From: Zhi-Qiang Lei <zhiqiang.lei@...>
Date: 2010-11-05T23:58:22+09:00
Subject: Re: Convert Array to nested Hash

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