how do I invert my hash (switch key/value) and group by value

Question

I have a hash in rails like so:

{"unique_id" => "1",
 "unique_id2" => "2",
 "unique_id3" => "n"}

Each unique key has a count that can be a number 1-20. What I would like to do is have a hash that looks like this:

{"1" => ["unique_id", "unique_id2"],
 "2" => ["unique_id3"],
 "3" => ["unique_id4", "unique_id5", "uniqueid6"]}

How would I go about doing that with a hash?

Answer 1

Not too hard!

hash = { "unique_id" => "1",
  "unique_id2" => "2",
  "unique_id3" => "n"
}
new_hash = hash.each_with_object({}) { |(k,v), h| (h[v] ||= []) << k }

each_with_object({}) is just an each loop with a blank hash

||= [] means if the hash doesn't have a value for v , set it equal to an empty array

<< k pushes the key onto the array

Answer 2

Try this:

  h.group_by{|k,v| v }.each{|k,v| v.map!(&:first) }

group_by takes your hash and groups it by value

each iterates over the result hash

map! maps the first elements of the result value arrays, since group_by on a Hash returns two-dimensional Array s with the structure [key, value]