How to Get Hash Map Keys of the Max Value in Ruby Including Duplicates?

In Ruby, if you want to get the key(s) of the largest value in a hash map, including any possible duplicates, you can do either the following:

1. Use Enumerable#select;
2. Use Hash#each.

Please note that these methods will return the key of the largest value as well as any duplicates of it. If you want to just return the first match instead, then you can use alternative approaches.

Using Enumerable#select

With Enumerable#select, you can follow these steps to get key(s) of the largest value in a hash map, including duplicates:

1. Use the Enumerable#select (or Enumerable#filter) method to remove values from the hash that don't match the largest value;
2. Return an array of keys from the filtered hash using the Hash#keys method.

For example, you can implement this using the Enumerable#select method in the following way:

def max_value_keys(hash)
  max_value = hash.values.max
  max_values = hash.select { | key, value | value == max_value }
  max_values.keys
end

print max_value_keys({ a: 20, b: 50, c: 30 }) #=> [:b]
print max_value_keys({ a: 20, b: 50, c: 50 }) #=> [:b, :c]
print max_value_keys({ a: -20, b: -50, c: -30 }) #=> [:a]
print max_value_keys({}) #=> []

This method involves multiple iterations over the hash map, returning an array of keys corresponding to the matching values. If an empty hash is provided as input, the method will return an empty array.

You may substitute Enumerable#select with Enumerable#filter as they are aliases of each other, and can be used interchangeably.

Using Hash#each

You can use the Hash#each method to iterate over each item in the hash map, and determine the key(s) of the largest value:

def max_value_keys(hash)
  max_value = nil
  keys = []

  hash.each do | key, value |
    if max_value.nil? || value > max_value
      max_value = value
      keys = [key]
    elsif value == max_value
      keys << key
    end
  end

  keys
end

print max_value_keys({ a: 20, b: 50, c: 30 }) #=> [:b]
print max_value_keys({ a: 20, b: 50, c: 50 }) #=> [:b, :c]
print max_value_keys({ a: -20, b: -50, c: -30 }) #=> [:a]
print max_value_keys({}) #=> []

This approach iterates over the hash map only once, keeping track of the maximum value encountered so far and accumulating keys that have the same maximum value. if an empty hash is provided as input, the method will return an empty array.