# About 'dup'

**URL:** <https://www.ruby-forum.com/t/about-dup/170100>\
**Category:** Ruby\
**Created:** [June 12, 2009, 9:41am UTC](https://www.ruby-forum.com/t/about-dup/170100 "2009-06-12T09:41:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![kyoichi886](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kyoichi886](https://www.ruby-forum.com/u/kyoichi886)\
**Post date:** [June 12, 2009, 9:41am UTC](https://www.ruby-forum.com/t/about-dup/170100/1 "2009-06-12T09:41:25Z")

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Hello,

mainarray=Hash.new(0)  
subarray=Hash.new(0)

thirdarray={  
‘a’ =\> ‘0’,  
‘b’ =\> ‘0’,  
‘c’ =\> ‘0’,  
}

subarray[‘first’]=thirdarray.dup  
subarray[‘second’]=thirdarray.dup

mainarray[‘x’]=subarray.dup  
mainarray[‘y’]=subarray.dup

mainarray[‘x’][‘first’][‘c’]=56  
mainarray[‘y’][‘first’][‘c’]=73  
p mainarray

the result is:  
{“x”=\>{“second”=\>{“a”=\>“0”, “b”=\>“0”, “c”=\>“0”}, “first”=\>{“a”=\>“0”,  
“b”=\>“0”, “c”=\>73}},  
“y”=\>{“second”=\>{“a”=\>“0”, “b”=\>“0”, “c”=\>“0”}, “first”=\>{“a”=\>“0”,  
“b”=\>“0”, “c”=\>73}}}

Why did the mainarray[‘x’][‘first’][‘c’] become 73,too?  
dup does not work if I use it twice?

---

<div class="post-metadata">

**Author:** ![kyoichi886](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kyoichi886](https://www.ruby-forum.com/u/kyoichi886)\
**Post date:** [June 12, 2009, 10:04am UTC](https://www.ruby-forum.com/t/about-dup/170100/2 "2009-06-12T10:04:40Z")

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On Friday 12 June 2009, Wu Xiaoyi wrote:

> |  
> |  
> |the result is:  
> |{“x”=\>{“second”=\>{“a”=\>“0”, “b”=\>“0”, “c”=\>“0”}, “first”=\>{“a”=\>“0”,  
> |“b”=\>“0”, “c”=\>73}},  
> | “y”=\>{“second”=\>{“a”=\>“0”, “b”=\>“0”, “c”=\>“0”}, “first”=\>{“a”=\>“0”,  
> |“b”=\>“0”, “c”=\>73}}}  
> |  
> |Why did the mainarray[‘x’][‘first’][‘c’] become 73,too?  
> |dup does not work if I use it twice?

The problem is that dup only does a shallow copy of the object. In your  
case,  
this means it creates a new hash but fills it with the same contents of  
the  
original one. This means that mainarray[‘x’][‘first’] contains the same  
object  
as mainarray[‘y’][‘first’] (you can see this by checking the object\_id  
of the  
two). If you want the hashes in mainarray[‘x’] and mainarray[‘y’] to  
have  
different contents, you need to do a deep copy of subarray. The simpler  
way to  
do this is to use Marshal.dup and Marshal.load:

mainarray[‘x’] = Marshal.load(Marshal.dump(subarray))  
mainarray[‘y’] = Marshal.load(Marshal.dump(subarray))

Of course, this will fail if subarray contains an object which can’t be  
marshalled (see the documentation for the Marshal module to see which  
they  
are). In this case you’ll need to define a more specific method to do  
this.

I hope this helps

Stefano

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<div class="post-metadata">

**Author:** ![kyoichi886](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kyoichi886](https://www.ruby-forum.com/u/kyoichi886)\
**Post date:** [June 12, 2009, 10:25am UTC](https://www.ruby-forum.com/t/about-dup/170100/3 "2009-06-12T10:25:29Z")

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Stefano C. wrote:

> On Friday 12 June 2009, Wu Xiaoyi wrote:  
> I hope this helps  
> Stefano

Hello,Stefano:  
It really worked!Thank you very much!!  
It’s very kind of you to offer to help me.I really appreciate you!!