
> => "Hello World."

An infinite variation of this is a two liner.

hinit = proc { |hash, key| hash[key] = Hash.new(&hinit) }
foo = Hash.new(&hinit)

foo["ii"]["jj"]["kk"] = "Hi, James!"

require 'pp'
p foo

-austin
--
Austin Ziegler * halostatue@gmail.com
* Alternate: austin@halostatue.ca

Belorion

6/16/2005 4:18:00 PM

0

On 6/16/05, James Edward Gray II <james@grayproductions.net> wrote:
> On Jun 16, 2005, at 11:07 AM, Belorion wrote:
>
> > I want a Hash of Hashes. Furthermore, I want it so that if a key for
> > the first has does not exist, the default value is a new hash.
>
> irb(main):001:0> foo = Hash.new { |hash, key| hash[key] = Hash.new }
> => {}
> irb(main):002:0> foo["ii"]["jj"] = "Hello World."
> => "Hello World."
> irb(main):003:0> foo["ii"]
> => {"jj"=>"Hello World."}
> irb(main):004:0> foo["ii"]["jj"]
> => "Hello World."
>
> Hope that helps.

Aha! Perfect!