How to swap keys and values in a hash

hashmap, ruby

Solution

Ruby has a helper method for Hash that lets you treat a Hash as if it was inverted (in essence, by letting you access keys through values):

{a: 1, b: 2, c: 3}.key(1)
=> :a

If you want to keep the inverted hash, then Hash#invert should work for most situations:

{a: 1, b: 2, c: 3}.invert
=> {1=>:a, 2=>:b, 3=>:c}

BUT...

If you have duplicate values, `invert` will discard all but the last occurrence of your values (because it will keep replacing new value for that key during iteration). Likewise, `key` will only return the first match:

{a: 1, b: 2, c: 2}.key(2)
=> :b

{a: 1, b: 2, c: 2}.invert
=> {1=>:a, 2=>:c}

So, if your values are unique you can use `Hash#invert`. If not, then you can keep all the values as an array, like this:

class Hash
  # like invert but not lossy
  # {"one"=>1,"two"=>2, "1"=>1, "2"=>2}.inverse => {1=>["one", "1"], 2=>["two", "2"]} 
  def safe_invert
    each_with_object({}) do |(key,value),out| 
      out[value] ||= []
      out[value] << key
    end
  end
end

Note: This code with tests is now on GitHub.

Or:

class Hash
  def safe_invert
    self.each_with_object({}){|(k,v),o|(o[v]||=[])<<k}
  end
end

Problem

How do I swap keys and values in a Hash? I have the following Hash: ``` {:a=>:one, :b=>:two, :c=>:three} ``` that I want to transform into: ``` {:one=>:a, :two=>:b, :three=>:c} ``` Using `map` seems rather tedious. Is there a shorter solution?

Original source