Hash.keys / values / has_key? / has_value?
Methods for retrieving a list of keys or values from a hash, and for checking whether a specific key or value exists. Frequently used for data inspection and debugging.
Syntax
# Returns an array of all keys. hash.keys # Returns an array of all values. hash.values # Checks whether a key exists (several aliases available). hash.has_key?(key) hash.key?(key) hash.include?(key) hash.member?(key) # Checks whether a value exists. hash.has_value?(value) hash.value?(value) # Returns an array of values for the specified keys. hash.values_at(key1, key2, ...)
Method list
| Method | Description |
|---|---|
| keys | Returns all keys in the hash as an array. Insertion order is preserved. |
| values | Returns all values in the hash as an array. Insertion order is preserved. |
| has_key?(key) | Returns true if the specified key exists. key?, include?, and member? are aliases. |
| has_value?(val) | Returns true if the specified value exists. value? is an alias. |
| values_at(k1, k2, ...) | Returns an array of values for the specified keys. Missing keys produce nil. |
| key(value) | Returns the first key that maps to the specified value. Returns nil if not found. |
| size / length | Returns the number of key-value pairs in the hash. |
| empty? | Returns true if the hash contains no entries. |
Sample code
person = { name: 'Taro', age: 30, city: 'Tokyo' }
# List keys and values
puts person.keys.inspect # [:name, :age, :city]
puts person.values.inspect # ["Taro", 30, "Tokyo"]
# Check for key existence
puts person.has_key?(:name) # true
puts person.key?(:email) # false (key not found)
puts person.include?(:age) # true (alias of has_key?)
# Check for value existence
puts person.has_value?('Taro') # true
puts person.value?(99) # false
# Retrieve values for multiple keys at once
puts person.values_at(:name, :city).inspect # ["Taro", "Tokyo"]
puts person.values_at(:name, :email).inspect # ["Taro", nil] (missing key returns nil)
# Look up a key by value
puts person.key('Taro') # name
puts person.key('unknown') # nil
# Size and empty check
puts person.size # 3
puts person.empty? # false
puts {}.empty? # true
# Compare hash keys using set intersection
config1 = { host: 'localhost', port: 3000 }
config2 = { host: 'example.com', port: 80, ssl: true }
common_keys = config1.keys & config2.keys
puts common_keys.inspect # [:host, :port] (keys shared by both hashes)
Notes
Since Ruby 1.9, hashes maintain insertion order, so the arrays returned by keys and values reflect the order in which entries were added.
has_key? has three aliases: key?, include?, and member?. All four behave identically, but the Ruby style guide often recommends the shorter key?. Pick one and use it consistently across your codebase.
values_at is convenient when you need to retrieve several values at once. Because missing keys return nil, check for key existence with has_key? beforehand, or use fetch to supply a default value.
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