# Bug or not a bug? array\*=int **URL:** \ **Category:** Ruby\ **Created:** [October 29, 2008, 4:31pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059 "2008-10-29T16:31:35Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 29, 2008, 4:31pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/1 "2008-10-29T16:31:35Z")
So I’m wondering, is this a bug, or not a bug, or a type-o in the docs? According to [class Array - RDoc Documentation](http://www.ruby-doc.org/core/classes/Array.html) array\*int “[R]eturns a new array built by concatenating the int copies of self.” Copies should be brand new independent copies, right? foo=[[nil,nil,nil]]returns a new array built by concatenating the int copies of self. foo\*=3 #Foo is now. #[[nil,nil,nil], # [nil,nil,nil], # [nil,nil,nil]] foo[1][1]=12 #Foo is now #[[nil,12,nil], # [nil,12,nil], # [nil,12,nil]] #But I would have expected it to be #[[nil,nil,nil], # [nil,12,nil], # [nil,nil,nil]] ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 29, 2008, 5:13pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/2 "2008-10-29T17:13:54Z")
On Wed, Oct 29, 2008 at 8:30 AM, Kyle S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > So I’m wondering, is this a bug, or not a bug, or a type-o in the docs? > > According to [class Array - RDoc Documentation](http://www.ruby-doc.org/core/classes/Array.html) > array\*int “[R]eturns a new array built by concatenating the int > copies of self.” > Copies should be brand new independent copies, right? No, copies are shallow copies. Consider, for example irb(main):002:0\> class A; attr\_accessor :a; end =\> nil irb(main):003:0\> a = A.new =\> #\ irb(main):004:0\> a.a = 10 =\> 10 irb(main):005:0\> foo = [a] =\> [#\] irb(main):006:0\> foo \*= 3 =\> [#\, #\, #\] irb(main):007:0\> a.a = 20 =\> 20 irb(main):008:0\> foo =\> [#\, #\, #\] martin ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 29, 2008, 5:29pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/3 "2008-10-29T17:29:08Z")
Ahh. And upon not being lazy and looking at the C, I see this… ary2 = ary\_new(rb\_obj\_class(ary), len); Which I’m guessing is equivalent to this in ruby (which I already knew would return the same array len times) ary2=Array.new(ary,len) Any chance the document maintainer could slip the word “shallow” into the description of array\*? 🙂 ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 29, 2008, 6:38pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/4 "2008-10-29T18:38:42Z")
Robert K. wrote: > deep copy is more complex … and in many cases impossible (e.g. open files/sockets; procs/blocks; any object with a singleton class) ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 29, 2008, 5:55pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/5 "2008-10-29T17:55:32Z")
On 29.10.2008 17:26, Kyle S. wrote: > Ahh. > And upon not being lazy and looking at the C, I see this… > ary2 = ary\_new(rb\_obj\_class(ary), len); > > Which I’m guessing is equivalent to this in ruby (which I already knew > would return the same array len times) > ary2=Array.new(ary,len) > > Any chance the document maintainer could slip the word “shallow” into > the description of array\*? Dunno. But all (or almost all) copies (e.g. dup, clone, Enumerable#map, Enumerable#select) do shallow copying only. So shallow is the rule and not particularly exceptional - as it is also more efficient and deep copy is more complex (i.e. deciding when to create a new object or just copy the reference). There are quite a few discussions of this topic in the archives… For deep copies the idiom probably most often used is copy = Marshal.load(Marshal.dump(obj)) Kind regards robert ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 30, 2008, 7:56pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/6 "2008-10-30T19:56:09Z")
On Thu, Oct 30, 2008 at 7:22 AM, Stefan R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > As mentioned by others, shallow is the default (also to my knowledge in > most OO languages). > The moment you wish for deep copy is usually the moment you should > consider writing a proper class instead of deeply nesting > arrays/hashes/… > > Regards > Stefan Stefan, while I agree that in general you should write a proper class, the case I was talking about is a completely valid and normal use of multi-dimensional arrays in ruby. I’m guessing that duplicating a row in an array is not a rare occurrence either. I was just saying that maybe a mention of this should be added to the standard docs, as it’s a common pitfall. Bits are cheap 🙂 common pitfalls should be noted all over the place in the docs. –Kyle ---
**Author:** ![kyle\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/f6c823/32.png) [@kyle\_s](https://www.ruby-forum.com/u/kyle_s)\ **Post date:** [October 30, 2008, 1:23pm UTC](https://www.ruby-forum.com/t/bug-or-not-a-bug-array-int/152059/7 "2008-10-30T13:23:50Z")
Kyle S. wrote: > Ahh. > And upon not being lazy and looking at the C, I see this… > ary2 = ary\_new(rb\_obj\_class(ary), len); > > Which I’m guessing is equivalent to this in ruby (which I already knew > would return the same array len times) > ary2=Array.new(ary,len) > > Any chance the document maintainer could slip the word “shallow” into > the description of array\*? > > 🙂 As mentioned by others, shallow is the default (also to my knowledge in most OO languages). The moment you wish for deep copy is usually the moment you should consider writing a proper class instead of deeply nesting arrays/hashes/… Regards Stefan