From: Gyoung-Yoon Noh <nohmad@...>
Date: 2005-05-12T07:49:23+09:00
Subject: Re: object loops and what they return

I've posted similar code to this list a few days ago. See ruby-talk:140818
Sorry for duplicating.

 >> [ ].each do |e|
 >> end.any? or "empty!"
#=> "empty!"
 >> [1].each do |e|
 >> end.any? or puts "empty!"
#=> true


On 5/12/05, Gavin Kistner <Gavin.Kistner@refinery.com> wrote:
> Yes. Not often, but occasionally the return value is useful.
> 
> For example, there was a solution like this posted to the rails mailing list, on how to iterate over all the items in a record, or print out a 'no records found' type line if none existed:
> 
> if my_array.each do |item|
>    puts "item"
> end.empty?
>    puts "(no items found)"
> end
> 
> ________________________________
> 
> From: Eric Mahurin [mailto:eric_mahurin@yahoo.com]
> Sent: Wed 5/11/2005 3:31 PM
> To: ruby-talk ML
> Subject: Re: object loops and what they return
> 
> 
> > > Consider these loops:
> > >
> > > <object>.<loop-method> { <code> }
> > >
> > > where loop method is each, each_with_index, upto, downto,
> > step,
> > > and probably others.
> > >
> > > Although it is not documented, all of these look to return
> > the
> > > origninal object (collection or int).  Does anybody find
> > this
> > > useful??  If not, I would propose that these return nil
> > just
> > > like loop, while, until, begin/end while, and begin/end
> > until.
> > > I've never found the return value of these methods useful,
> > but
> > > I have found the the built-in loops returning nil useful.
> > Here
> > > are a couple:
> > >
> > > # find first index where you find the object obj in array
> > > index = array.each_with_index do |i,x|
> > >    break(i) if obj.equal?(x)
> > > end
> > >
> > > # find last index where you find the object obj in array
> > > index = (array.size-1).downto(0) do |i,x|
> > >    break(i) if obj.equal?(x)
> > > end
> >
> > >> a=%w{a b c d e f g ab c}
> > => ["a", "b", "c", "d", "e", "f", "g", "ab", "c"]
> > >> a.index "c"
> > => 2
> > >> a.rindex "c"
> > => 8
> > >> a.index "foo"
> > => nil
> 
> I was wanting to compare the objects with equal? (compares
> object ids) not == (what index/rindex use).
> 
> > > The problem with the above now is that index will be the
> > loop
> > > object (array and array.size-1 from above) when you don't
> > find
> > > the obj.  Instead of the above, I end up using old-style
> > loops
> > > to accomplish what I want.
> > >
> > > With "each" returning nil, you can also see that many of
> > the
> > > derived loops in Enumerable become trival almost to where
> > you
> > > don't need them.
> >
> > Interesting aspect.  I assume the return behavior is from a
> > time where break
> > could not return a value so your constructions weren't
> > possible.
> >
> > Typically I put such functionality into methods and then I
> > use "return" to
> > short circuit:
> >
> > module Enumerable
> >   def find_pos(x)
> >     each_with_index {|e,i| return i if x == e}
> >     nil
> >   end
> >
> >   def find_cond
> >     each_with_index {|e,i| return i if yield e}
> >     nil
> >   end
> > end
> 
> The loops I describe above were in some method.  I didn't want
> to create a new method just for those loops.  That would be
> kind of silly.  I used traditional loops instead.  Also, with
> what I am proposing you wouldn't need the "nil" in the above 2
> methods.
> 
> Does anybody find a use for these loop methods returning the
> original object (undocumented) instead of nil (like other
> loops)?  If not, I would like to make an RCR for this.
> 
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> 


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