From: Todd Benson <caduceass@...>
Date: 2008-11-18T14:44:11+09:00
Subject: Re: function to select only certain key/value pairs from hash?

On Mon, Nov 17, 2008 at 11:23 PM, Einar Magnús Boson
<einarmagnus@tistron.se> wrote:
>
> 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).

Good point.  The use of #inject, though, may cloud that performance analysis.

> so if the hash is big it should make a difference.
>
> If the elements always are just a few it doesn't really matter.

I'm an #inject sort of guy so I like the way you approach this.  But,
there must be a reason why you don't prefer the negative #reject that
seems to work for most people.

I might benchmark this, but I think object creation and destruction
might outweigh the O(f*h).

Todd