From: Eric Mahurin <eric_mahurin@...>
Date: 2005-05-12T08:01:14+09:00
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
> > original object (collection or int).
> 
> I think the return values are documented in ri for all of
> these.

Enumerable says nothing about the return value or each. 
Array.each says it returns an Array, but not what's in that
Array.  Before testing, I was thinking it might be the same as
map/collect.  The integer loops also say they return an
integer, but not what it is.

> > 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.
> 
> On the other hand, if you were to do this (looking for nil):
> 
>    obj = array.each {|e| break(e) if e.nil? }
> 
> and each returned nil, you wouldn't know whether you'd
> succeeded or
> not.

Then you do it another way.  Differentiating between the
element nil in a collection and nil meaning nothing is a common
problem, I think.  You have the same problem using find:

obj = array.find{|e| not e.kind_of?Integer }

If you were looking for the first non-integer in array and it
happened to be nil, you couldn't tell whether nil was that
object or there were no non-integers.

> The various find and include? and any? and index methods are
> probably
> your best bet for this kind of operation.

In the particular code I was looking at, none of them worked (I
think I looked at all of them).  I needed a different compare
(equal?) and in one case I wanted to go in reverse order.



		
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