Robert Klemme
6/13/2005 7:38:00 AM
Martin DeMello wrote:
> ee <erik.eide@gmail.com> wrote:
>> Hi
>>
>> Is there a simple way of searching the keys of a Hash to find a
>> subset. I have a set of keys with two values indicating group and
>> sub-group, eg:
>>
>> group_1_item_1
>> group_2_item_2
>> group_1_item_2
>> group_1_item_4
>> group_2_item_3
>>
>> I'd like to divide them into groups, but without having to loop
>> through the entire collection and having to test each key
>
> You can't get away without testing each key under the hood (this is
> intrinsically an O(n) problem),
Definitely!
> but this should make it syntactically
> convenient at least:
>
> hash.partition {|k,v| k =~ /group_1/ }.map {|i| Hash[*i.flatten]}
>
> Enumerable#partition divides your collection into two parts, based on
> whether the block returns true or false, so it doesn't scale to more
> than two subsets, but for your example it should work nicely.
IMHO we should have a general partitioning method (probably in Enumerable)
that does partitioning into several buckets and not just two. Something
like this maybe:
module Enumerable
def general_partition
parts = Hash.new {|h,k| h[k] = self.class.new}
each do |x|
parts[yield(x)] << x
end
parts
end
end
class Hash
def << (x) self[x[0]] = x[1] end
end
>> h={1=>1,2=>2,3=>3}
=> {1=>1, 2=>2, 3=>3}
>> h.general_partition {|k,v| k % 3}
=> {0=>{3=>3}, 1=>{1=>1}, 2=>{2=>2}}
Opinions?
Kind regards
robert