Convert array of 2-element arrays into a hash, where duplicate keys append additional values
Using functional baby steps:
irb:01.0> array = [[:a,:b],[:a,:c],[:c,:b]]
#=> [[:a, :b], [:a, :c], [:c, :b]]
irb:02.0> array.group_by(&:first)
#=> {:a=>[[:a, :b], [:a, :c]], :c=>[[:c, :b]]}
irb:03.0> array.group_by(&:first).map{ |k,a| [k,a.map(&:last)] }
#=> [[:a, [:b, :c]], [:c, [:b]]]
irb:04.0> Hash[ array.group_by(&:first).map{ |k,a| [k,a.map(&:last)] } ]
#=> {:a=>[:b, :c], :c=>[:b]}
Using imperative style programming:
irb:10.0> h = Hash.new{ |h,k| h[k]=[] }
#=> {}
irb:11.0> array.each{ |k,v| h[k] << v }
#=> [[:a, :b], [:a, :c], [:c, :b]]
irb:12.0> h
#=> {:a=>[:b, :c], :c=>[:b]}
As an imperative one-liner:
irb:13.0> h = Hash.new{ |h,k| h[k]=[] }.tap{ |h| array.each{ |k,v| h[k] << v } }
#=> {:a=>[:b, :c], :c=>[:b]}
Or using everyone's favorite inject
:
irb:14.0> array.inject(Hash.new{ |h,k| h[k]=[] }){ |h,(k,v)| h[k] << v; h }
#=> {:a=>[:b, :c], :c=>[:b]}
If you really want to have single values not collided as an array, you can either un-array them as a post-processing step, or use a different hash accumulation strategy that only creates an array upon collision. Alternatively, wrap your head around this:
irb:17.0> hashes = array.map{ |pair| Hash[*pair] } # merge many mini hashes
#=> [{:a=>:b}, {:a=>:c}, {:c=>:b}]
irb:18.0> hashes.inject{ |h1,h2| h1.merge(h2){ |*a| a[1,2] } }
#=> {:a=>[:b, :c], :c=>:b}
EDIT: In Ruby 2.1+, you can use Array#to_h
pry(main)> [[:a,:b],[:a,:c],[:c,:b]].to_h
=> {:a=>:c, :c=>:b}
END EDIT
The public [] method on the Hash class accepts a key-value pair array and returns a hash with the first element of the array as key and the second as value.
The last value in the key-value pair will be the actual value when there are key duplicates.
Hash[[[:a,:b],[:a,:c],[:c,:b]]]
=> {:a=>:c, :c=>:b}
This syntax is valid in 1.9.3+ ; I'm not sure about earlier Ruby versions (it's not valid in 1.8.7)
ref: http://www.ruby-doc.org/core-2.1.0/Hash.html#method-c-5B-5D
Another interesting way of doing it would be using the inject method: (obviously the method above is more succinct and recommended for this specific problem)
[ [:a, :b], [:a, :c], [:c, :b] ].inject({}) { |memo, obj|
memo[obj.first] = obj.last
memo
}
=> {:a=>:c, :c=>:b}
inject iterates over the enumerable, your array in this case, starting with the injected parameter, in this case the empty hash {}.
For each object in the enumerable, the block is called with the variables memo and obj:
obj is the current object in the array
memo is the value that has been returned by your block's last iteration (for the first iteration, it's what you inject)