How to Get the Key of the Largest Value in a Ruby Hash Map?

You can get the key of the largest numeric value in a Ruby hash map in the following ways:

1. Using Enumerable#max_by;
2. Using Hash#each;
3. Using Enumerable#max.

Please note that these methods will return only the first match in case of duplicates of the maximum value. However, you can use alternative approaches if you want to have an array of all matches returned instead.

Using Enumerable#max_by

You can do the following:

1. Iterate over the key-value pairs of the hash map using Enumerable#max_by;
2. Specify the value in the block to identify the pair with the maximum value, and return it as an array;
3. Call the Enumerable#first method with the safe navigation operator (&.) to safely return the key (which would be the first element of the resulting array).

For example, you can implement this in the following way:

def max_value_key(hash)
  max_hash = hash.max_by { | key, value | value }
  max_hash&.first
end

print max_value_key({ a: 20, b: 50, c: 30 }) #=> b
print max_value_key({ a: 20, b: 50, c: 50 }) #=> b
print max_value_key({ a: -20, b: -50, c: -30 }) #=> a
print max_value_key({}) #=> nil

You may shorten this to the following one-liner:

def max_value_key(hash)
  hash.max_by(&:last)&.first
end

print max_value_key({ a: 20, b: 50, c: 30 }) #=> b
print max_value_key({ a: 20, b: 50, c: 50 }) #=> b
print max_value_key({ a: -20, b: -50, c: -30 }) #=> a
print max_value_key({}) #=> nil

In this shorthand, the &:last symbol simply returns the value from the key-value pair passed to the block.

Using Hash#each

You can do the following:

1. Iterate over the key-value pairs of the hash map using Hash#each;
2. Compare value of each item to find the largest value and its corresponding key;
3. When the iteration finishes, return the key for the largest value.

def max_value_key(hash)
  max_key = nil
  max_value = nil

  hash.each do | key, value |
    if max_value.nil? || value > max_value
      max_key = key
      max_value = value
    end
  end

  max_key
end

print max_value_key({ a: 20, b: 50, c: 30 }) #=> b
print max_value_key({ a: 20, b: 50, c: 50 }) #=> b
print max_value_key({ a: -20, b: -50, c: -30 }) #=> a
print max_value_key({}) #=> nil

Using Enumerable#max

You can do the following:

1. Return an array of all the values using Hash#values;
2. Find the maximum value using Enumerable#max;
3. Use the Hash#key method to locate the corresponding key.

For example, you can implement this in the following way:

def max_value_key(hash)
  hash.key(hash.values.max)
end

print max_value_key({ a: 20, b: 50, c: 30 }) #=> b
print max_value_key({ a: 20, b: 50, c: 50 }) #=> b
print max_value_key({ a: -20, b: -50, c: -30 }) #=> a
print max_value_key({}) #=> nil

It's worth mentioning that this method involves multiple iterations/passes through the enumerable.