From: Kev Jackson <kevin.jackson@...>
Date: 2005-10-12T12:46:52+09:00
Subject: Re: small array/hash question

Ara.T.Howard wrote:

> On Wed, 12 Oct 2005, Kev Jackson wrote:
>
>> Hi all,
>>
>> I'm trying to loop across a dataset and create a hash where each 
>> value is an array so that later I can loop over the hash and for each 
>> key (it's important I store the key), I can loop over the array 
>> contained and spit out some results.
>>
>> Without looking at the docs I wanted to do something like...
>>
>> (in pseudo-code)
>> loop across data here
>> work_types[nsc_id]=do |types|
>>      types << data[7]
>> end
>> end loop
>> where work_types is the hash and types is the array I want to 
>> accumulate data in
>>
>> This doesn't work, so I'm wondering what the ruby idiom for this kind 
>> of thing would be.  Essentially for each piece of data I want to get 
>> the appropriate value from the Hash and append the value on to the 
>> end of the array associated with the key, or if it doesn't exist in 
>> the Hash, create a new entry with a new array populated with the value.
>>
>> I'm sure there's a very simple way of doing this, but I can't see the 
>> method I want in the standard library docs - I thought it might be 
>> collect, but it doesn't look like it
>>
>> Thanks
>> Kev
>
>
> i think you want something like this:
>
>   harp:~ > irb
>   irb(main):001:0> work_types = Hash::new{|h,k| h[k] = []}
>   => {}
>   irb(main):002:0> work_types[ 'foo' ] << 42
>   => [42]
>   irb(main):003:0> work_types[ 'foo' ] << 42
>   => [42, 42]
>   irb(main):004:0> work_types[ 'bar' ] << 'forty-two'
>   => ["forty-two"]
>   irb(main):005:0> work_types
>   => {"foo"=>[42, 42], "bar"=>["forty-two"]}
>
> if not you'll have to post more about your exact problem and some 
> sample data.
>
> hth.
>
> -a

I got the output I wanted with this

work_types = Hash.new
if work_types.has_key?(nsc_id) then
    work_types[nsc_id]= work_types[nsc_id].include?(work_type) ? 
work_types[nsc_id] : work_types[nsc_id] << work_type
else
    work_types[nsc_id]= [work_type]
end

So the problem is solved, but I wonder if there's a more elegant way of 
doing it (especially the check to see if the value is already in the 
array).  My first assumption was that assignment to a Hash took a block 
(hence the pseudo code), I was actually a little suprised that it didn't ;)

Kev