From: dblack@...
Date: 2003-08-12T22:37:24+09:00
Subject: Re: hash default value

Hi --

On Tue, 12 Aug 2003, Emmanuel Touzery wrote:

> Hello,
>
>     I'm bitten by a hash default value problem. I wanted to avoid the
> myHash[a] = {} if !myHash[a]; myHash[a]["blue"] = 1
>     pattern by using hash default values, but i get an unexpected (for
> me) result:
>
> irb(main):001:0> myHash = {}
> {}
> irb(main):002:0> myHash["blue"]["red"] = 1
> NameError: undefined method `[]=' for nil
>         from (irb):2
> irb(main):003:0> myOtherHash = Hash.new({})
> {}
> irb(main):004:0> myOtherHash["blue"]["red"] = 1
> 1
> irb(main):005:0> p myOtherHash
> {}
> nil
>
> i expected that myOtherHash would be {"blue"=>{"red"=>1}}

No, there's no autovivification in the Perl manner :-)

> > what am i missing here? is there a workaround to the ugly syntax i said
> i want to avoid? (you have the right to say "store your data in a decent
> manner")

The most common technique for this is:

  my_hash = {}
  my_hash["blue"] ||= {}       # init to hash if not init'd
  my_hash["blue"]["red"] = 1

which can be trimmed down to:

  my_hash = {}
  (my_hash["blue"] ||= {})["red"] = 1


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav