# Creating Hash of Arrays

**URL:** <https://www.ruby-forum.com/t/creating-hash-of-arrays/61300>\
**Category:** Ruby\
**Created:** [May 22, 2006, 10:30pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300 "2006-05-22T22:30:58Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:30pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/1 "2006-05-22T22:30:58Z")

</div>

How can I create a hash that contains an array for its value? How can I  
then  
loop through the hash keys and intern the contained arrays.

## Pseudo Code

myhash[‘customercode’][0]=Firstname  
myhash[‘customercode’][1]=M.I  
myhash[‘customercode’][2]=Lastname

myhash.each do |k,v|  
myhash[‘k’].each do |elm|  
puts “#{k} -\> elm”  
end  
end

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:38pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/2 "2006-05-22T22:38:05Z")

</div>

myhash[‘customercode’] = Array.new

> myhash[‘customercode’][0]=Firstname  
> myhash[‘customercode’][1]=M.I  
> myhash[‘customercode’][2]=Lastname
> 
> myhash.each do |el|  
> el.each do |elm|  
> puts “#{k} -\> elm”  
> end  
> end

Should work. Also, if you want to have every item of the hash default  
to an array, something like this to construct the Hash is what you  
want:

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

Douglas

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:38pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/3 "2006-05-22T22:38:14Z")

</div>

Paul D. Kraus schrieb:

> myhash.each do |k,v|  
> myhash[‘k’].each do |elm|  
> puts “#{k} -\> elm”  
> end  
> end

## ruby code:

myhash = Hash.new{|h, k| h[k] = [nil] \* 3}

myhash[‘customercode’][0]=‘Firstname’  
myhash[‘customercode’][1]=‘M.I’  
myhash[‘customercode’][2]=‘Lastname’

myhash.each do |k,v|  
myhash[k].each do |elm|  
puts “#{k} -\> #{elm}”  
end  
end

## output:

customercode -\> Firstname  
customercode -\> M.I  
customercode -\> Lastname

does that help?

Simon

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:42pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/4 "2006-05-22T22:42:15Z")

</div>

On May 22, 2006, at 4:28 PM, Paul D. Kraus wrote:

> myhash.each do |k,v|  
> myhash[‘k’].each do |elm|  
> puts “#{k} -\> elm”  
> end  
> end

hsh = Hash.new { |this\_hash, key\_that\_didnt\_exist| this\_hash  
[key\_that\_didnt\_exist] = [] }

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:45pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/5 "2006-05-22T22:45:59Z")

</div>

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

Newbie here but what exactly does this like do.  
Can you someone break it down for me thanks.

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:53pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/6 "2006-05-22T22:53:05Z")

</div>

On 5/22/06, Paul D. Kraus [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > h = Hash.new { |hash, key| hash[key] = Array.new }
> 
> Newbie here but what exactly does this like do.  
> Can you someone break it down for me thanks.

This creates a new hash with a “default block” which will be called  
whenever someone tries to get a value for a non-existent key. When the  
block is called a new Array is created for the given key value and  
stored in the hash. Therefore you can do things like the following and  
they will just magically work:

h[:people] \<\< “Paul”  
h[:people] \<\< “Douglas”  
h[:people] \<\< “Simon”  
h[:animals] \<\< “Dog”  
h[:animals] \<\< “Cat”  
h[:animals] \<\< “Armadillo”

Ryan

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 22, 2006, 10:56pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/7 "2006-05-22T22:56:31Z")

</div>

2006/5/22, Paul D. Kraus [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> > h = Hash.new { |hash, key| hash[key] = Array.new }
> 
> Newbie here but what exactly does this like do.  
> Can you someone break it down for me thanks.

From the docs: [class Hash - RDoc Documentation](http://www.ruby-doc.org/core/classes/Hash.html#M000886)

If you try and get the value using a hash key which does not exist,  
then the value of the block when called with the hash and the new key  
will be returned instead.

Douglas

---

<div class="post-metadata">

**Author:** ![Paul\_DSKraus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ed655f/32.png) [@Paul\_DSKraus](https://www.ruby-forum.com/u/Paul_DSKraus)\
**Post date:** [May 23, 2006, 6:06pm UTC](https://www.ruby-forum.com/t/creating-hash-of-arrays/61300/8 "2006-05-23T18:06:13Z")

</div>

2006/5/22, Paul D. Kraus [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> How can I create a hash that contains an array for its value? How can I then  
> loop through the hash keys and intern the contained arrays.
> 
> ## Pseudo Code
> 
> myhash[‘customercode’][0]=Firstname  
> myhash[‘customercode’][1]=M.I  
> myhash[‘customercode’][2]=Lastname

Another solution picking up Daniel’s suggestion:

> > Item = Struct.new :first\_name, :mi, :last\_name  
> > =\> Item  
> > myhash = Hash.new {|h,k| h[k]=Item.new}  
> > =\> {}  
> > myhash[‘code’].first\_name = “first”  
> > =\> “first”  
> > myhash[‘code’].mi = “mi”  
> > =\> “mi”  
> > myhash[‘code’].last\_name = “last”  
> > =\> “last”  
> > myhash[‘code’]  
> > =\> #\<struct Item first\_name=“first”, mi=“mi”, last\_name=“last”\>

> myhash.each do |k,v|  
> myhash[‘k’].each do |elm|  
> puts “#{k} → elm”  
> end  
> end

You’re making things more complicated than necessary. This will work  
(with you array approach and with my suggestion):

> > myhash.each do |k, v|  
> > ?\> v.each {|elm| puts “#{k} → #{elm}” }  
> > end  
> > code → first  
> > code → mi  
> > code → last  
> > =\> {“code”=\>#\<struct Item first\_name=“first”, mi=“mi”,  
> > last\_name=“last”\>}

alternative

> > myhash.each {|k,it| it.each {|val| print k, " → ", val, “\n”} }  
> > code → first  
> > code → mi  
> > code → last  
> > =\> {“code”=\>#\<struct Item first\_name=“first”, mi=“mi”,  
> > last\_name=“last”\>}

Btw, also you use ‘k’ as a key in your iteration - you must remove the  
single quotes to make it work the way you want.

Kind regards

robert