From: Rob Biedenharn <Rob@...>
Date: 2010-11-06T00:34:13+09:00
Subject: Re: Convert Array to nested Hash

On Nov 5, 2010, at 10:58 AM, Zhi-Qiang Lei wrote:
> On Oct 24, 2010, at 7:07 PM, Robert Klemme wrote:
> On Sun, Oct 24, 2010 at 11:27 AM, Sandor Szuecs
>> <sandor.szuecs@fu-berlin.de> wrote:
>>> On Oct 24, 2010, at 7:25 AM, Zhi-Qiang Lei wrote:
>>>> Dear All,
>>>>
>>>> I'm seeking a method to convert an array like:
>>>>
>>>> [
>>>>      ["A", "a", 1]
>>>>      ["A", "b", 2]
>>>>      ["B", "a", 1]
>>>>
>>>> ]
>>>>
>>>> into a hash as follow:
>>>> {
>>>>      {"A" => {"a" => 1, "b" => 2}},
>>>>      {"B" => {"a" => 1}}
>>>> }
>>>>
>>>> Do you have any idea on this? Thanks in advance.
>>>
>>> I don't think that you really want to add 3 layers of dictionary.
>>> If you really want this, then you think about your abstractions to
>>> get a simpler solution.
>>>
>>> a = [
>>>   ["A", "a", 1],
>>>   ["A", "b", 2],
>>>   ["B", "a", 1]
>>> ]
>>>
>>> def a_to_h(a)
>>> h={}
>>> a.map do |nested|
>>>   key = nested.shift
>>>   h[key] ||= {}
>>>   h[key].merge!({ nested.shift => nested.shift })
>>> end
>>> h
>>> end
>>>
>>> irb:0> a_to_h(a)
>>> => {"A"=>{"a"=>1, "b"=>2}, "B"=>{"a"=>1}}
>>
>> Here's another way:
>>
>> irb(main):019:0> [
>> irb(main):020:1*   ["A", "a", 1],
>> irb(main):021:1*   ["A", "b", 2],
>> irb(main):022:1*   ["B", "a", 1],
>> irb(main):023:1*
>> irb(main):024:1* ].inject({}) do |h,(k1,k2,v)|
>> irb(main):025:1*   (h[k1] ||= {})[k2] = v
>> irb(main):026:1>   h
>> irb(main):027:1> end
>> => {"A"=>{"a"=>1, "b"=>2}, "B"=>{"a"=>1}}
>>
>> and another one
>>
>> irb(main):028:0> [
>> irb(main):029:1*   ["A", "a", 1],
>> irb(main):030:1*   ["A", "b", 2],
>> irb(main):031:1*   ["B", "a", 1],
>> irb(main):032:1*
>> irb(main):033:1* ].inject(Hash.new {|h,k|
>> h[k]=Hash.new(&h.default_proc)}) do |h,(k1,k2,v)|
>> irb(main):034:1*   h[k1][k2] = v
>> irb(main):035:1>   h
>> irb(main):036:1> end
>> => {"A"=>{"a"=>1, "b"=>2}, "B"=>{"a"=>1}}
>>
>> But I totally agree: why build up a strcuture of nested Arrays and
>> then convert it into the nested Hash structure?  Is is much more
>> efficient to build up the Hash structure initially.  One might even
>> create a Struct for values in the top level Hash.
>>
>> Cheers
>>
>> robert
>>
>> --  
>> remember.guy do |as, often| as.you_can - without end
>> http://blog.rubybestpractices.com/
>>
>
>
> I'm sorry to bring this old topic out. But there is a question make  
> me confused.
>
> irb(main):019:0> [
> irb(main):020:1*   ["A", "a", 1],
> irb(main):021:1*   ["A", "b", 2],
> irb(main):022:1*   ["B", "a", 1],
> irb(main):023:1*
> irb(main):024:1* ].inject({}) do |h,(k1,k2,v)|
> irb(main):025:1*   (h[k1] ||= {})[k2] = v
> irb(main):026:1>   h
> irb(main):027:1> end
> => {"A"=>{"a"=>1, "b"=>2}, "B"=>{"a"=>1}}
>
> Is the tuple (k1, k2, v) in block arguments an array? What's the  
> meaning of () in block arguments? Thanks.
>
> Best regards,
> Zhi-Qiang Lei
> zhiqiang.lei@gmail.com

The inject() will pass two arguments to the block.  The accumulated  
value and the new element.  In the use of the parentheses in the block  
arguments acts like an array assignment so the individual values of  
the array element are available with more meaningful names.

.inject({}) do |h,array|
   k1, k2, v = array
   (h[k1] ||= {})[k2] = v
   h
end

is the same thing without using the block's ability to "unpack" the  
array elements.

-Rob

Rob Biedenharn		
Rob@AgileConsultingLLC.com	http://AgileConsultingLLC.com/
rab@GaslightSoftware.com		http://GaslightSoftware.com/