From: "David A. Black" <dblack@...>
Date: 2009-09-16T03:53:25+09:00
Subject: Re: .map.with_object(3){|v|v+3} #=> 3 Is this a bug?

Hi --

On Wed, 16 Sep 2009, 7stud -- wrote:

> David A. Black wrote:
>> Hi --
>>
>> On Tue, 15 Sep 2009, 7stud -- wrote:
>>
>>> ------------------------------------------------- Enumerator#with_object
>>>     e.with_object(obj) {|(*args), memo_obj| ... }
>>>     e.with_object(obj)
>>>
>>>     From Ruby 1.9.1
>>> ------------------------------------------------------------------------
>>>     Iterates the given block for each element with an arbitrary object
>>>     given, and returns the initially given object.
>>>
>>>     If no block is given, returns an enumerator.
>>
>> That's pretty much what it does. It's kind of like inject, except it
>> uses the same object each time through instead of assigning the
>> accumulator slot to the value of the block. Thus:
>>
>>    array = [1,2,3,4,5]
>>    tens_hash = array.each_with_object({}) {|n,obj| obj[n] = n * 10 }
>>      # => {1=>10, 2=>20, 3=>30, 4=>40, 5=>50}
>>
>> It saves you having to do that awkward:
>>
>>    {|h,n| h[n] = n * 10; h }
>>
>> thing to feed the object back into the loop.
>>
>> Without a block, it returns an enumerator, which will then do a
>> "with"-style double iteration (like with_index) based on whatever you
>> call on it:
>>
>>>> array = [1,2,3,4,5]
>> => [1, 2, 3, 4, 5]
>>>> enum =  array.each_with_object({})
>> => #<Enumerator:0x1f525c>
>>>> tens_hash = enum.each {|n,obj| obj[n] = n * 10 }
>> => {1=>10, 2=>20, 3=>30, 4=>40, 5=>50}
>>
>>
>> David
>>>
>
>
> Thanks for the response.
>
> In the definition:
>
>> e.with_object(obj) {|(*args), memo_obj| ... }
>
> 1) Why *args?
>
> 2) Why (*args) ?

I read it as just shorthand for "miscellaneous other arguments". I
don't know whether there's a better way to denote that.

> In the definition:
>
>> e.with_object(obj) {|(*args), memo_obj| ... }
>
> 3) Why 'memo_obj' and not 'obj'?
>
> Even though two different variable names makes it hard to track what is
> going on, the following assignment takes place:
>
> memo_obj = obj

I think it's schematic. obj is not necessarily the variable "obj";
it's just an object:

   e.with_object(<some object here>)

while memo_obj indicates that the last parameter will bind to that
object (and could serve as a variable name).


David

-- 
David A. Black, Director
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