Hash

Class

A Hash maps each of its unique keys to a specific value.

A Hash has certain similarities to an Array, but:

  • An Array index is always an Integer.

  • A Hash key can be (almost) any object.

Hash Data Syntax

The older syntax for Hash data uses the “hash rocket,” =>:

h = {:foo => 0, :bar => 1, :baz => 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

Alternatively, but only for a Hash key that’s a Symbol, you can use a newer JSON-style syntax, where each bareword becomes a Symbol:

h = {foo: 0, bar: 1, baz: 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

You can also use a String in place of a bareword:

h = {'foo': 0, 'bar': 1, 'baz': 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

And you can mix the styles:

h = {foo: 0, :bar => 1, 'baz': 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

But it’s an error to try the JSON-style syntax for a key that’s not a bareword or a String:

# Raises SyntaxError (syntax error, unexpected ':', expecting =>):
h = {0: 'zero'}

Hash value can be omitted, meaning that value will be fetched from the context by the name of the key:

x = 0
y = 100
h = {x:, y:}
h # => {:x=>0, :y=>100}

Common Uses

You can use a Hash to give names to objects:

person = {name: 'Matz', language: 'Ruby'}
person # => {:name=>"Matz", :language=>"Ruby"}

You can use a Hash to give names to method arguments:

def some_method(hash)
  p hash
end
some_method({foo: 0, bar: 1, baz: 2}) # => {:foo=>0, :bar=>1, :baz=>2}

Note: when the last argument in a method call is a Hash, the curly braces may be omitted:

some_method(foo: 0, bar: 1, baz: 2) # => {:foo=>0, :bar=>1, :baz=>2}

You can use a Hash to initialize an object:

class Dev
  attr_accessor :name, :language
  def initialize(hash)
    self.name = hash[:name]
    self.language = hash[:language]
  end
end
matz = Dev.new(name: 'Matz', language: 'Ruby')
matz # => #<Dev: @name="Matz", @language="Ruby">

Creating a Hash

You can create a Hash object explicitly with:

  • A hash literal.

You can convert certain objects to Hashes with:

You can create a Hash by calling method Hash.new.

Create an empty Hash:

h = Hash.new
h # => {}
h.class # => Hash

You can create a Hash by calling method Hash.[].

Create an empty Hash:

h = Hash[]
h # => {}

Create a Hash with initial entries:

h = Hash[foo: 0, bar: 1, baz: 2]
h # => {:foo=>0, :bar=>1, :baz=>2}

You can create a Hash by using its literal form (curly braces).

Create an empty Hash:

h = {}
h # => {}

Create a Hash with initial entries:

h = {foo: 0, bar: 1, baz: 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

Hash Value Basics

The simplest way to retrieve a Hash value (instance method []):

h = {foo: 0, bar: 1, baz: 2}
h[:foo] # => 0

The simplest way to create or update a Hash value (instance method []=):

h = {foo: 0, bar: 1, baz: 2}
h[:bat] = 3 # => 3
h # => {:foo=>0, :bar=>1, :baz=>2, :bat=>3}
h[:foo] = 4 # => 4
h # => {:foo=>4, :bar=>1, :baz=>2, :bat=>3}

The simplest way to delete a Hash entry (instance method delete):

h = {foo: 0, bar: 1, baz: 2}
h.delete(:bar) # => 1
h # => {:foo=>0, :baz=>2}

Entry Order

A Hash object presents its entries in the order of their creation. This is seen in:

  • Iterative methods such as each, each_key, each_pair, each_value.

  • Other order-sensitive methods such as shift, keys, values.

  • The String returned by method inspect.

A new Hash has its initial ordering per the given entries:

h = Hash[foo: 0, bar: 1]
h # => {:foo=>0, :bar=>1}

New entries are added at the end:

h[:baz] = 2
h # => {:foo=>0, :bar=>1, :baz=>2}

Updating a value does not affect the order:

h[:baz] = 3
h # => {:foo=>0, :bar=>1, :baz=>3}

But re-creating a deleted entry can affect the order:

h.delete(:foo)
h[:foo] = 5
h # => {:bar=>1, :baz=>3, :foo=>5}

Hash Keys

Hash Key Equivalence

Two objects are treated as the same hash key when their hash value is identical and the two objects are eql? to each other.

Modifying an Active Hash Key

Modifying a Hash key while it is in use damages the hash’s index.

This Hash has keys that are Arrays:

a0 = [ :foo, :bar ]
a1 = [ :baz, :bat ]
h = {a0 => 0, a1 => 1}
h.include?(a0) # => true
h[a0] # => 0
a0.hash # => 110002110

Modifying array element a0[0] changes its hash value:

a0[0] = :bam
a0.hash # => 1069447059

And damages the Hash index:

h.include?(a0) # => false
h[a0] # => nil

You can repair the hash index using method rehash:

h.rehash # => {[:bam, :bar]=>0, [:baz, :bat]=>1}
h.include?(a0) # => true
h[a0] # => 0

A String key is always safe. That’s because an unfrozen String passed as a key will be replaced by a duplicated and frozen String:

s = 'foo'
s.frozen? # => false
h = {s => 0}
first_key = h.keys.first
first_key.frozen? # => true

User-Defined Hash Keys

To be useable as a Hash key, objects must implement the methods hash and eql?. Note: this requirement does not apply if the Hash uses compare_by_identity since comparison will then rely on the keys’ object id instead of hash and eql?.

Object defines basic implementation for hash and eq? that makes each object a distinct key. Typically, user-defined classes will want to override these methods to provide meaningful behavior, or for example inherit Struct that has useful definitions for these.

A typical implementation of hash is based on the object’s data while eql? is usually aliased to the overridden == method:

class Book
  attr_reader :author, :title

  def initialize(author, title)
    @author = author
    @title = title
  end

  def ==(other)
    self.class === other &&
      other.author == @author &&
      other.title == @title
  end

  alias eql? ==

  def hash
    [self.class, @author, @title].hash
  end
end

book1 = Book.new 'matz', 'Ruby in a Nutshell'
book2 = Book.new 'matz', 'Ruby in a Nutshell'

reviews = {}

reviews[book1] = 'Great reference!'
reviews[book2] = 'Nice and compact!'

reviews.length #=> 1

Default Values

The methods [], values_at and dig need to return the value associated to a certain key. When that key is not found, that value will be determined by its default proc (if any) or else its default (initially ‘nil`).

You can retrieve the default value with method default:

h = Hash.new
h.default # => nil

You can set the default value by passing an argument to method Hash.new or with method default=

h = Hash.new(-1)
h.default # => -1
h.default = 0
h.default # => 0

This default value is returned for [], values_at and dig when a key is not found:

counts = {foo: 42}
counts.default # => nil (default)
counts[:foo] = 42
counts[:bar] # => nil
counts.default = 0
counts[:bar] # => 0
counts.values_at(:foo, :bar, :baz) # => [42, 0, 0]
counts.dig(:bar) # => 0

Note that the default value is used without being duplicated. It is not advised to set the default value to a mutable object:

synonyms = Hash.new([])
synonyms[:hello] # => []
synonyms[:hello] << :hi # => [:hi], but this mutates the default!
synonyms.default # => [:hi]
synonyms[:world] << :universe
synonyms[:world] # => [:hi, :universe], oops
synonyms.keys # => [], oops

To use a mutable object as default, it is recommended to use a default proc

Default Proc

When the default proc for a Hash is set (i.e., not nil), the default value returned by method [] is determined by the default proc alone.

You can retrieve the default proc with method default_proc:

h = Hash.new
h.default_proc # => nil

You can set the default proc by calling Hash.new with a block or calling the method default_proc=

h = Hash.new { |hash, key| "Default value for #{key}" }
h.default_proc.class # => Proc
h.default_proc = proc { |hash, key| "Default value for #{key.inspect}" }
h.default_proc.class # => Proc

When the default proc is set (i.e., not nil) and method [] is called with with a non-existent key, [] calls the default proc with both the Hash object itself and the missing key, then returns the proc’s return value:

h = Hash.new { |hash, key| "Default value for #{key}" }
h[:nosuch] # => "Default value for nosuch"

Note that in the example above no entry for key :nosuch is created:

h.include?(:nosuch) # => false

However, the proc itself can add a new entry:

synonyms = Hash.new { |hash, key| hash[key] = [] }
synonyms.include?(:hello) # => false
synonyms[:hello] << :hi # => [:hi]
synonyms[:world] << :universe # => [:universe]
synonyms.keys # => [:hello, :world]

Note that setting the default proc will clear the default value and vice versa.

Be aware that a default proc that modifies the hash is not thread-safe in the sense that multiple threads can call into the default proc concurrently for the same key.

What’s Here

First, what’s elsewhere. Class Hash:

Here, class Hash provides methods that are useful for:

Class Hash also includes methods from module Enumerable.

Methods for Creating a Hash

  • ::[]: Returns a new hash populated with given objects.

  • ::new: Returns a new empty hash.

  • ::try_convert: Returns a new hash created from a given object.

Methods for Setting Hash State

  • compare_by_identity: Sets self to consider only identity in comparing keys.

  • default=: Sets the default to a given value.

  • default_proc=: Sets the default proc to a given proc.

  • rehash: Rebuilds the hash table by recomputing the hash index for each key.

Methods for Querying

  • any?: Returns whether any element satisfies a given criterion.

  • compare_by_identity?: Returns whether the hash considers only identity when comparing keys.

  • default: Returns the default value, or the default value for a given key.

  • default_proc: Returns the default proc.

  • empty?: Returns whether there are no entries.

  • eql?: Returns whether a given object is equal to self.

  • hash: Returns the integer hash code.

  • has_value?: Returns whether a given object is a value in self.

  • include?, has_key?, member?, key?: Returns whether a given object is a key in self.

  • length, size: Returns the count of entries.

  • value?: Returns whether a given object is a value in self.

Methods for Comparing

  • <: Returns whether self is a proper subset of a given object.

  • <=: Returns whether self is a subset of a given object.

  • ==: Returns whether a given object is equal to self.

  • >: Returns whether self is a proper superset of a given object

  • >=: Returns whether self is a superset of a given object.

Methods for Fetching

  • []: Returns the value associated with a given key.

  • assoc: Returns a 2-element array containing a given key and its value.

  • dig: Returns the object in nested objects that is specified by a given key and additional arguments.

  • fetch: Returns the value for a given key.

  • fetch_values: Returns array containing the values associated with given keys.

  • key: Returns the key for the first-found entry with a given value.

  • keys: Returns an array containing all keys in self.

  • rassoc: Returns a 2-element array consisting of the key and value of the first-found entry having a given value.

  • values: Returns an array containing all values in self/

  • values_at: Returns an array containing values for given keys.

Methods for Assigning

  • []=, store: Associates a given key with a given value.

  • merge: Returns the hash formed by merging each given hash into a copy of self.

  • merge!, update: Merges each given hash into self.

  • replace: Replaces the entire contents of self with the contents of a given hash.

Methods for Deleting

These methods remove entries from self:

  • clear: Removes all entries from self.

  • compact!: Removes all nil-valued entries from self.

  • delete: Removes the entry for a given key.

  • delete_if: Removes entries selected by a given block.

  • filter!, select!: Keep only those entries selected by a given block.

  • keep_if: Keep only those entries selected by a given block.

  • reject!: Removes entries selected by a given block.

  • shift: Removes and returns the first entry.

These methods return a copy of self with some entries removed:

  • compact: Returns a copy of self with all nil-valued entries removed.

  • except: Returns a copy of self with entries removed for specified keys.

  • filter, select: Returns a copy of self with only those entries selected by a given block.

  • reject: Returns a copy of self with entries removed as specified by a given block.

  • slice: Returns a hash containing the entries for given keys.

Methods for Iterating

  • each, each_pair: Calls a given block with each key-value pair.

  • each_key: Calls a given block with each key.

  • each_value: Calls a given block with each value.

Methods for Converting

  • inspect, to_s: Returns a new String containing the hash entries.

  • to_a: Returns a new array of 2-element arrays; each nested array contains a key-value pair from self.

  • to_h: Returns self if a Hash; if a subclass of Hash, returns a Hash containing the entries from self.

  • to_hash: Returns self.

  • to_proc: Returns a proc that maps a given key to its value.

Methods for Transforming Keys and Values

Other Methods

  • flatten: Returns an array that is a 1-dimensional flattening of self.

  • invert: Returns a hash with the each key-value pair inverted.

Class Methods

Hash[] → new_empty_hash

Hash[hash] → new_hash

Hash[ [*2_element_arrays] ] → new_hash

Hash[*objects] → new_hash
::

Returns a new Hash object populated with the given objects, if any. See Hash::new.

With no argument, returns a new empty Hash.

When the single given argument is a Hash, returns a new Hash populated with the entries from the given Hash, excluding the default value or proc.

h = {foo: 0, bar: 1, baz: 2}
Hash[h] # => {:foo=>0, :bar=>1, :baz=>2}

When the single given argument is an Array of 2-element Arrays, returns a new Hash object wherein each 2-element array forms a key-value entry:

Hash[ [ [:foo, 0], [:bar, 1] ] ] # => {:foo=>0, :bar=>1}

When the argument count is an even number; returns a new Hash object wherein each successive pair of arguments has become a key-value entry:

Hash[:foo, 0, :bar, 1] # => {:foo=>0, :bar=>1}

Raises an exception if the argument list does not conform to any of the above.

Hash.ruby2_keywords_hash(hash) → hash
::

Duplicates a given hash and adds a ruby2_keywords flag. This method is not for casual use; debugging, researching, and some truly necessary cases like deserialization of arguments.

h = {k: 1}
h = Hash.ruby2_keywords_hash(h)
def foo(k: 42)
  k
end
foo(*[h]) #=> 1 with neither a warning or an error

Hash.ruby2_keywords_hash?(hash) → true or false
::

Checks if a given hash is flagged by Module#ruby2_keywords (or Proc#ruby2_keywords). This method is not for casual use; debugging, researching, and some truly necessary cases like serialization of arguments.

ruby2_keywords def foo(*args)
  Hash.ruby2_keywords_hash?(args.last)
end
foo(k: 1)   #=> true
foo({k: 1}) #=> false

Hash.try_convert(obj) → obj, new_hash, or nil
::

If obj is a Hash object, returns obj.

Otherwise if obj responds to :to_hash, calls obj.to_hash and returns the result.

Returns nil if obj does not respond to :to_hash

Raises an exception unless obj.to_hash returns a Hash object.

Instance Methods

hash < other_hash → true or false
#

Returns true if hash is a proper subset of other_hash, false otherwise:

h1 = {foo: 0, bar: 1}
h2 = {foo: 0, bar: 1, baz: 2}
h1 < h2 # => true
h2 < h1 # => false
h1 < h1 # => false

hash <= other_hash → true or false
#

Returns true if hash is a subset of other_hash, false otherwise:

h1 = {foo: 0, bar: 1}
h2 = {foo: 0, bar: 1, baz: 2}
h1 <= h2 # => true
h2 <= h1 # => false
h1 <= h1 # => true

hash == object → true or false
#

Returns true if all of the following are true:

  • object is a Hash object.

  • hash and object have the same keys (regardless of order).

  • For each key key, hash[key] == object[key].

Otherwise, returns false.

Equal:

h1 = {foo: 0, bar: 1, baz: 2}
h2 = {foo: 0, bar: 1, baz: 2}
h1 == h2 # => true
h3 = {baz: 2, bar: 1, foo: 0}
h1 == h3 # => true

hash > other_hash → true or false
#

Returns true if hash is a proper superset of other_hash, false otherwise:

h1 = {foo: 0, bar: 1, baz: 2}
h2 = {foo: 0, bar: 1}
h1 > h2 # => true
h2 > h1 # => false
h1 > h1 # => false

hash >= other_hash → true or false
#

Returns true if hash is a superset of other_hash, false otherwise:

h1 = {foo: 0, bar: 1, baz: 2}
h2 = {foo: 0, bar: 1}
h1 >= h2 # => true
h2 >= h1 # => false
h1 >= h1 # => true

hash[key] → value
#

Returns the value associated with the given key, if found:

h = {foo: 0, bar: 1, baz: 2}
h[:foo] # => 0

If key is not found, returns a default value (see Default Values):

h = {foo: 0, bar: 1, baz: 2}
h[:nosuch] # => nil

hash[key] = value → value
#

Associates the given value with the given key; returns value.

If the given key exists, replaces its value with the given value; the ordering is not affected (see Entry Order):

h = {foo: 0, bar: 1}
h[:foo] = 2 # => 2
h.store(:bar, 3) # => 3
h # => {:foo=>2, :bar=>3}

If key does not exist, adds the key and value; the new entry is last in the order (see Entry Order):

h = {foo: 0, bar: 1}
h[:baz] = 2 # => 2
h.store(:bat, 3) # => 3
h # => {:foo=>0, :bar=>1, :baz=>2, :bat=>3}

hash.any? → true or false

hash.any?(object) → true or false

hash.any? {|key, value| ... } → true or false
#

Returns true if any element satisfies a given criterion; false otherwise.

If self has no element, returns false and argument or block are not used.

With no argument and no block, returns true if self is non-empty; false if empty.

With argument object and no block, returns true if for any key key h.assoc(key) == object:

h = {foo: 0, bar: 1, baz: 2}
h.any?([:bar, 1]) # => true
h.any?([:bar, 0]) # => false
h.any?([:baz, 1]) # => false

With no argument and a block, calls the block with each key-value pair; returns true if the block returns any truthy value, false otherwise:

h = {foo: 0, bar: 1, baz: 2}
h.any? {|key, value| value < 3 } # => true
h.any? {|key, value| value > 3 } # => false

Related: Enumerable#any?

hash.assoc(key) → new_array or nil
#

If the given key is found, returns a 2-element Array containing that key and its value:

h = {foo: 0, bar: 1, baz: 2}
h.assoc(:bar) # => [:bar, 1]

Returns nil if key key is not found.

hash.clear → self
#

Removes all hash entries; returns self.

hash.compact → new_hash
#

Returns a copy of self with all nil-valued entries removed:

h = {foo: 0, bar: nil, baz: 2, bat: nil}
h1 = h.compact
h1 # => {:foo=>0, :baz=>2}

hash.compact! → self or nil
#

Returns self with all its nil-valued entries removed (in place):

h = {foo: 0, bar: nil, baz: 2, bat: nil}
h.compact! # => {:foo=>0, :baz=>2}

Returns nil if no entries were removed.

hash.compare_by_identity → self
#

Sets self to consider only identity in comparing keys; two keys are considered the same only if they are the same object; returns self.

By default, these two object are considered to be the same key, so s1 will overwrite s0:

s0 = 'x'
s1 = 'x'
h = {}
h.compare_by_identity? # => false
h[s0] = 0
h[s1] = 1
h # => {"x"=>1}

After calling #compare_by_identity, the keys are considered to be different, and therefore do not overwrite each other:

h = {}
h.compare_by_identity # => {}
h.compare_by_identity? # => true
h[s0] = 0
h[s1] = 1
h # => {"x"=>0, "x"=>1}

hash.compare_by_identity? → true or false
#

Returns true if compare_by_identity has been called, false otherwise.

hash.default → object

hash.default(key) → object
#

Returns the default value for the given key. The returned value will be determined either by the default proc or by the default value. See Default Values.

With no argument, returns the current default value:

h = {}
h.default # => nil

If key is given, returns the default value for key, regardless of whether that key exists:

h = Hash.new { |hash, key| hash[key] = "No key #{key}"}
h[:foo] = "Hello"
h.default(:foo) # => "No key foo"

hash.default = value → object
#

Sets the default value to value; returns value:

h = {}
h.default # => nil
h.default = false # => false
h.default # => false

See Default Values.

hash.default_proc → proc or nil
#

Returns the default proc for self (see Default Values):

h = {}
h.default_proc # => nil
h.default_proc = proc {|hash, key| "Default value for #{key}" }
h.default_proc.class # => Proc

hash.default_proc = proc → proc
#

Sets the default proc for self to proc (see Default Values):

h = {}
h.default_proc # => nil
h.default_proc = proc { |hash, key| "Default value for #{key}" }
h.default_proc.class # => Proc
h.default_proc = nil
h.default_proc # => nil

hash.delete(key) → value or nil

hash.delete(key) {|key| ... } → object
#

Deletes the entry for the given key and returns its associated value.

If no block is given and key is found, deletes the entry and returns the associated value:

h = {foo: 0, bar: 1, baz: 2}
h.delete(:bar) # => 1
h # => {:foo=>0, :baz=>2}

If no block given and key is not found, returns nil.

If a block is given and key is found, ignores the block, deletes the entry, and returns the associated value:

h = {foo: 0, bar: 1, baz: 2}
h.delete(:baz) { |key| raise 'Will never happen'} # => 2
h # => {:foo=>0, :bar=>1}

If a block is given and key is not found, calls the block and returns the block’s return value:

h = {foo: 0, bar: 1, baz: 2}
h.delete(:nosuch) { |key| "Key #{key} not found" } # => "Key nosuch not found"
h # => {:foo=>0, :bar=>1, :baz=>2}

hash.delete_if {|key, value| ... } → self

hash.delete_if → new_enumerator
#

If a block given, calls the block with each key-value pair; deletes each entry for which the block returns a truthy value; returns self:

h = {foo: 0, bar: 1, baz: 2}
h.delete_if {|key, value| value > 0 } # => {:foo=>0}

If no block given, returns a new Enumerator:

h = {foo: 0, bar: 1, baz: 2}
e = h.delete_if # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:delete_if>
e.each { |key, value| value > 0 } # => {:foo=>0}

hash.dig(key, *identifiers) → object
#

Finds and returns the object in nested objects that is specified by key and identifiers. The nested objects may be instances of various classes. See Dig Methods.

Nested Hashes:

h = {foo: {bar: {baz: 2}}}
h.dig(:foo) # => {:bar=>{:baz=>2}}
h.dig(:foo, :bar) # => {:baz=>2}
h.dig(:foo, :bar, :baz) # => 2
h.dig(:foo, :bar, :BAZ) # => nil

Nested Hashes and Arrays:

h = {foo: {bar: [:a, :b, :c]}}
h.dig(:foo, :bar, 2) # => :c

This method will use the default values for keys that are not present:

h = {foo: {bar: [:a, :b, :c]}}
h.dig(:hello) # => nil
h.default_proc = -> (hash, _key) { hash }
h.dig(:hello, :world) # => h
h.dig(:hello, :world, :foo, :bar, 2) # => :c

hash.each {|key, value| ... } → self

hash.each → new_enumerator
#
An alias for each_pair

hash.each_key {|key| ... } → self

hash.each_key → new_enumerator
#

Calls the given block with each key; returns self:

h = {foo: 0, bar: 1, baz: 2}
h.each_key {|key| puts key }  # => {:foo=>0, :bar=>1, :baz=>2}

Output:

foo
bar
baz

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.each_key # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:each_key>
h1 = e.each {|key| puts key }
h1 # => {:foo=>0, :bar=>1, :baz=>2}

Output:

foo
bar
baz

hash.each_pair {|key, value| ... } → self

hash.each_pair → new_enumerator
#

Calls the given block with each key-value pair; returns self:

h = {foo: 0, bar: 1, baz: 2}
h.each_pair {|key, value| puts "#{key}: #{value}"} # => {:foo=>0, :bar=>1, :baz=>2}

Output:

foo: 0
bar: 1
baz: 2

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.each_pair # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:each_pair>
h1 = e.each {|key, value| puts "#{key}: #{value}"}
h1 # => {:foo=>0, :bar=>1, :baz=>2}

Output:

foo: 0
bar: 1
baz: 2

hash.each_value {|value| ... } → self

hash.each_value → new_enumerator
#

Calls the given block with each value; returns self:

h = {foo: 0, bar: 1, baz: 2}
h.each_value {|value| puts value } # => {:foo=>0, :bar=>1, :baz=>2}

Output:

0
1
2

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.each_value # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:each_value>
h1 = e.each {|value| puts value }
h1 # => {:foo=>0, :bar=>1, :baz=>2}

Output:

0
1
2

hash.empty? → true or false
#

Returns true if there are no hash entries, false otherwise:

{}.empty? # => true
{foo: 0, bar: 1, baz: 2}.empty? # => false

hash.eql?(object) → true or false
#

Returns true if all of the following are true:

  • object is a Hash object.

  • hash and object have the same keys (regardless of order).

  • For each key key, h[key].eql?(object[key]).

Otherwise, returns false.

h1 = {foo: 0, bar: 1, baz: 2}
h2 = {foo: 0, bar: 1, baz: 2}
h1.eql? h2 # => true
h3 = {baz: 2, bar: 1, foo: 0}
h1.eql? h3 # => true

hsh.except(*keys) → a_hash
#

Returns a new Hash excluding entries for the given keys:

h = { a: 100, b: 200, c: 300 }
h.except(:a)          #=> {:b=>200, :c=>300}

Any given keys that are not found are ignored.

hash.fetch(key) → object

hash.fetch(key, default_value) → object

hash.fetch(key) {|key| ... } → object
#

Returns the value for the given key, if found.

h = {foo: 0, bar: 1, baz: 2}
h.fetch(:bar) # => 1

If key is not found and no block was given, returns default_value:

{}.fetch(:nosuch, :default) # => :default

If key is not found and a block was given, yields key to the block and returns the block’s return value:

{}.fetch(:nosuch) {|key| "No key #{key}"} # => "No key nosuch"

Raises KeyError if neither default_value nor a block was given.

Note that this method does not use the values of either default or default_proc.

hash.fetch_values(*keys) → new_array

hash.fetch_values(*keys) {|key| ... } → new_array
#

Returns a new Array containing the values associated with the given keys *keys:

h = {foo: 0, bar: 1, baz: 2}
h.fetch_values(:baz, :foo) # => [2, 0]

Returns a new empty Array if no arguments given.

When a block is given, calls the block with each missing key, treating the block’s return value as the value for that key:

h = {foo: 0, bar: 1, baz: 2}
values = h.fetch_values(:bar, :foo, :bad, :bam) {|key| key.to_s}
values # => [1, 0, "bad", "bam"]

When no block is given, raises an exception if any given key is not found.

filter
#
An alias for select

filter!
#
An alias for select!

hash.flatten → new_array

hash.flatten(level) → new_array
#

Returns a new Array object that is a 1-dimensional flattening of self.

By default, nested Arrays are not flattened:

h = {foo: 0, bar: [:bat, 3], baz: 2}
h.flatten # => [:foo, 0, :bar, [:bat, 3], :baz, 2]

Takes the depth of recursive flattening from Integer argument level:

h = {foo: 0, bar: [:bat, [:baz, [:bat, ]]]}
h.flatten(1) # => [:foo, 0, :bar, [:bat, [:baz, [:bat]]]]
h.flatten(2) # => [:foo, 0, :bar, :bat, [:baz, [:bat]]]
h.flatten(3) # => [:foo, 0, :bar, :bat, :baz, [:bat]]
h.flatten(4) # => [:foo, 0, :bar, :bat, :baz, :bat]

When level is negative, flattens all nested Arrays:

h = {foo: 0, bar: [:bat, [:baz, [:bat, ]]]}
h.flatten(-1) # => [:foo, 0, :bar, :bat, :baz, :bat]
h.flatten(-2) # => [:foo, 0, :bar, :bat, :baz, :bat]

When level is zero, returns the equivalent of to_a :

h = {foo: 0, bar: [:bat, 3], baz: 2}
h.flatten(0) # => [[:foo, 0], [:bar, [:bat, 3]], [:baz, 2]]
h.flatten(0) == h.to_a # => true

hash.has_key?(key) → true or false
#
An alias for include?

hash.has_value?(value) → true or false
#

Returns true if value is a value in self, otherwise false.

hash.hash → an_integer
#

Returns the Integer hash-code for the hash.

Two Hash objects have the same hash-code if their content is the same (regardless of order):

h1 = {foo: 0, bar: 1, baz: 2}
h2 = {baz: 2, bar: 1, foo: 0}
h2.hash == h1.hash # => true
h2.eql? h1 # => true

hash.include?(key) → true or false
#

Returns true if key is a key in self, otherwise false.

hash.replace(other_hash) -> self
#

Replaces the entire contents of self with the contents of other_hash; returns self:

h = {foo: 0, bar: 1, baz: 2}
h.replace({bat: 3, bam: 4}) # => {:bat=>3, :bam=>4}

hash.inspect → new_string
#

Returns a new String containing the hash entries:

h = {foo: 0, bar: 1, baz: 2}
h.inspect # => "{:foo=>0, :bar=>1, :baz=>2}"

hash.invert → new_hash
#

Returns a new Hash object with the each key-value pair inverted:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.invert
h1 # => {0=>:foo, 1=>:bar, 2=>:baz}

Overwrites any repeated new keys: (see Entry Order):

h = {foo: 0, bar: 0, baz: 0}
h.invert # => {0=>:baz}

hash.keep_if {|key, value| ... } → self

hash.keep_if → new_enumerator
#

Calls the block for each key-value pair; retains the entry if the block returns a truthy value; otherwise deletes the entry; returns self.

h = {foo: 0, bar: 1, baz: 2}
h.keep_if { |key, value| key.start_with?('b') } # => {:bar=>1, :baz=>2}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.keep_if # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:keep_if>
e.each { |key, value| key.start_with?('b') } # => {:bar=>1, :baz=>2}

hash.key(value) → key or nil
#

Returns the key for the first-found entry with the given value (see Entry Order):

h = {foo: 0, bar: 2, baz: 2}
h.key(0) # => :foo
h.key(2) # => :bar

Returns nil if no such value is found.

hash.key?(key) → true or false
#
An alias for include?

hash.keys → new_array
#

Returns a new Array containing all keys in self:

h = {foo: 0, bar: 1, baz: 2}
h.keys # => [:foo, :bar, :baz]

hash.length → integer
#
An alias for size

hash.member?(key) → true or false
#
An alias for include?

hash.merge → copy_of_self

hash.merge(*other_hashes) → new_hash

hash.merge(*other_hashes) { |key, old_value, new_value| ... } → new_hash
#

Returns the new Hash formed by merging each of other_hashes into a copy of self.

Each argument in other_hashes must be a Hash.

With arguments and no block:

  • Returns the new Hash object formed by merging each successive Hash in other_hashes into self.

  • Each new-key entry is added at the end.

  • Each duplicate-key entry’s value overwrites the previous value.

Example:

h = {foo: 0, bar: 1, baz: 2}
h1 = {bat: 3, bar: 4}
h2 = {bam: 5, bat:6}
h.merge(h1, h2) # => {:foo=>0, :bar=>4, :baz=>2, :bat=>6, :bam=>5}

With arguments and a block:

  • Returns a new Hash object that is the merge of self and each given hash.

  • The given hashes are merged left to right.

  • Each new-key entry is added at the end.

  • For each duplicate key:

    • Calls the block with the key and the old and new values.

    • The block’s return value becomes the new value for the entry.

Example:

h = {foo: 0, bar: 1, baz: 2}
h1 = {bat: 3, bar: 4}
h2 = {bam: 5, bat:6}
h3 = h.merge(h1, h2) { |key, old_value, new_value| old_value + new_value }
h3 # => {:foo=>0, :bar=>5, :baz=>2, :bat=>9, :bam=>5}

With no arguments:

  • Returns a copy of self.

  • The block, if given, is ignored.

Example:

h = {foo: 0, bar: 1, baz: 2}
h.merge # => {:foo=>0, :bar=>1, :baz=>2}
h1 = h.merge { |key, old_value, new_value| raise 'Cannot happen' }
h1 # => {:foo=>0, :bar=>1, :baz=>2}

hash.merge! → self

hash.merge!(*other_hashes) → self

hash.merge!(*other_hashes) { |key, old_value, new_value| ... } → self
#
An alias for update

hash.rassoc(value) → new_array or nil
#

Returns a new 2-element Array consisting of the key and value of the first-found entry whose value is == to value (see Entry Order):

h = {foo: 0, bar: 1, baz: 1}
h.rassoc(1) # => [:bar, 1]

Returns nil if no such value found.

hash.rehash → self
#

Rebuilds the hash table by recomputing the hash index for each key; returns self.

The hash table becomes invalid if the hash value of a key has changed after the entry was created. See Modifying an Active Hash Key.

hash.reject {|key, value| ... } → new_hash

hash.reject → new_enumerator
#

Returns a new Hash object whose entries are all those from self for which the block returns false or nil:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.reject {|key, value| key.start_with?('b') }
h1 # => {:foo=>0}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.reject # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:reject>
h1 = e.each {|key, value| key.start_with?('b') }
h1 # => {:foo=>0}

hash.reject! {|key, value| ... } → self or nil

hash.reject! → new_enumerator
#

Returns self, whose remaining entries are those for which the block returns false or nil:

h = {foo: 0, bar: 1, baz: 2}
h.reject! {|key, value| value < 2 } # => {:baz=>2}

Returns nil if no entries are removed.

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.reject! # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:reject!>
e.each {|key, value| key.start_with?('b') } # => {:foo=>0}

hash.replace(other_hash) → self
#
An alias for initialize_copy

hash.select {|key, value| ... } → new_hash

hash.select → new_enumerator
#

Returns a new Hash object whose entries are those for which the block returns a truthy value:

h = {foo: 0, bar: 1, baz: 2}
h.select {|key, value| value < 2 } # => {:foo=>0, :bar=>1}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.select # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:select>
e.each {|key, value| value < 2 } # => {:foo=>0, :bar=>1}

hash.select! {|key, value| ... } → self or nil

hash.select! → new_enumerator
#

Returns self, whose entries are those for which the block returns a truthy value:

h = {foo: 0, bar: 1, baz: 2}
h.select! {|key, value| value < 2 }  => {:foo=>0, :bar=>1}

Returns nil if no entries were removed.

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.select!  # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:select!>
e.each { |key, value| value < 2 } # => {:foo=>0, :bar=>1}

hash.shift → [key, value] or nil
#

Removes the first hash entry (see Entry Order); returns a 2-element Array containing the removed key and value:

h = {foo: 0, bar: 1, baz: 2}
h.shift # => [:foo, 0]
h # => {:bar=>1, :baz=>2}

Returns nil if the hash is empty.

hash.size → integer
#

Returns the count of entries in self:

{foo: 0, bar: 1, baz: 2}.length # => 3

hash.slice(*keys) → new_hash
#

Returns a new Hash object containing the entries for the given keys:

h = {foo: 0, bar: 1, baz: 2}
h.slice(:baz, :foo) # => {:baz=>2, :foo=>0}

Any given keys that are not found are ignored.

hash.store(key, value)
#
An alias for []=

hash.to_a → new_array
#

Returns a new Array of 2-element Array objects; each nested Array contains a key-value pair from self:

h = {foo: 0, bar: 1, baz: 2}
h.to_a # => [[:foo, 0], [:bar, 1], [:baz, 2]]

hash.to_h → self or new_hash

hash.to_h {|key, value| ... } → new_hash
#

For an instance of Hash, returns self.

For a subclass of Hash, returns a new Hash containing the content of self.

When a block is given, returns a new Hash object whose content is based on the block; the block should return a 2-element Array object specifying the key-value pair to be included in the returned Array:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.to_h {|key, value| [value, key] }
h1 # => {0=>:foo, 1=>:bar, 2=>:baz}

hash.to_hash → self
#

Returns self.

hash.to_proc → proc
#

Returns a Proc object that maps a key to its value:

h = {foo: 0, bar: 1, baz: 2}
proc = h.to_proc
proc.class # => Proc
proc.call(:foo) # => 0
proc.call(:bar) # => 1
proc.call(:nosuch) # => nil

to_s
#
An alias for inspect

hash.transform_keys {|key| ... } → new_hash

hash.transform_keys(hash2) → new_hash

hash.transform_keys(hash2) {|other_key| ...} → new_hash

hash.transform_keys → new_enumerator
#

Returns a new Hash object; each entry has:

  • A key provided by the block.

  • The value from self.

An optional hash argument can be provided to map keys to new keys. Any key not given will be mapped using the provided block, or remain the same if no block is given.

Transform keys:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.transform_keys {|key| key.to_s }
h1 # => {"foo"=>0, "bar"=>1, "baz"=>2}

h.transform_keys(foo: :bar, bar: :foo)
#=> {bar: 0, foo: 1, baz: 2}

h.transform_keys(foo: :hello, &:to_s)
#=> {:hello=>0, "bar"=>1, "baz"=>2}

Overwrites values for duplicate keys:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.transform_keys {|key| :bat }
h1 # => {:bat=>2}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.transform_keys # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:transform_keys>
h1 = e.each { |key| key.to_s }
h1 # => {"foo"=>0, "bar"=>1, "baz"=>2}

hash.transform_keys! {|key| ... } → self

hash.transform_keys!(hash2) → self

hash.transform_keys!(hash2) {|other_key| ...} → self

hash.transform_keys! → new_enumerator
#

Same as Hash#transform_keys but modifies the receiver in place instead of returning a new hash.

hash.transform_values {|value| ... } → new_hash

hash.transform_values → new_enumerator
#

Returns a new Hash object; each entry has:

  • A key from self.

  • A value provided by the block.

Transform values:

h = {foo: 0, bar: 1, baz: 2}
h1 = h.transform_values {|value| value * 100}
h1 # => {:foo=>0, :bar=>100, :baz=>200}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.transform_values # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:transform_values>
h1 = e.each { |value| value * 100}
h1 # => {:foo=>0, :bar=>100, :baz=>200}

hash.transform_values! {|value| ... } → self

hash.transform_values! → new_enumerator
#

Returns self, whose keys are unchanged, and whose values are determined by the given block.

h = {foo: 0, bar: 1, baz: 2}
h.transform_values! {|value| value * 100} # => {:foo=>0, :bar=>100, :baz=>200}

Returns a new Enumerator if no block given:

h = {foo: 0, bar: 1, baz: 2}
e = h.transform_values! # => #<Enumerator: {:foo=>0, :bar=>100, :baz=>200}:transform_values!>
h1 = e.each {|value| value * 100}
h1 # => {:foo=>0, :bar=>100, :baz=>200}

hash.merge! -> self hash.merge!(*other_hashes) -> self hash.merge!(*other_hashes) { |key, old_value, new_value| ... } -> self
#

Merges each of other_hashes into self; returns self.

Each argument in other_hashes must be a Hash.

With arguments and no block:

  • Returns self, after the given hashes are merged into it.

  • The given hashes are merged left to right.

  • Each new entry is added at the end.

  • Each duplicate-key entry’s value overwrites the previous value.

Example:

h = {foo: 0, bar: 1, baz: 2}
h1 = {bat: 3, bar: 4}
h2 = {bam: 5, bat:6}
h.merge!(h1, h2) # => {:foo=>0, :bar=>4, :baz=>2, :bat=>6, :bam=>5}

With arguments and a block:

  • Returns self, after the given hashes are merged.

  • The given hashes are merged left to right.

  • Each new-key entry is added at the end.

  • For each duplicate key:

    • Calls the block with the key and the old and new values.

    • The block’s return value becomes the new value for the entry.

Example:

h = {foo: 0, bar: 1, baz: 2}
h1 = {bat: 3, bar: 4}
h2 = {bam: 5, bat:6}
h3 = h.merge!(h1, h2) { |key, old_value, new_value| old_value + new_value }
h3 # => {:foo=>0, :bar=>5, :baz=>2, :bat=>9, :bam=>5}

With no arguments:

  • Returns self, unmodified.

  • The block, if given, is ignored.

Example:

h = {foo: 0, bar: 1, baz: 2}
h.merge # => {:foo=>0, :bar=>1, :baz=>2}
h1 = h.merge! { |key, old_value, new_value| raise 'Cannot happen' }
h1 # => {:foo=>0, :bar=>1, :baz=>2}

hash.value?(value) → true or false
#
An alias for has_value?

hash.values → new_array
#

Returns a new Array containing all values in self:

h = {foo: 0, bar: 1, baz: 2}
h.values # => [0, 1, 2]

hash.values_at(*keys) → new_array
#

Returns a new Array containing values for the given keys:

h = {foo: 0, bar: 1, baz: 2}
h.values_at(:baz, :foo) # => [2, 0]

The default values are returned for any keys that are not found:

h.values_at(:hello, :foo) # => [nil, 0]