From: "Einar Magnús Boson" <einarmagnus@...>
Date: 2008-11-18T14:23:50+09:00
Subject: Re: function to select only certain key/value pairs from hash?


On 18.11.2008, at 05:00 , Todd Benson wrote:

> On Mon, Nov 17, 2008 at 7:44 PM, Einar Magnús Boson
> <einarmagnus@tistron.se> wrote:
>> This is what I meant to do, a lot more efficient to look up the  
>> values
>> you're lookin for than looping through all keys for every element.
>>
>> hash = {:a => 1,
>>                       :b => 2,
>>                       :c => 3,
>>                       :d => 4,
>>                       :str => "test"}
>>
>> selected = [:a, :d, :str].inject({}){|result, key| result[key]=
>> hash[key];result}
>>
>>
>> p selected
>>
>> # >> {:str=>"test", :a=>1, :d=>4}
>
> You will get nils for none existing keys that way.
>
> selected = [:whatever].inject({}){|result, key| = hash[key]; result}
> #=> {:whatever => nil}
>
> If that's what you want, great.
>
> Todd
>

if that is a problem it's easy to fix


hash = {:a => 1,
         :b => 2,
         :c => 3,
         :d => 4,
         :str => "test"}

find = [:a, :d, :str, :extra]

selected = find.inject({}) {
			|result, key|
			val=hash[key]
			result[key]=val if val
			result }

p selected
# >> {:str=>"test", :a=>1, :d=>4}

given: f, h = find.size, hash.size
This method is O(f) because hash lookup is O(1)
The other way it's O(f*h).
so if the hash is big it should make a difference.

If the elements always are just a few it doesn't really matter.

einarmagnus