# Appending an array to a frozen array

**URL:** <https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838>\
**Category:** Ruby\
**Created:** [March 25, 2009, 11:34am UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838 "2009-03-25T11:34:02Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![kofboss](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/d2c977/32.png) [@kofboss](https://www.ruby-forum.com/u/kofboss)\
**Post date:** [March 25, 2009, 11:34am UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838/1 "2009-03-25T11:34:02Z")

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Morning all,

@dependencies.freeze  
@dependencies += [“blah”, “blah”]

does not error out due to modifying a frozen array (1.8.7 p72).

Is this expected behaviour? Up until now I’ve considered freezing an  
array as making the array completely read only.

Cheers,  
James

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**Author:** ![kofboss](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/d2c977/32.png) [@kofboss](https://www.ruby-forum.com/u/kofboss)\
**Post date:** [March 25, 2009, 11:52am UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838/2 "2009-03-25T11:52:12Z")

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On Wed, Mar 25, 2009 at 4:00 PM, James F.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Morning all,
> 
> @dependencies.freeze  
> @dependencies += [“blah”, “blah”]
> 
> does not error out due to modifying a frozen array (1.8.7 p72).  
> […]

+= creates a new array and assigns it to @dependencies. The original  
array remains frozen but @dependencies no longer refers to it.

To illustrate:  
$: irb  
01\> a = b = [1, 2]  
 → [1, 2]  
02\> a.freeze  
 → [1, 2]  
03\> b.frozen?  
 → true  
04\> b += [3, 4]  
 → [1, 2, 3, 4]  
05\> a.frozen?  
 → true

solidarity,  
lasitha

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**Author:** ![kofboss](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/d2c977/32.png) [@kofboss](https://www.ruby-forum.com/u/kofboss)\
**Post date:** [March 25, 2009, 12:06pm UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838/3 "2009-03-25T12:06:16Z")

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Ah, I understand - thanks. My next question is - is it possible to  
prevent this?

* * *

From: lasitha [removed\_email\_address@domain.invalid]  
Sent: 25 March 2009 10:48  
To: ruby-talk ML  
Subject: Re: appending an array to a frozen array

On Wed, Mar 25, 2009 at 4:00 PM, James F.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Morning all,
> 
> @dependencies.freeze  
> @dependencies += [“blah”, “blah”]
> 
> does not error out due to modifying a frozen array (1.8.7 p72).  
> […]

+= creates a new array and assigns it to @dependencies. The original  
array remains frozen but @dependencies no longer refers to it.

To illustrate:  
$: irb  
01\> a = b = [1, 2]  
 → [1, 2]  
02\> a.freeze  
 → [1, 2]  
03\> b.frozen?  
 → true  
04\> b += [3, 4]  
 → [1, 2, 3, 4]  
05\> a.frozen?  
 → true

solidarity,  
lasitha

---

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**Author:** ![kofboss](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/d2c977/32.png) [@kofboss](https://www.ruby-forum.com/u/kofboss)\
**Post date:** [March 25, 2009, 5:54pm UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838/4 "2009-03-25T17:54:47Z")

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James F. wrote:

> Ah, I understand - thanks. My next question is - is it possible to prevent this?  
> …
> 
> > @dependencies.freeze  
> > @dependencies += [“blah”, “blah”]
> > 
> > does not error out due to modifying a frozen array (1.8.7 p72).

Use an attr\_reader rather than an instance variable?

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**Author:** ![kofboss](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/d2c977/32.png) [@kofboss](https://www.ruby-forum.com/u/kofboss)\
**Post date:** [March 25, 2009, 5:56pm UTC](https://www.ruby-forum.com/t/appending-an-array-to-a-frozen-array/163838/5 "2009-03-25T17:56:33Z")

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2009/3/25 James F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> Ah, I understand - thanks. My next question is - is it possible to prevent this?

Since you are using an instance variable you control how it is  
accessed. So, yes. If you want to know whether there is a hard way to  
prevent reassignment to instance variables, the only safe way I know  
off the top of my head is to freeze the owner:

irb(main):001:0\> o=Object.new  
=\> #Object:0x10171ed8  
irb(main):002:0\> o.instance\_variable\_set “@foo”, 1  
=\> 1  
irb(main):003:0\> o  
=\> #\<Object:0x10171ed8 @foo=1\>  
irb(main):004:0\> o.freeze  
=\> #\<Object:0x10171ed8 @foo=1\>  
irb(main):005:0\> o.instance\_variable\_set “@foo”, 2  
RuntimeError: can’t modify frozen object  
from (irb):5:in `instance_variable_set' from (irb):5 from /opt/bin/irb19:12:in `’  
irb(main):006:0\>

Kind regards

robert