From: Robert Klemme <shortcutter@...>
Date: 2007-05-29T21:25:16+09:00
Subject: Re: Underscore

On 29.05.2007 14:11, Robert Klemme wrote:
> On 29.05.2007 13:59, Chris Carter wrote:
>> On 5/29/07, Robert Klemme <shortcutter@googlemail.com> wrote:
>>> On 29.05.2007 13:07, Jano Svitok wrote:
>>> > On 5/29/07, Jon Harrop <jon@ffconsultancy.com> wrote:
>>> >> Michael Fellinger wrote:
>>> >> > some people use _ as a temporary meaningless variable, just a
>>> >> > throw-away so to say.
>>> >> > In this case something like
>>> >> >
>>> >> > hash = {:a => :b, :c => :d}
>>> >> >
>>> >> > and you are not interested in the :b and :d
>>> >> >
>>> >> > hash.each do |key, _|
>>> >> >   p key
>>> >> > end
>>> >> >
>>> >> > I'm not necessarily a friend of this technique, but it seems 
>>> easy on
>>> >> > the minds of some people.
>>> >>
>>> >> Right, this is exactly what I guessed it was doing (it is the same in
>>> >> SML/OCaml/F#) but what value was being thrown away in the Ruby 
>>> program
>>> >> and
>>> >> where did it come from?
>>> >>
>>> >>   (1..n).inject(x) { |acc, _| yield(acc) }
>>> >>
>>> >> --
>>> >> Dr Jon D Harrop, Flying Frog Consultancy
>>> >> The F#.NET Journal
>>> >> http://www.ffconsultancy.com/products/fsharp_journal/?usenet
>>> >
>>> > inject takes a block with two parameters. classic example is a sum 
>>> of an
>>> > array:
>>> >
>>> > array.inject(0) {|sum, item| sum + item }
>>> >
>>> > so, in this case, item is not needed, so it is replaced by a variable
>>> > with name of "_"
>>> > that by convention means "temporary", "throw away"
>>> >
>>> > it can be anything else:
>>> > (1..n).inject(x) { |acc, i_dont_need_this| yield(acc) }
>>>
>>> Actually, #inject does not really make sense in this case.  All that
>>> happens here is that some value is yielded n times to a block.  That
>>> could have been done much more concise like this:
>>>
>>> n.times { yield x }
>>>
>>> Kind regards
>>>
>>>         robert
>>>
>>>
>>
>> Actually, If it does what I think it does, the inject is needed
>> because as we know, inject sets the value returned from the block as
>> the accumulator for the next round.  Therefore:
>>>> nest(2) {|x| p x; [22] }
>> 2
>> [22]
>> => [22]
> 
> Stupid me.  Of course you are right.  I should have taken more time to 
> digest this - or have more coffee.  Thank you for correcting me!

I was too fast (again).  Even though the return value is used, I'd rather do

n.times { x = yield x }

than using #inject which does more than needed in this case.  :-)

Now, did I look at all aspects...?

Kind regards

	robert