From: Robert Klemme <bob.news@...>
Date: 2005-10-27T17:22:08+09:00
Subject: Re: Enumerable#build

Eric Hodel wrote:
> On Oct 26, 2005, at 4:47 AM, Robert Klemme wrote:
>
>> Martin DeMello wrote:
>>
>>> class Enumerable
>>>   def build(seed)
>>>     each {|i|
>>>       seed.add(yield i)
>>>     }
>>>   end
>>> end
>>>
>>> and
>>>
>>> Array#build == Array#<<
>>> Hash#build(k,v) == Hash#[]
>>> etc
>>>
>>> So you could say [1,2,3].build({}) {|i| [i, 1]} for the question
>>> that prompted the to_hash thread, but also, in general, you can
>>> collect into non-array structures (binary trees, e.g.) as long as
>>> they implement a suitable #add method.
>>>
>>> (Of course, you can do ary.inject(seed) {|i| seed.add(f(i)); seed}
>>> right now but a #build method makes the common case pretty.)
>>>
>>
>> Looks good.  But,
>>
>>  - Enumerable is a module... :-)
>>
>>  - You should add seed as return value.
>>
>>  - Your first example won't work as Hash#add is nonexistent
>>
>> An alternative approach would be to reverse the logic and have
>> Enumerable#populate:
>>
>> module Enumerable
>>   def populate(*enum)
>>     enum = enum.first if enum.size == 1
>>     enum.each {|*a| add(yield(*a))}
>>     self
>>   end
>> end
>>
>> class Hash
>>   def populate(*enum)
>>     enum = enum.first if enum.size == 1
>>     enum.each {|*a| send(:[]=, *yield(*a))}
>>     self
>>   end
>> end
>>
>>
>>>> h={}.populate(1,2,3) {|x| [x, "val #{x}"]}
>>>>
>> => {1=>"val 1", 2=>"val 2", 3=>"val 3"}
>>
>> What do you think?
>
> I think you have too much magic.

Is this good or bad? :-))

> Enumerable#to_a, Array#fill, Array#flatten and Hash::[] do this
> pretty handily and much more cleanly:
>
> Hash[*ary.fill { |i| [i, "val #{i}"]}.flatten]
>
> $ ruby -e 'p Hash[*(0..3).to_a.fill { |i| [i, "val #{i}"] }.flatten ]'
> {0=>"val 0", 1=>"val 1", 2=>"val 2", 3=>"val 3"}

I'm not sure about cleanness but this is certainly less memory efficient
as it creates relatively large temporary data structures.

Kind regards

    robert