# Weird result when updating an array element **URL:** \ **Category:** ruby-talk\ **Created:** [7 September 2005 19:38 UTC](https://rubytalk.org/t/weird-result-when-updating-an-array-element/20670 "2005-09-07T19:38:49Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Nelson\_Owen](https://avatars.discourse-cdn.com/v4/letter/n/958977/32.png) [@Nelson\_Owen](https://rubytalk.org/u/Nelson_Owen)\ **Post date:** [7 September 2005 19:38 UTC](https://rubytalk.org/t/weird-result-when-updating-an-array-element/20670/1 "2005-09-07T19:38:49Z")
Given the code below, only one element in @ver should be changed, however when I execute the code all the elements are changed. Can anyone explain why this is happening? class SomeClass     def initialize         @ver = Array.new(2, {'pos' =\> Array.new(3), 'col' =\> Array.new(4)})         @ver[0]['pos'][0] = 0         puts @ver.inspect     end end SomeClass.new actual result: [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}] expected result: [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[nil, nil, nil]}] ---
**Author:** ![Simon\_Kroger](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Simon\_Kroger](https://rubytalk.org/u/Simon_Kroger)\ **Post date:** [7 September 2005 19:54 UTC](https://rubytalk.org/t/weird-result-when-updating-an-array-element/20670/2 "2005-09-07T19:54:28Z")
Nelson Owen wrote: > Given the code below, only one element in @ver should be changed, however when I execute the code all the elements are changed. Can anyone explain why this is happening? > > class SomeClass >    def initialize >        @ver = Array.new(2, {'pos' =\> Array.new(3), 'col' =\> Array.new(4)}) >        @ver[0]['pos'][0] = 0 >        puts @ver.inspect >    end > end > SomeClass.new > > actual result: > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}] > > expected result: > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[nil, nil, nil]}] from the Array docs: ...it is created with size copies of obj (that is, size references to the same obj). So ver[0] and ver[1] are in fact pointing to the same object. You can use: @ver = Array.new(2) {{'pos' =\> Array.new(3), 'col' =\> Array.new(4)}} wich looks odd, but works 🙂 cheers Simon ---
**Author:** ![Robert](https://avatars.discourse-cdn.com/v4/letter/r/e95f7d/32.png) [@Robert](https://rubytalk.org/u/Robert)\ **Post date:** [7 September 2005 19:56 UTC](https://rubytalk.org/t/weird-result-when-updating-an-array-element/20670/3 "2005-09-07T19:56:29Z")
> Given the code below, only one element in @ver should be changed, > however when I execute the code all the elements are changed. Can > anyone explain why this is happening? > > class SomeClass >    def initialize >        @ver = Array.new(2, {'pos' =\> Array.new(3), 'col' =\> >        Array.new(4)}) @ver[0]['pos'][0] = 0 >        puts @ver.inspect >    end > end > SomeClass.new > > actual result: > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, > nil, nil, nil], "pos"=\>[0, nil, nil]}] > > expected result: > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, > nil, nil, nil], "pos"=\>[nil, nil, nil]}] You are creating a two element array where the same hash is referenced for each array element: > > obj = Object.new =\> #\ > > obj.object\_id =\> 135042152 > > a=Array.new(3,obj) =\> [#\, #\, #\] > > a.map {|o| o.object\_id} =\> [135042152, 135042152, 135042152] > > a.map {|o| o.object\_id}.uniq =\> [135042152] You want the block form of array init: > > a = Array.new(3) { Object.new } =\> [#\, #\, #\] > > a.map {|o| o.object\_id} =\> [134992532, 134992448, 134992376] > > a.map {|o| o.object\_id}.uniq =\> [134992532, 134992448, 134992376] In your case @ver = Array.new(2){ {'pos' =\> Array.new(3), 'col' =\> Array.new(4)} } Kind regards     robert > **···** > > Nelson Owen \ wrote: ---
**Author:** ![Nelson\_Owen](https://avatars.discourse-cdn.com/v4/letter/n/958977/32.png) [@Nelson\_Owen](https://rubytalk.org/u/Nelson_Owen)\ **Post date:** [7 September 2005 20:17 UTC](https://rubytalk.org/t/weird-result-when-updating-an-array-element/20670/4 "2005-09-07T20:17:55Z")
Simon Kröger wrote: > Nelson Owen wrote: > > > Given the code below, only one element in @ver should be changed, however when I execute the code all the elements are changed. Can anyone explain why this is happening? > > > > class SomeClass > >    def initialize > >        @ver = Array.new(2, {'pos' =\> Array.new(3), 'col' =\> Array.new(4)}) > >        @ver[0]['pos'][0] = 0 > >        puts @ver.inspect > >    end > > end > > SomeClass.new > > > > actual result: > > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}] > > > > expected result: > > [{"col"=\>[nil, nil, nil, nil], "pos"=\>[0, nil, nil]}, {"col"=\>[nil, nil, nil, nil], "pos"=\>[nil, nil, nil]}] > > from the Array docs: > > ...it is created with size copies of obj (that is, size references to the same obj). > > So ver[0] and ver[1] are in fact pointing to the same object. > > You can use: > > @ver = Array.new(2) {{'pos' =\> Array.new(3), 'col' =\> Array.new(4)}} > > wich looks odd, but works 🙂 > > cheers > > Simon > > . Ah, I see now. Thanks!