From: Glenn <glenn_ritz@...>
Date: 2009-01-05T10:47:16+09:00
Subject: Re: Default values of hashes

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Hi David,

In your other post, there is no default value for the innermost hash.

hproc = lambda {|h,k| h[k] = Hash.new(&hproc) }
hash = Hash.new(&hproc)

So if I try to increment on a key that isn't there yet, like this:

hash[1][2][3] += 1

I get an undefined method for hash error.

Am I using it correctly if I want the inner hash to have a default?  Please forgive me if I have missed something in your example.  I am still relatively new to Ruby and am trying to get to the next level by writing more sophisticated programs.

Thanks,

Glenn



________________________________
From: David A. Black <dblack@rubypal.com>
To: ruby-talk ML <ruby-talk@ruby-lang.org>
Sent: Sunday, January 4, 2009 4:22:37 PM
Subject: Re: Default values of hashes

Hi --

On Mon, 5 Jan 2009, Sebastian Hungerecker wrote:

> Glenn wrote:
>> In other words, if I need 3 levels, it creates a hash within hash object 3
>> levels deep (like in the line of code above) and if I need 5, it creates
>> one 5 levels deep.  I do not know how to do this (or even if it is
>> possible).  Any suggestions on this one?
>
> def nested_hash(levels, def_value=nil, &def_proc)
>  inner_hash = def_proc ? Hash.new(&def_proc) : Hash.new(def_value)
>  if levels == 1
>    inner_hash
>  else
>    l = [lambda {|h,k| h[k] = inner_hash}]
>    (levels-2).times do |i|
>      l[i+1] = lambda {|h,k| h[k] = Hash.new( &l[i] ) }
>    end
>    Hash.new(&l[-1])
>  end
> end
>
> This creates a nested hash levels levels deep, where the innermost hash can
> optionally have a default value or proc.
>
> Usage:
> h = nested_hash(1)
> # Same as h = Hash.new
>
> h = nested_hash(1) {|h,k| h[k] = Array.new}
> # Same as h = Hash.new {|h,k| h[k] = Array.new}
>
> h = nested_hash(3)
> # Same as h = Hash.new {|h,k| h[k] = Hash.new {|h,k| h[k] = Hash.new}}
>
> h = nested_hash(3, :foo)
> # Same as h = Hash.new {|h,k| h[k] = Hash.new {|h,k| h[k] = Hash.new(:foo)}}

Why not make it just keep going automatically? (See my other post.)


David

-- 
David A. Black / Ruby Power and Light, LLC
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