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comp.lang.ruby

Hashtable of sets

Trevor Harmon

4/30/2006 8:14:00 PM

Hi,

I'm new to Ruby and need a data structure that consists of a hashtable
of sets. While writing some code for this, I've run into some odd Ruby
behavior that I'm hoping someone can explain to me.

I started with a hashtable:

h = Hash.new([])

Here, I made the default value an empty set instead of nil. That way,
if I wanted to add a new element "q" to a set mapped to the key 'a', I
could do so in just one line. For example:

h['a'] = %w{q r s}
h['a'] << "t"

That seemed to work:

> h['a']
=> ["q", "r", "s", "t"]
> h['b']
=> []

Great! Now let me add an element to the set mapped to 'b':

h['b'] << "x"

That seemed to work, too:

> h['b']
=> ["x"]

But wait, now all the other keys are mapped to the same thing!

> h['c']
=> ["x"]
> h['d']
=> ["x"]

Can someone please explain how this happened? I don't understand it. Thanks,

Trevor

P.S. I'm using Ruby 1.8.4.
4 Answers

Marcin Mielzynski

4/30/2006 9:36:00 PM

0

Trevor Harmon wrote:
> Hi,
>
> I'm new to Ruby and need a data structure that consists of a hashtable
> of sets. While writing some code for this, I've run into some odd Ruby
> behavior that I'm hoping someone can explain to me.
>
> I started with a hashtable:
>
> h = Hash.new([])
>

This supplies the default object value when there is no key in a hash
and this object will not be copied, so:

h['foo'] << 4
h['blah'] << "ddd"

>> h['bar']
=> [4, "ddd"]

the same instance is referenced...

In order to create a hash that behaves the way you expect, is to create
a hash with a block that returns default value (in this case a separate
instance will be created):


h = Hash.new{[]}
h['foo'] << 4
h['blah'] << "ddd"

>> h['bar']
=> []

lopex

Trevor Harmon

5/1/2006 1:19:00 AM

0

On 2006-04-30 14:35:37 -0700, Marcin MielżyÅ?ski <lopexx@autograf.pl> said:

> In order to create a hash that behaves the way you expect, is to create
> a hash with a block that returns default value (in this case a separate
> instance will be created):
>
> h = Hash.new{[]}
> h['foo'] << 4
> h['blah'] << "ddd"
>
> >> h['bar']
> => []

That fixes the same-instance problem, but it still doesn't work right
because h['foo'] and h['blah'] also return [].

I could fix this by special-handling the case where the hashtable entry
is empty, but that would defeat the purpose of the Hash.new{[]} trick.
Isn't there some one-liner I could use to add elements to this data
structure? Thanks,

Trevor

Daniel Mitterdorfer

5/1/2006 8:59:00 AM

0

Trevor Harmon schrieb:
> On 2006-04-30 14:35:37 -0700, Marcin MielżyÅ?ski <lopexx@autograf.pl> said:
> Isn't there some one-liner I could use to add elements to this data
> structure? Thanks,
>
> Trevor
>

Gregory Brown discusses this issue in his NubyGems series. See [1]

hth

Daniel

[1]
http://www.oreillynet.com/ruby/blog/2006/04/nubygems_hash_initializat...

Robert Klemme

5/1/2006 9:54:00 AM

0

Marcin Mielzynski wrote:
> Trevor Harmon wrote:
>> Hi,
>>
>> I'm new to Ruby and need a data structure that consists of a hashtable
>> of sets. While writing some code for this, I've run into some odd Ruby
>> behavior that I'm hoping someone can explain to me.
>>
>> I started with a hashtable:
>>
>> h = Hash.new([])
>>
>
> This supplies the default object value when there is no key in a hash
> and this object will not be copied, so:
>
> h['foo'] << 4
> h['blah'] << "ddd"
>
> >> h['bar']
> => [4, "ddd"]
>
> the same instance is referenced...

Correct.

> In order to create a hash that behaves the way you expect, is to create
> a hash with a block that returns default value (in this case a separate
> instance will be created):
>
>
> h = Hash.new{[]}
> h['foo'] << 4
> h['blah'] << "ddd"
>
> >> h['bar']
> => []

No, you need this

require 'set'
h = Hash.new {|ha,k| ha[k] = Set.new}

1. Class Set implements the requirement of Trevor who wants a hash of sets.

2. Assignment is needed in the block because otherwise a new Set / Array
is returned every time a key is not found.

Kind regards

robert