From: Todd Benson <caduceass@...>
Date: 2008-11-18T19:40:46+09:00
Subject: Re: function to select only certain key/value pairs from hash?

On Mon, Nov 17, 2008 at 11:04 PM, Todd Benson <caduceass@gmail.com> 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}

I'm not apologizing for the missing result[key] on the lhs, though I
know it's pretty important to many to have working code in a message.
Anyone with a half a brain knows what I meant, but just to be clean...

h = Hash[:a, 1, :b, 2]
p [:missing_key].inject({}) {|result, key| result[key] = h[key]; result}
=> {:missing_key => nil}

In any case, reducing a hash (being selective) I think is best done
using a reject with a negative in the block (as per Chris Shea's
second example).

Todd