From: Ezra Zygmuntowicz <ezmobius@...>
Date: 2007-11-11T06:23:20+09:00
Subject: Re: equivalent injecting implementations?


On Nov 10, 2007, at 7:50 AM, Rick DeNatale wrote:

> On Nov 10, 2007 9:37 AM, Trans <transfire@gmail.com> wrote:
>>
>>
>> On Nov 10, 8:50 am, "Rick DeNatale" <rick.denat...@gmail.com> wrote:
>>> On 11/9/07, Trans <transf...@gmail.com> wrote:
>>>
>>>
>>>
>>>> Are these strictly equivalent? I get the feeling no, but I haven't
>>>> been able to demonstrate it yet.
>>>
>>>>  def injecting(s)
>>>>    inject(s) do |k, i|
>>>>      yield(k, i); k
>>>>    end
>>>>  end
>>>
>>>>  def injecting(res, &block)
>>>>    ([res]*length).zip(to_a).each(&block)
>>>>    res
>>>>  end
>>>
>>>> Thanks,
>>>> T.
>>>
>>> I can't see how in general they can be.
>>>
>>> In which class or module are you defining these?  Since you are  
>>> using
>>> inject and each, I'm guessing that it's either Enumerable or some
>>> subclass,
>>>
>>> But then depending on which subclass you don't have a length  
>>> method so...
>>>
>>> Perhaps some more context would help.
>>
>> Oh sorry, I meant to put that. These are Enumerable methods.
>>
> module Enumerable
>  def injecting(s)
>     inject(s) do |k, i|
>       yield(k, i); k
>     end
>   end
>
>   def injecting2(res, &block)
>     ([res]*length).zip(to_a).each(&block)
>     res
>   end
> end
>
> (1..3).injecting(0) {|k,i| k + i} # => 0
> (1..3).injecting2(0) {|k,i| k + i} # =>
> # ~> -:9:in `injecting2': undefined local variable or method `length'
> for 1..3:Range (NameError)
> # ~> 	from -:15
>
> Which illustrates my point.
>
> Just what is injecting supposed to mean anyway? The first
> implementation uses inject and takes a two argument block like inject
> does, but seems to be going out of it's way to disregard the value of
> the argument block.
>
> This doesn't seem much like inject to me.


injecting is similar to returning in rails. There is a common pattern  
of accumulating a hash via inject that looks like this:

[1,2,3].inject({}) { |memo,n| memo[n] = n ;  memo}
#=> {1=>1, 3=>3,2=>2}

Note the ugly trailing ;memo, this is so the fll accumulated hash is  
the final return result.

injecting is just a little bit nicer way to do the same thing:

[1,2,3].injecting({}) { |memo,n| memo[n] = n }
#=> {1=>1, 3=>3,2=>2}

Cheers-
-Ezra