# Array in a Hash

**URL:** <https://www.ruby-forum.com/t/array-in-a-hash/154959>\
**Category:** Ruby\
**Created:** [December 5, 2008, 4:32pm UTC](https://www.ruby-forum.com/t/array-in-a-hash/154959 "2008-12-05T16:32:49Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![damaris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/2bfe46/32.png) [@damaris](https://www.ruby-forum.com/u/damaris)\
**Post date:** [December 5, 2008, 4:32pm UTC](https://www.ruby-forum.com/t/array-in-a-hash/154959/1 "2008-12-05T16:32:49Z")

</div>

Hi you all,

I have the following code,where I have a hash whose entries are  
initalized by an empty Array:

h = Hash.new(Array.new(0))  
h[“a”] \<\< 0  
h[“b”] \<\< 6  
h[“a”] \<\< 5  
h[“b”] \<\< 10  
p h[“a”]  
p h[“b”]

Both “p” prints the same: [0,6,5,10]  
What am I doing wrong? (I want h[“a”] to be [0,5] and h[“b”]=[6,10]

---

<div class="post-metadata">

**Author:** ![damaris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/2bfe46/32.png) [@damaris](https://www.ruby-forum.com/u/damaris)\
**Post date:** [December 5, 2008, 4:42pm UTC](https://www.ruby-forum.com/t/array-in-a-hash/154959/2 "2008-12-05T16:42:21Z")

</div>

On Dec 5, 2008, at 9:26 AM, Damaris F. wrote:

> I have the following code,where I have a hash whose entries are  
> initalized by an empty Array:
> 
> h = Hash.new(Array.new(0))

h = Hash.new { |hash, key| hash[key] = Array.new }

> h[“a”] \<\< 0  
> h[“b”] \<\< 6  
> h[“a”] \<\< 5  
> h[“b”] \<\< 10  
> p h[“a”]  
> p h[“b”]
> 
> Both “p” prints the same: [0,6,5,10]  
> What am I doing wrong? (I want h[“a”] to be [0,5] and h[“b”]=[6,10]

If you just pass an object to Hash.new() the exact same object is used  
to initialize all entries. That’s okay with something like a Fixnum  
which will have to be replaced, but not intended with a mutable object  
like Array. Switching to the block form as I did above forces Ruby to  
run the code each time, getting us a new Array.

James Edward G. II

---

<div class="post-metadata">

**Author:** ![damaris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/2bfe46/32.png) [@damaris](https://www.ruby-forum.com/u/damaris)\
**Post date:** [December 5, 2008, 4:48pm UTC](https://www.ruby-forum.com/t/array-in-a-hash/154959/3 "2008-12-05T16:48:49Z")

</div>

On Fri, Dec 5, 2008 at 9:26 AM, Damaris F.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> p h[“a”]  
> p h[“b”]
> 
> Both “p” prints the same: [0,6,5,10]  
> What am I doing wrong? (I want h[“a”] to be [0,5] and h[“b”]=[6,10]

That form of Hash.new does not create a different new Array for each  
new hash key, it creates a single Array instance to which all new hash  
keys refer. You might prefer the block form of Hash.new.

irb(main):001:0\> h = Hash.new(Array.new(0))  
=\> {}  
irb(main):002:0\> h[:a] \<\< 0  
=\> [0]  
irb(main):003:0\> h[:b] \<\< 1  
=\> [0, 1]  
irb(main):004:0\> h[:a].object\_id  
=\> 1073560370  
irb(main):005:0\> h[:b].object\_id  
=\> 1073560370  
irb(main):006:0\> g = Hash.new{|hash, key| hash[key] = Array.new(0)}  
=\> {}  
irb(main):007:0\> g[:a] \<\< 0  
=\> [0]  
irb(main):008:0\> g[:b] \<\< 1  
=\> [1]  
irb(main):009:0\> g[:a].object\_id  
=\> 1073475250  
irb(main):010:0\> g[:b].object\_id  
=\> 1073464620  
irb(main):011:0\> g[:a] \<\< 4  
=\> [0, 4]  
irb(main):012:0\> g[:b] \<\< 5  
=\> [1, 5]

-Michael C. Libby