将哈希数组反向映射到哈希

Question

I have an Array that contains Hash with only one key and value as an Array .

Eg:

a = [{1 => ["foo", "bar"]}, {2 => ["hello"]}, {3 => ["world", "bar"]}]

Now I want to create a Hash having values of above Hash es as key and their keys as values.

Eg: (desired result)

res =  {"foo"=>[1], "bar"=>[1, 3], "hello"=>[2], "world"=>[3]}

I have solved this is following way:

b =  Hash.new { |h, k| h[k] = [] }
a.each { |hash|
  hash.each { |key, vals|
    vals.each { |val|
      b[val] << key
    }
  }
}
b
# => {"foo"=>[1], "bar"=>[1, 3], "hello"=>[2], "world"=>[3]}

It works fine but there should be a better, shorter way to do this than iterating so many times. Please suggest.

Answer 1

One could invert keys and values and then collect elements as needed:

a = [{1 => ["foo", "bar"]}, {2 => ["hello"]}, {3 => ["world", "bar"]}]

a.map(&:invert).inject({}) { |memo,el| 
    el = el.flatten   # hash of size 1 ⇒ array [k ⇒ v]
    el.first.each { |k| (memo[k] ||= []) << el.last }
    memo 
}

#⇒ {
#  "bar" => [
#    [0] 1,
#    [1] 3
#  ],
#  "foo" => [
#    [0] 1
#  ],
#  "hello" => [
#    [0] 2
#  ],
#  "world" => [
#    [0] 3
#  ]
# }

Hope it helps.

Answer 2

I'd use group_by , but you need to massage your data:

a.flat_map(&:to_a) # => [[1, ["foo", "bar"]], [2, ["hello"]], [3, ["world", "bar"]]]
.flat_map{|key, values| values.map{|v| [key, v]}} # => [[1, "foo"], [1, "bar"], [2, "hello"], [3, "world"], [3, "bar"]]
.group_by(&:last) # => {"foo"=>[[1, "foo"]], "bar"=>[[1, "bar"], [3, "bar"]], "hello"=>[[2, "hello"]], "world"=>[[3, "world"]]}
.map{|key, values| [key, values.map(&:first)]} # =>  [["foo", [1]], ["bar", [1, 3]], ["hello", [2]], ["world", [3]]]
.to_h # => {"foo"=>[1], "bar"=>[1, 3], "hello"=>[2], "world"=>[3]}