# Strange behaviour with array of arrays
**URL:** https://rubytalk.org/t/strange-behaviour-with-array-of-arrays/34169
**Category:** ruby-talk
**Created:** [2 January 2007 21:14 UTC](https://rubytalk.org/t/strange-behaviour-with-array-of-arrays/34169 "2007-01-02T21:14:54Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Dan\_Stevens\_IAmAI](https://rubytalk.org/u/Dan_Stevens_IAmAI)
#### Post date: [2 January 2007 21:14 UTC](https://rubytalk.org/t/strange-behaviour-with-array-of-arrays/34169/1 "2007-01-02T21:14:54Z")
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.
---
### Author:  [@James\_Edward\_Gray\_II](https://rubytalk.org/u/James_Edward_Gray_II)
#### Post date: [2 January 2007 21:21 UTC](https://rubytalk.org/t/strange-behaviour-with-array-of-arrays/34169/2 "2007-01-02T21:21:21Z")
> 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.
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 Gray II
> **···**
>
> On Jan 2, 2007, at 3:14 PM, Dan Stevens (IAmAI) wrote:
---
### Author:  [@Tim\_Pease](https://rubytalk.org/u/Tim_Pease)
#### Post date: [2 January 2007 21:22 UTC](https://rubytalk.org/t/strange-behaviour-with-array-of-arrays/34169/3 "2007-01-02T21:22:54Z")
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
> **···**
>
> On 1/2/07, Dan Stevens (IAmAI) \ wrote:
>
> > 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.