From: mental@...
Date: 2005-12-01T07:29:00+09:00
Subject: Re: Help constructing interesting hash?

Quoting Chris McMahon <christopher.mcmahon@gmail.com>:

> Suppose I have an array of arrays like
>
> [[A, B, C][1,2,3]]
>
> I can easily make a hash (using each_with_index) where one value
> is
> the lookup value for the other value:
>
> 1=>A
> 2=>B
> 3=>C

 def build( values, keys )
   hash = {}
   values.zip( keys ) do |value, key|
     hash[key] = value
   end
   hash
 end

 build(*[[A, B, C], [1, 2, 3]]) #=> {1=>A, 2=>B, 3=>C}

> Now suppose I have a AoA like
>
> [[A, B, C,][1,2,2]]
>
> Is there a readable way to construct a hash like
>
> 1=>[A]
> 2=>[B, C]

 def build2( values, keys )
   hash = {}
   values.zip( keys ) do |value, key|
     ( hash[key] ||= [] ).push value
   end
   hash
 end

 build2(*[[A, B, C], [1, 2, 2]]) #=> {1=>[A], 2=>[B, C]}

In other words, you can probably just take the code you've got and
replace:

 hash[key] = value

with:

 ( hash[key] ||= [] ).push value

This bit can also be written in longer form as:

 hash[key] ||= []
 hash[key].push value

or even:

 hash[key] = [] unless hash[key]
 hash[key].push value

(but the shortest form saves you one or two hash lookups)

-mental