Order in Hashes


#1

Hello,

I am building a menu structure for rails that I’d like to store in a
simple
Hash.

Now I found out that ruby Hashes do not keep the order, like this
program:

b = {‘upkpgn’=>1,
‘jmay’=>2,
‘vkvxxm’=>3}

b.each_key {|k|
puts “%s => %s” % [k, b[k]]
}

…will output:

upkpgn => 1
vkvxxm => 3
jmay => 2

(vkvxxm and jmay are swapped)

I read an article describing this behavior, but it only mentions a
‘sort’
solution which is useless for me because my menu has a logical order:

http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/159776

Are there any workarounds for this? Or shall I rather write my own
containers?

BTW: Why is ruby doing this anyways…?!

Thanks,
Martin


#2

On Apr 4, 2006, at 7:43 AM, Martin B. wrote:

 'jmay'=>2,

jmay => 2

(vkvxxm and jmay are swapped)

b.keys.sort.each { |k|

}

I read an article describing this behavior, but it only mentions a
‘sort’
solution which is useless for me because my menu has a logical order:

http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/159776

Are there any workarounds for this?

You’ve mentioned the first one, you can sort when you access the values.

You could also switch away from a Hash and use something like an
Array of Arrays. Lookup the method Array#assoc and Array#rassoc,
which might be helpful with this.

Or shall I rather write my own containers?

I’m pretty sure there is an OrderedHash on the RAA.

BTW: Why is ruby doing this anyways…?!

A Hash is, by definition, an unordered construct.

James Edward G. II


#3

BTW: Why is ruby doing this anyways…?!

A Hash is an unsorted container; It is a widely-used algorithm that
makes no guarantees about the order of keys to optimise reading times at
the expense of insertion.

As with all algorithms, if the container doesn’t have the
characteristics you want, then you are using the wrong container.

There’s a Wikipedia article here on Hash Tables:

http://en.wikipedia.org/wiki/Hash_table

I would suggest that what you’re looking for is not a Hash at all,
but simply a List.

Martin


#4

A hash has no ‘order’ requirement. Items are added in an
‘implementation efficiency’ order - which may be different on
different underlying platforms.

If you want to retain a particular order, use an array.

Bob G


#5

Martin B. wrote:

Hello,

I am building a menu structure for rails that I’d like to store in a
simple
Hash.

Now I found out that ruby Hashes do not keep the order, like this
program:

b = {‘upkpgn’=>1,
‘jmay’=>2,
‘vkvxxm’=>3}

b.each_key {|k|
puts “%s => %s” % [k, b[k]]
}

…will output:

upkpgn => 1
vkvxxm => 3
jmay => 2

(vkvxxm and jmay are swapped)

I read an article describing this behavior, but it only mentions a
‘sort’
solution which is useless for me because my menu has a logical order:

http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/159776

Are there any workarounds for this? Or shall I rather write my own
containers?

If all you’re using it for is to test existence of keys, then Hash can
be replaced with SortedSet. See ‘ri Set’ for more info (the two classes
are in the same file in the standard distribution). Otherwise, you will
need to find or implement a Hash that maintains key order.


Toby DiPasquale


#6

On Tue, Apr 04, 2006 at 11:13:47PM +0900, Dave B. wrote:

Martin B. wrote:

I am building a menu structure for rails that I’d like to store in a simple
Hash.

Now I found out that ruby Hashes do not keep the order, like this program:

Are there any workarounds for this? Or shall I rather write my own containers?

You can use SortedHash. You can get it from the RAA. It preserves sort
order.

I assume a SortedHash is a tree of some kind? Probably Red-Black Tree,
or similar. In which case, this is not what he wants. He wants the keys
to be read out in the order in which they were inserted, he doesn’t want
sorting per se, at all.

As far as I can tell, what he wants is a list.

Martin


#7

If you need one, have a look at Facets’ Dictionary class (also called
OrderedHash).

http://facets.rubyforge.org

T.


#8

Martin B. wrote:

I am building a menu structure for rails that I’d like to store in a simple
Hash.

Now I found out that ruby Hashes do not keep the order, like this program:

Are there any workarounds for this? Or shall I rather write my own containers?

You can use SortedHash. You can get it from the RAA. It preserves sort
order.

Cheers,
Dave


#9

removed_email_address@domain.invalid wrote:

On Tue, Apr 04, 2006 at 11:13:47PM +0900, Dave B. wrote:

You can use SortedHash. You can get it from the RAA. It preserves sort
order.

I assume a SortedHash is a tree of some kind? Probably Red-Black Tree,
or similar. In which case, this is not what he wants. He wants the keys
to be read out in the order in which they were inserted, he doesn’t want
sorting per se, at all.

Sorry about the error. The name of the package I’m thinking of is
actually OrderedHash, and it is what he wants - it preserves (insertion)
order.

http://raa.ruby-lang.org/project/orderedhash/

Cheers,
Dave


#10

On Wed, 5 Apr 2006, Dave B. wrote:

Sorry about the error. The name of the package I’m thinking of is
actually OrderedHash, and it is what he wants - it preserves (insertion)
order.

http://raa.ruby-lang.org/project/orderedhash/

that one has some errors. i’ve patched it in my alib, also on the raa.
feel
free to lift it.

fyi.

-a


#11

On Tue, 4 Apr 2006, Martin B. wrote:

(vkvxxm and jmay are swapped)

I read an article describing this behavior, but it only mentions a ‘sort’
solution which is useless for me because my menu has a logical order:

http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/159776

Are there any workarounds for this? Or shall I rather write my own containers?

use an Array and Array#assoc

 harp:~ > cat a.rb
 b =
   %w( upkpgn 1 ),
   %w( jmay 2 ),
   %w( vkvxxm 3 )

 b.each{|kv| puts "%s => %s" % kv}

 puts b.assoc('upkpgn').last
 puts b.assoc('jmay').last
 puts b.assoc('vkvxxm').last


 harp:~ > ruby a.rb
 upkpgn => 1
 jmay => 2
 vkvxxm => 3
 1
 2
 3

BTW: Why is ruby doing this anyways…?!

hashes are, by definition, unsorted containers.

regards.

-a