# \[\]= and two dimensional arrays

**URL:** <https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380>\
**Category:** Ruby\
**Created:** [June 3, 2006, 3:17pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380 "2006-06-03T15:17:44Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Son\_SonOfLilit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c57346/32.png) [@Son\_SonOfLilit](https://www.ruby-forum.com/u/Son_SonOfLilit)\
**Post date:** [June 3, 2006, 3:17pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380/1 "2006-06-03T15:17:44Z")

</div>

I expect:  
irb(main):002:0\> a = Array.new.fill(Array.new.fill(“x”, 0, 4), 0, 4)  
=\> [[“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”],  
[“x”,  
“x”, “x”, “x”]]  
irb(main):003:0\> p a  
[[“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”,  
“x”, “x”, “x”]]  
=\> nil  
irb(main):004:0\> a[2][3] = “a”  
=\> “a”  
irb(main):005:0\> p a  
[[“x”, “x”, “x”, “a”], [“x”, “x”, “x”, “a”], [“x”, “x”, “x”, “a”], [“x”,  
“x”, “x”, “a”]]  
=\> nil  
irb(main):006:0\>

========================

I get:  
irb(main):002:0\> a = Array.new.fill(Array.new.fill(“x”, 0, 4), 0, 4)  
=\> [[“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”],  
[“x”,  
“x”,  
“x”, “x”]]  
irb(main):003:0\> p a  
[[“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”,  
“x”, "x  
", “x”]]  
=\> nil  
irb(main):004:0\> a[2][3] = “a”  
=\> “a”  
irb(main):005:0\> p a  
[[“x”, “x”, “x”, “a”], [“x”, “x”, “x”, “a”], [“x”, “x”, “x”, “a”], [“x”,  
“x”, "x  
", “a”]]  
=\> nil

=========================

I spend hours debugging and when I find it out, I have no clue how to  
solve  
it.

Could somebody clue me as to a solution?

Thanks,

SonOfLilit

---

<div class="post-metadata">

**Author:** ![Son\_SonOfLilit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c57346/32.png) [@Son\_SonOfLilit](https://www.ruby-forum.com/u/Son_SonOfLilit)\
**Post date:** [June 3, 2006, 3:20pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380/2 "2006-06-03T15:20:51Z")

</div>

Oi, got that all wrong. Correct version:

I expect:

> [[“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”, “x”, “x”, “x”], [“x”,  
> “x”, “x”]]  
> ", “a”]]
> 
> SonOfLilit

---

<div class="post-metadata">

**Author:** ![Son\_SonOfLilit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c57346/32.png) [@Son\_SonOfLilit](https://www.ruby-forum.com/u/Son_SonOfLilit)\
**Post date:** [June 3, 2006, 3:20pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380/3 "2006-06-03T15:20:59Z")

</div>

Oh, solved. Solution:  
a = Array.new.fill(Array.new.fill(“x”, 0, 4).dup, 0, 4)

I was storing references to the same array.

Is there a better idiom for bulding 2d arrays and initializing them to a  
constant value?

---

<div class="post-metadata">

**Author:** ![Son\_SonOfLilit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c57346/32.png) [@Son\_SonOfLilit](https://www.ruby-forum.com/u/Son_SonOfLilit)\
**Post date:** [June 3, 2006, 3:36pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380/4 "2006-06-03T15:36:16Z")

</div>

Son SonOfLilit wrote:

> > irb(main):004:0\> a[2][3] = “a”  
> > irb(main):002:0\> a = Array.new.fill(Array.new.fill(“x”, 0, 4), 0, 4)  
> > irb(main):005:0\> p a  
> > Could somebody clue me as to a solution?  
> > OK, here’s the clue. You only have two arrays. Scroll down for spoilers.

OK, here’s the key.

a = Array.new.fill(Array.new.fill(“x”, 0, 4), 0, 4)

1. create an array (the inner “Array.new”)
2. fill it with 4 "x"s
3. create another array (the outer “Array.new”)
4. fill it with 4 references to the first array

What you want is probably:

a = Array.new(4) { Array.new(4, “x”) }

or

a = Array.new(4) { Array.new(4) { “x” }}

That will create a new array for each of the four elements it  
initializes the outer array with.

The second version will give you 16 strings instead of just 4.

Cheers,  
Dave

---

<div class="post-metadata">

**Author:** ![Son\_SonOfLilit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c57346/32.png) [@Son\_SonOfLilit](https://www.ruby-forum.com/u/Son_SonOfLilit)\
**Post date:** [June 3, 2006, 4:21pm UTC](https://www.ruby-forum.com/t/and-two-dimensional-arrays/62380/5 "2006-06-03T16:21:41Z")

</div>

Thank you very much. Didn’t know I could do a = Array.new(4) {  
Array.new(4)  
{ “x” }} 🙂

Nive to learn something new.

SonOfLilit