How to find a hash key containing a matching value?

In this article, we will discuss how to find a hash key containing a matching value in ruby. We can find a hash key containing a matching value through different methods ranging from using  Hash#key method and Hash#select method to any? method

Finding a hash key containing a matching value using the Hash#key

The Hash#key method returns the first key in the hash that has the matching value

Syntax:

hash.key(value)

Example: In this example, we use Hash#key to search for the key containing the value 2 in the example_hash where it returns the first key found, which is :b.

Ruby

# Define a method to find a hash key containing a matching value using Hash#key.
def find_key_with_value(hash, value)
  # Use the key method to find the key containing the specified value.
  hash.key(value)
end

# Example hash
example_hash = { a: 1, b: 2, c: 3, d: 1 }

# Find the key containing the value 2 in the example hash
result = find_key_with_value(example_hash, 2)

# Output the result
puts "Key containing value 2 is #{result}"

 

Output

Key containing value 2 is b

Find a hash key containing a matching value using Hash#select

The Hash#select method returns a new hash consisting of key-value pairs for which the block returns true.

Syntax:

hash.select { |key, value| value == desired_value }.keys

Example: In this example,we use Hash#select  method to searches for keys containing the value 1 in the example_hash and thus returning array of keys that match the condition, which are [:a, :d].

Ruby

 

# Define a method to find a hash key containing a matching value using Hash#select.
def find_key_with_value(hash, value)
  # Use select to filter key-value pairs where the value matches the desired value, then extract the keys.
  hash.select { |key, val| val == value }.keys
end

# Example hash
example_hash = { a: 1, b: 2, c: 3, d: 1 }

# Find the key(s) containing the value 1 in the example hash
result = find_key_with_value(example_hash, 1)

# Output the result
puts "Key(s) containing value 1 are #{result}"

 

Output

Key(s) containing value 1 are [:a, :d]

Find a hash key containing a matching value using Enumerable#find

The Enumerable#find method returns the first key-value pair for which the block returns true

Syntax:

hash.find { |key, value| value == desired_value }&.first

Example: In this example we use the Enumerable#find method is used to search for the first key containing the value 3 in the example_hash. It returns the first key found, which is :c.

Ruby

# Define a method to find a hash key containing a matching value using Enumerable#find.
def find_key_with_value(hash, value)
  # Use find to search for the first key-value pair where the value matches the desired value, then extract the key.
  hash.find { |key, val| val == value }&.first
end

# Example hash
example_hash = { a: 1, b: 2, c: 3, d: 1 }

# Find the first key containing the value 3 in the example hash
result = find_key_with_value(example_hash, 3)

# Output the result
puts "First key containing value 3 is #{result}"

Output

Method 3: First key containing value 3 is c