# What is wrong with this few lines of code

**URL:** <https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458>\
**Category:** Ruby\
**Created:** [April 23, 2010, 2:49pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458 "2010-04-23T14:49:07Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 2:49pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/1 "2010-04-23T14:49:07Z")

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ar = Array.new( 2,[])  
%w(a b).each do |e|  
ar[0] \<\< e  
ar[1] \<\< e  
end  
ar.each do |e|  
e.each {|line| p line}  
end

Returns  
“a”  
“a”  
“b”  
“b”  
“a”  
“a”  
“b”  
“b”

I would expect:  
“a”  
“b”  
“a”  
“b”

by  
TheR

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**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 3:27pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/2 "2010-04-23T15:27:46Z")

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Hi Damjan

The code ar = Array.new(n, []) actually makes n copies of the same  
object  
(empty array), so each time you do either ar[0] \<\< e or ar[1] \<\< e you  
are  
appending elements to the same array.

What I guess you want is to different objects, and you can do that by  
e.g.  
ar = Array.new(2){[]}  
(or just ar = [[],[]] …or many other ways :o) )

Best regards,  
Rolf

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<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 3:35pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/3 "2010-04-23T15:35:22Z")

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At 2010-04-23 08:49AM, “Damjan R.” wrote:

> ar = Array.new( 2,[])

ar[0] and ar[1] both refer to the same (empty) array at this point

```
ar = Array.new(2, []) # => [[], []]
ar[0].object_id # => 136474520
ar[1].object_id # => 136474520

```

You want the block syntax for Array.new

```
ar = Array.new(2) {[]} # => [[], []]
ar[0].object_id # => 136514378
ar[1].object_id # => 136514364
```

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<div class="post-metadata">

**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 4:20pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/4 "2010-04-23T16:20:28Z")

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Hi,

Am 23.04.2010 14:49, schrieb Damjan R.:

> Returns  
> “a”  
> “b”  
> “a”  
> “b”

works as expected with ruby 1.9.1p376 (2009-12-07 revision 26041)  
[i386-cygwin].

Kind regards

Waldemar

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**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 4:38pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/5 "2010-04-23T16:38:02Z")

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2010/4/23 Rolf P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> The code ar = Array.new(n, []) actually makes n copies of the same object  
> (empty array), so each time you do either ar[0] \<\< e or ar[1] \<\< e you are  
> appending elements to the same array.

That description is inconsistent and might confuse one or the other  
reader: there are no copies made of the _object_ (Array in this case)  
but only of the _reference_!

irb(main):001:0\> a = Array.new(3,[])  
=\> [[], [], []]  
irb(main):002:0\> a.map {|x| x.object\_id}  
=\> [135000124, 135000124, 135000124]  
irb(main):003:0\> a.map {|x| x.object\_id}.uniq  
=\> [135000124]

The idiom you describe when you say “copy” is the block form of  
Arra.new:

irb(main):004:0\> a = Array.new(3) { [] }  
=\> [[], [], []]  
irb(main):005:0\> a.map {|x| x.object\_id}  
=\> [135669212, 135669198, 135669184]  
irb(main):006:0\> a.map {|x| x.object\_id}.uniq  
=\> [135669212, 135669198, 135669184]

> What I guess you want is to different objects, and you can do that by e.g.  
> ar = Array.new(2){[]}  
> (or just ar = [[],[]] …or many other ways :o) )

Exactly.

Kind regards

robert

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**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 4:50pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/6 "2010-04-23T16:50:20Z")

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Hi –

On Fri, 23 Apr 2010, Waldemar Dick wrote:

> > e.each {|line| p line}  
> > “b”
> > 
> > I would expect:  
> > “a”  
> > “b”  
> > “a”  
> > “b”
> 
> works as expected with ruby 1.9.1p376 (2009-12-07 revision 26041)  
> [i386-cygwin].

Are you saying that you’re getting the second result from exactly the  
same code? (It should give the first result.)

David

–  
David A. Black, Senior Developer, Cyrus Innovation Inc.

THE Ruby training with Black/Brown/McAnally  
COMPLEAT Coming to Chicago area, June 18-19, 2010!  
RUBYIST [http://www.compleatrubyist.com](http://www.compleatrubyist.com)

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**Author:** ![ther](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@ther](https://www.ruby-forum.com/u/ther)\
**Post date:** [April 23, 2010, 5:13pm UTC](https://www.ruby-forum.com/t/what-is-wrong-with-this-few-lines-of-code/187458/7 "2010-04-23T17:13:45Z")

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Hi,

Am 23.04.2010 16:47, schrieb David A. Black:  
[…]

> Are you saying that you’re getting the second result from exactly the  
> same code? (It should give the first result.)

sorry, I must have missed the second assignment (ar[1]\<\< e), so I got  
the wrong result.

Waldemar