What is wrong with this few lines of code

Ruby
1

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

2

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

3

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
4

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

5

2010/4/23 Rolf P. [email protected]:

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

6

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

7

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