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.
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
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.
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.
···
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
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.