From: Gennady Bystritsky <gennady-ruby@...>
Date: 2012-09-20T12:43:14+09:00
Subject: Re: behavior of iterator methods

On Sep 19, 2012, at 8:20 PM, takanobu maekawa wrote:

> Hi all,
> 
> 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?

Just because you presented 2 totally different cases: in the first one you deal with strings, while in the second you have Fixnums. If you re-wrote your first case with iterators, you would realize that everything is exactly the same as with the direct modification:

s = [ 'a', 'b', 'c' ]
s.each do |item|
  item.upcase!
end

However, when you work with Fixnum literals, several things needs to be kept in mind:

- Fixnums are immutable, there's no bang (!) methods to change them in-place like you can do for strings with update!()

- Construct i += 1 in ruby is a syntax sugar for i = i + 1, so the result is a completely different Fixnum instance. Same is true for strings: 

s = "a"
s += "a"

After the above s will be pointing to another instance of String class with value "aa", while

s << 'a'

will be pointing to the same instance of String with updated value 'aa'

Hope this makes sense to you,
Gennady.