Re: Default values of hashes

Glenn,

A Hash can either have a default value:
   h1 = Hash.new(:foo)
   h1['new']
   => :foo
or a default proc
   h2 = Hash.new { :foo }
   h2['new']
   => :foo
but the proc doesn't create the entry in the hash
   h2.keys
   => []
See my first response or the rdoc for Hash.new

If you want to create the entry, you need to use the hash and key  
values passed to the block:
   h3 = Hash.new {|h,k| h[k] = :foo }
   h3['new']
   => :foo
   h3.keys
   => ["new"]
   h3
   => {"new"=>:foo}

You're trying to eat your cake and have it, too.  An initialization  
similar to your recent post is needed.

def deep_hash(levels=1, default=nil)
   if levels == 0
     if default
       Hash.new { |h,k| h[k] = default }
     else
       Hash.new
     end
   else
     Hash.new { |h,k| h[k] = deep_hash(levels-1, default) }
   end
end

if File.expand_path($0) == File.expand_path(__FILE__)
   require 'test/unit'

   class DeepHashTest < Test::Unit::TestCase
     def test_zero_levels_and_no_default_is_normal_hash
       expected = Hash.new
       actual = deep_hash(0)
       assert_equal expected, actual
       assert_nil actual[:key]
       assert_equal [], actual.keys
     end

     def test_zero_levels_and_default_is_normal_hash
       expected = Hash.new(42)
       actual = deep_hash(0,42)
       assert_equal expected, actual
       assert_equal 42, actual[:key]
       assert_equal [:key], actual.keys
     end

     def test_one_level_and_no_default_is_simple_nested_hash
       expected = Hash.new {|h,k| h[k] = Hash.new}
       actual = deep_hash(1)
       assert_equal expected, actual
       assert_nil actual[1][:key]
       assert_equal [1], actual.keys
       assert_equal [], actual[1].keys
     end

     def test_one_level_and_default_is_simple_nested_hash
       expected = Hash.new {|h,k| h[k] = Hash.new(43)}
       actual = deep_hash(1,43)
       assert_equal expected, actual
       assert_equal 43, actual[1][:key]
       assert_equal [1], actual.keys
       assert_equal [:key], actual[1].keys
     end

     def test_three_levels_and_default_for_glenn
       expected = Hash.new {|h3,k3| h3[k3] = Hash.new {|h2,k2| h2[k2]  
= Hash.new {|h,k| h[k] = Hash.new(44) } } }
       actual = deep_hash(3, 44)
       assert_equal expected, actual
       assert(expected == actual, "Using ==")
       assert_equal 44, actual[:a][:b][:c][:d]
       actual[:a][:b][:c][:new] += 1
       assert_equal 45, actual[:a][:b][:c][:new]
       assert_equal [:d,:new], actual[:a][:b][:c].keys
     end
   end
end

__END__

The meaning of a certain number of levels is perhaps off-by-one  
relative to what you want and I'm not trying to allow the inner-most  
(deepest) hash to have a default_proc since it would need to have  
something like lambda {|h,k|h[k]=block.call}.

I suppose that can be left as an exercise for the reader. ;-)

-Rob

Rob Biedenharn		http://agileconsultingllc.com
[e-mail]

On Jan 4, 2009, at 8:47 PM, Glenn wrote:

> 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 <[e-mail]>
> To: ruby-talk ML <[e-mail]>
> 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
> Ruby/Rails consulting & training: http://www.rubypal.com
> Coming in 2009: The Well-Grounded Rubyist (http://manning.com/black2)
>
> http://www.wishsight.com => Independent, social wishlist management!