From: Robert Klemme <bob.news@...>
Date: 2005-05-12T06:05:45+09:00
Subject: Re: object loops and what they return


"Eric Mahurin" <eric_mahurin@yahoo.com> schrieb im Newsbeitrag 
news:20050511195851.99230.qmail@web41121.mail.yahoo.com...
> 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

> 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

Kind regards

    robert