如何在 ruby​​ 中的嵌套散列中获取某个值的键？

Question

This is a follow question to another topic I posted earlier.

Say I have the following hash.

my_hash = {
  'array1' => %w[
    value1
    value2
  ],
  'array2' => %w[
    value3
    value4
  ]
}

What is the fastest or simplest way to get the key for a certain value in one of the arrays. For example, I want to get the key for value2, and get array1, or get the key for value3 and get array2

Answer 1

The fastest you can go with the existing data structure is to scan all arrays in the Hash:

key, _ = my_hash.find{|k, v| v.include? "value3" }

If this lookup is an operation that is performed many times and you need to go faster than this, you may consider a data structure that allows for faster lookups, like one of the following:

• convert the inner arrays into Sets
• construct a reverse lookup Hash, where the values ( value1 , value2 , etc.) would point to the corresponding key in the initial hash (if all the values are unique).

Answer 2

my_hash.detect { |_, v| v.include? 'value2' }.first

Please note, that if there are many keys, having value2 in the values array, this approach will return the very first one.

To receive all keys, one might use:

my_hash.select { |_, v| v.include? 'value2' }.to_h.keys

or:

my_hash.map { |k, v| k if v.include?('value2') }.compact