behavior of iterator methods

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?

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