# Nested Arrays in Method Definitions--a puzzle

**URL:** <https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328>\
**Category:** Ruby\
**Created:** [August 20, 2008, 1:20am UTC](https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328 "2008-08-20T01:20:51Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Jonah\_Bloch-Johnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ce7236/32.png) [@Jonah\_Bloch-Johnson](https://www.ruby-forum.com/u/Jonah_Bloch-Johnson)\
**Post date:** [August 20, 2008, 1:20am UTC](https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328/1 "2008-08-20T01:20:51Z")

</div>

Hello there Ruby-minded people!

```
 I am writing because I am somewhat stumped by why the following

```

discrepency occurs; having defined the following:

class Warfare  
def haves  
x = [1]  
y = []  
y \<\< x  
x = [2] # (compare this)  
y  
end  
def havenots  
x = [1]  
y = []  
y \<\< x  
x[0] = 2 # (to this)  
y  
end  
end

I get this in irb:

> > load ‘snah.rb’  
> > =\> true
> 
> > a = Warfare.new  
> > =\> #Warfare:0x33a93c
> 
> > a.haves  
> > =\> [[1]]
> 
> > a.havenots  
> > =\> [[2]]

Why does meddling with the innards of the “x” have retroactive  
repercussions, whereas redefining it outright does not?

Any insight would be greatly appreciated.

-Jonah

---

<div class="post-metadata">

**Author:** ![Jonah\_Bloch-Johnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ce7236/32.png) [@Jonah\_Bloch-Johnson](https://www.ruby-forum.com/u/Jonah_Bloch-Johnson)\
**Post date:** [August 20, 2008, 1:31am UTC](https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328/2 "2008-08-20T01:31:59Z")

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On Tue, Aug 19, 2008 at 4:16 PM, Jonah Bloch-Johnson  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> repercussions, whereas redefining it outright does not?  
> When you say ‘x = [2]’ you aren’t redefining the object x, you are  
> taking the variable ‘x’ and binding it to a new object. When you say y  
> \<\< x, you are inserting the object that x was pointing to at that  
> moment into y, not the variable x. Think of a variable as an alias for  
> an object - what is actually passed around are the objects themselves,  
> but you can refer to them by any of their current aliases.

martin

---

<div class="post-metadata">

**Author:** ![Jonah\_Bloch-Johnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ce7236/32.png) [@Jonah\_Bloch-Johnson](https://www.ruby-forum.com/u/Jonah_Bloch-Johnson)\
**Post date:** [August 20, 2008, 4:43am UTC](https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328/3 "2008-08-20T04:43:15Z")

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Many many many thanks, Adam and Martin! I do think this clears it up.

Quoting A. Shelly [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

---

<div class="post-metadata">

**Author:** ![Jonah\_Bloch-Johnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ce7236/32.png) [@Jonah\_Bloch-Johnson](https://www.ruby-forum.com/u/Jonah_Bloch-Johnson)\
**Post date:** [August 20, 2008, 1:57am UTC](https://www.ruby-forum.com/t/nested-arrays-in-method-definitions-a-puzzle/146328/4 "2008-08-20T01:57:08Z")

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On 8/19/08, Jonah Bloch-Johnson [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I am writing because I am somewhat stumped by why the following discrepency occurs…

Variables and Array entries hold references to objects.

irb(main):001:0\> x=[1]  
=\> [1]  
irb(main):002:0\> x.object\_id  
=\> 20774910

irb(main):003:0\> y=[];y\<\<x  
=\> [[1]]  
irb(main):004:0\> y[0].object\_id  
=\> 20774910 #the same object that x referrs to

irb(main):005:0\> x=[2]  
=\> [2]  
irb(main):006:0\> x.object\_id  
=\> 21222360 #now x refers to the newly created array

irb(main):008:0\> y  
=\> [[1]]  
irb(main):007:0\> y[0].object\_id  
=\> 20774910 #but y still holds a reference to the old one.

irb(main):009:0\> y=[x]  
=\> [[2]]  
irb(main):010:0\> y[0].object\_id  
=\> 21222360 #y[0] is now a reference to the same Array as x

irb(main):011:0\> x[0]=4  
=\> 4  
irb(main):013:0\> y  
=\> [[4]] #so changes to the content of that Array are visible  
through y’s reference.

hope this helps,  
-Adam

> ```
> x = [1]
> 
> ```

[1] is an Array object, x is a variable referring to that array

> ```
> y = []
> 
> ```

y referrs to a 2nd Array

> ```
> y << x
> 
> ```

And now that array contains a reference to

> ```
> x = [2] # (compare this)
> 
> ```

Here you are creating an array, labeling it ‘x’, and storing a  
reference to it in the array labeled y. Then you reassign the ‘x’  
label to yet another array (the one containing 2).