# Strange behaviour with array of arrays

**URL:** <https://www.ruby-forum.com/t/strange-behaviour-with-array-of-arrays/80993>\
**Category:** Ruby\
**Created:** [January 2, 2007, 10:16pm UTC](https://www.ruby-forum.com/t/strange-behaviour-with-array-of-arrays/80993 "2007-01-02T22:16:11Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\
**Post date:** [January 2, 2007, 10:16pm UTC](https://www.ruby-forum.com/t/strange-behaviour-with-array-of-arrays/80993/1 "2007-01-02T22:16:11Z")

</div>

Please observe the following code:

# Create an array of arrays by explicitly stating values

test1 = [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]  
=\> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

# Create an array of arrays using new method

test2 = Array.new(3, Array.new(3))  
=\> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

#Both arrays appear to be equal  
test1 == test2 =\> true

# Modifying one of values behaves as expected

test1[1][1] = true =\> true  
test1 =\> [[nil, nil, nil], [nil, true, nil], [nil, nil, nil]]

# Modifying one of values behaves strangely

test2[1][1] = true =\> true  
test2 =\> [[nil, true, nil], [nil, true, nil], [nil, true, nil]]

Can anyone explain why this is happening as I don’t understand why the  
two array should behave differently? This is assuming this isn’t a  
bug.

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<div class="post-metadata">

**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\
**Post date:** [January 2, 2007, 10:22pm UTC](https://www.ruby-forum.com/t/strange-behaviour-with-array-of-arrays/80993/2 "2007-01-02T22:22:13Z")

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On Jan 2, 2007, at 3:14 PM, Dan Stevens (IAmAI) wrote:

> #Both arrays appear to be equal  
> Can anyone explain why this is happening as I don’t understand why the  
> two array should behave differently? This is assuming this isn’t a  
> bug.

Let me see if I can get Ruby to explain this for you:

> > a = [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]  
> > =\> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]
> 
> > a.map { |e| e.object\_id }  
> > =\> [3797062, 3797052, 3797042]
> 
> > a = Array.new(3, Array.new(3))  
> > =\> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]
> 
> > a.map { |e| e.object\_id }  
> > =\> [3779002, 3779002, 3779002]
> 
> > a = Array.new(3) { Array.new(3) }  
> > =\> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]
> 
> > a.map { |e| e.object\_id }  
> > =\> [3760242, 3760232, 3760222]

As you can see, the new(n, obj) constructor uses the same object over  
and over again. You want the block form (shown last) instead.

James Edward G. II

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<div class="post-metadata">

**Author:** ![Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ad7895/32.png) [@Dan\_Stevens\_SIAmAI](https://www.ruby-forum.com/u/Dan_Stevens_SIAmAI)\
**Post date:** [January 2, 2007, 10:24pm UTC](https://www.ruby-forum.com/t/strange-behaviour-with-array-of-arrays/80993/3 "2007-01-02T22:24:14Z")

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On 1/2/07, Dan Stevens (IAmAI) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> #Both arrays appear to be equal  
> Can anyone explain why this is happening as I don’t understand why the  
> two array should behave differently? This is assuming this isn’t a  
> bug.

The syntax you want is this …

test2 = Array.new(3) {Array.new(3)}

This will create a new array of three elements and then create a a new  
array object for each element of test2

test2 = Array.new(3, Array.new(3))

This will create a new array of three elements but use the same object  
for element of test2

Blessings,  
TwP