From: Jam Bees <jam@...>
Date: 2012-09-20T12:43:42+09:00
Subject: Re: the behavior of iterator methods

Some methods change objects in place, others return a changed object and leave the original in place.

 The bang is placed by convention at the end of methods that have some unusual behavior; most often, this unusual behavior is that a data structure is passed in by reference not by value. 

On Sep 19, 2012, at 9:32 PM, ten ten <lists@ruby-forum.com> wrote:

> Hi all,
> 
> Sorry, I deleted my previous post on the ruby forum by mistake.
> So I post it again.
> 
> 
> 
> I am a Ruby-beginner.
> I have some experience of C/C++ for several years.
> It has been only one week since I started to learn Ruby.
> Please tell me about the behavior of iterator methods.
> 
> At first, I did as bellow on the irb environment.
> -------------------------------------------------------
> irb(main):001:0> s = ["a", "b", "c"]
> => ["a", "b", "c"]
> irb(main):002:0> s[0].object_id
> => 36971304
> irb(main):003:0> s[1].object_id
> => 36971292
> irb(main):004:0> s[2].object_id
> => 36971280
> irb(main):005:0> s.each{|c| c.upcase!}
> => ["A", "B", "C"]
> irb(main):006:0> p s
> ["A", "B", "C"]
> => nil
> -------------------------------------------------------
> 
> Looking at this, I thought I can change the value of each element in
> Array object through iterator methods.
> 
> However, when I did next as bellow, that behavior looked different.
> -------------------------------------------------------
> irb(main):007:0> a = [1,2,3]
> => [1, 2, 3]
> irb(main):008:0> a[0].object_id
> => 3
> irb(main):009:0> a[1].object_id
> => 5
> irb(main):010:0> a[2].object_id
> => 7
> irb(main):011:0> a.each{|i| i += 1}
> => [1, 2, 3]
> irb(main):012:0> p a
> [1, 2, 3]
> => nil
> 
> irb(main):013:0> a.each{|i| p i.object_id}
> 3
> 5
> 7
> => [1, 2, 3]
> -------------------------------------------------------
> 
> I expected that Array#each method with the code block would change
> contents of the Array object variable named 'a' into [2,3,4].
> But it didn't. Why?
> 
> The object IDs of a[0], a[1], a[2] are shown through
> the the code block of Array#each. So I thought if variable named 'i'
> changed its value, that had to be reflected to the Array object 'a'.
> But it didn't.
> 
> 
> Why did this difference happened?
> 
> -- 
> Posted via http://www.ruby-forum.com/.
>