class Set

The Set class implements a collection of unordered values with no duplicates. It is a hybrid of Array’s intuitive inter-operation facilities and Hash’s fast lookup.

Set is easy to use with Enumerable objects (implementing each). Most of the initializer methods and binary operators accept generic Enumerable objects besides sets and arrays. An Enumerable object can be converted to Set using the to_set method.

Set uses a data structure similar to Hash for storage, except that it only has keys and no values.

Equality of elements is determined according to Object#eql? and Object#hash. Use Set#compare_by_identity to make a set compare its elements by their identity.
Set assumes that the identity of each element does not change while it is stored. Modifying an element of a set will render the set to an unreliable state.
When a string is to be stored, a frozen copy of the string is stored instead unless the original string is already frozen.

Comparison

The comparison operators <, >, <=, and >= are implemented as shorthand for the {proper_,}{subset?,superset?} methods. The <=> operator reflects this order, or returns nil for sets that both have distinct elements ({x, y} vs. {x, z} for example).

Example

s1 = Set[1, 2]                        #=> Set[1, 2]
s2 = [1, 2].to_set                    #=> Set[1, 2]
s1 == s2                              #=> true
s1.add("foo")                         #=> Set[1, 2, "foo"]
s1.merge([2, 6])                      #=> Set[1, 2, "foo", 6]
s1.subset?(s2)                        #=> false
s2.subset?(s1)                        #=> true

Contact

Akinori MUSHA knu@iDaemons.org (current maintainer)

Inheriting from `Set`

Before Ruby 4.0 (released December 2025), Set had a different, less efficient implementation. It was reimplemented in C, and the behavior of some of the core methods were adjusted.

To keep backward compatibility, when a class is inherited from Set, additional module Set::SubclassCompatible is included, which makes the inherited class behavior, as well as internal method names, closer to what it was before Ruby 4.0.

It can be easily seen, for example, in the inspect method behavior:

p Set[1, 2, 3]
# prints "Set[1, 2, 3]"

class MySet < Set
end
p MySet[1, 2, 3]
# prints "#<MySet: {1, 2, 3}>", like it was in Ruby 3.4

For new code, if backward compatibility is not necessary, it is recommended to instead inherit from Set::CoreSet, which avoids including the “compatibility” layer:

class MyCoreSet < Set::CoreSet
end
p MyCoreSet[1, 2, 3]
# prints "MyCoreSet[1, 2, 3]"

Set’s methods

First, what’s elsewhere. Class Set:

Inherits from class Object.
Includes module Enumerable, which provides dozens of additional methods.

In particular, class Set does not have many methods of its own for fetching or for iterating. Instead, it relies on those in Enumerable.

Here, class Set provides methods that are useful for:

Methods for Creating a `Set`

::[]: Returns a new set containing the given objects.
::new: Returns a new set containing either the given objects (if no block given) or the return values from the called block (if a block given).

Methods for `Set` Operations

| (aliased as union and +): Returns a new set containing all elements from self and all elements from a given enumerable (no duplicates).
& (aliased as intersection): Returns a new set containing all elements common to self and a given enumerable.
- (aliased as difference): Returns a copy of self with all elements in a given enumerable removed.
^: Returns a new set containing all elements from self and a given enumerable except those common to both.

Methods for Comparing

<=>: Returns -1, 0, or 1 as self is less than, equal to, or greater than a given object.
==: Returns whether self and a given enumerable are equal, as determined by Object#eql?.
compare_by_identity?: Returns whether the set considers only identity when comparing elements.

Methods for Querying

length (aliased as size): Returns the count of elements.
empty?: Returns whether the set has no elements.
include? (aliased as member? and #===): Returns whether a given object is an element in the set.
subset? (aliased as <=): Returns whether a given object is a subset of the set.
proper_subset? (aliased as <): Returns whether a given enumerable is a proper subset of the set.
superset? (aliased as >=): Returns whether a given enumerable is a superset of the set.
proper_superset? (aliased as >): Returns whether a given enumerable is a proper superset of the set.
disjoint?: Returns true if the set and a given enumerable have no common elements, false otherwise.
intersect?: Returns true if the set and a given enumerable: have any common elements, false otherwise.
compare_by_identity?: Returns whether the set considers only identity when comparing elements.

Methods for Assigning

add (aliased as <<): Adds a given object to the set; returns self.
add?: If the given object is not an element in the set, adds it and returns self; otherwise, returns nil.
merge: Merges the elements of each given enumerable object to the set; returns self.
replace: Replaces the contents of the set with the contents of a given enumerable.

Methods for Deleting

clear: Removes all elements in the set; returns self.
delete: Removes a given object from the set; returns self.
delete?: If the given object is an element in the set, removes it and returns self; otherwise, returns nil.
subtract: Removes each given object from the set; returns self.
delete_if - Removes elements specified by a given block.
select! (aliased as filter!): Removes elements not specified by a given block.
keep_if: Removes elements not specified by a given block.
reject! Removes elements specified by a given block.

Methods for Converting

classify: Returns a hash that classifies the elements, as determined by the given block.
collect! (aliased as map!): Replaces each element with a block return-value.
divide: Returns a hash that classifies the elements, as determined by the given block; differs from classify in that the block may accept either one or two arguments.
flatten: Returns a new set that is a recursive flattening of self.
flatten!: Replaces each nested set in self with the elements from that set.
inspect (aliased as to_s): Returns a string displaying the elements.
join: Returns a string containing all elements, converted to strings as needed, and joined by the given record separator.
to_a: Returns an array containing all set elements.
to_set: Returns self if given no arguments and no block; with a block given, returns a new set consisting of block return values.

Methods for Iterating

each: Calls the block with each successive element; returns self.

Other Methods

reset: Resets the internal state; useful if an object has been modified while an element in the set.

Public Class Methods

[]

[X] (*X) → Set[X]

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 225
def self.[]: [X] (*X) -> Set[X]

Returns a new Set object populated with the given objects, See Set::new.

json_create (object)

[A] (Hash[String, String | Array[A]] object) → Set[A]

Source

# File vendor/bundle/ruby/4.0.0/gems/json-2.21.0/lib/json/add/set.rb, line 9
def self.json_create(object)
  new object['a']
end

See as_json.

new

(_Each[A]) → untyped
[X] (_Each[X]) { (X) → A } → untyped
(?nil) → untyped

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 215
def initialize: (_Each[A]) -> untyped
                | [X] (_Each[X]) { (X) -> A } -> untyped
                | (?nil) -> untyped

Creates a new set containing the elements of the given enumerable object.

If a block is given, the elements of enum are preprocessed by the given block.

Set.new([1, 2])                       #=> Set[1, 2]
Set.new([1, 2, 1])                    #=> Set[1, 2]
Set.new([1, 'c', :s])                 #=> Set[1, "c", :s]
Set.new(1..5)                         #=> Set[1, 2, 3, 4, 5]
Set.new([1, 2, 3]) { |x| x * x }      #=> Set[1, 4, 9]

Public Instance Methods

(_Each[A]) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 237
def &: (_Each[A]) -> self

Returns a new set containing elements common to the set and the given enumerable object.

Set[1, 3, 5] & Set[3, 2, 1]             #=> Set[3, 1]
Set['a', 'b', 'z'] & ['a', 'b', 'c']    #=> Set["a", "b"]

Also aliased as: intersection

(_Each[A]) → self

Returns a new set built by merging the set and the elements of the given enumerable object.

Set[1, 2, 3] | Set[2, 4, 5]         #=> Set[1, 2, 3, 4, 5]
Set[1, 5, 'z'] | (1..6)             #=> Set[1, 5, "z", 2, 3, 4, 6]

Alias for: |

(_Each[A]) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 288
def -: (_Each[A]) -> self

Returns a new set built by duplicating the set, removing every element that appears in the given enumerable object.

Set[1, 3, 5] - Set[1, 5]                #=> Set[3]
Set['a', 'b', 'z'] - ['a', 'c']         #=> Set["b", "z"]

Also aliased as: difference

(self) → bool

Returns true if the set is a proper subset of the given set.

Alias for: proper_subset?

(A) → self

Adds the given object to the set and returns self. Use Set#merge to add many elements at once.

Set[1, 2].add(3)                    #=> Set[1, 2, 3]
Set[1, 2].add([3, 4])               #=> Set[1, 2, [3, 4]]
Set[1, 2].add(2)                    #=> Set[1, 2]

Alias for: add

(self) → bool

Returns true if the set is a subset of the given set.

Alias for: subset?

(self) → bool

Returns true if the set is a proper superset of the given set.

Alias for: proper_superset?

(self) → bool

Returns true if the set is a superset of the given set.

Alias for: superset?

(_Each[A]) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 395
def ^: (_Each[A]) -> self

Returns a new set containing elements exclusive between the set and the given enumerable object. (set ^ enum) is equivalent to ((set | enum) - (set & enum)).

Set[1, 2] ^ Set[2, 3]                   #=> Set[3, 1]
Set[1, 'b', 'c'] ^ ['b', 'd']           #=> Set["d", 1, "c"]

(_Each[A]) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 258
def |: (_Each[A]) -> self

Returns a new set built by merging the set and the elements of the given enumerable object.

Set[1, 2, 3] | Set[2, 4, 5]         #=> Set[1, 2, 3, 4, 5]
Set[1, 5, 'z'] | (1..6)             #=> Set[1, 5, "z", 2, 3, 4, 6]

Also aliased as: union, +

add

(A) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 310
def add: (A) -> self

Adds the given object to the set and returns self. Use Set#merge to add many elements at once.

Set[1, 2].add(3)                    #=> Set[1, 2, 3]
Set[1, 2].add([3, 4])               #=> Set[1, 2, [3, 4]]
Set[1, 2].add(2)                    #=> Set[1, 2]

Also aliased as: <<

add?

(A) → self?

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 333
def add?: (A) -> self?

Adds the given object to the set and returns self. If the object is already in the set, returns nil.

Set[1, 2].add?(3)                    #=> Set[1, 2, 3]
Set[1, 2].add?([3, 4])               #=> Set[1, 2, [3, 4]]
Set[1, 2].add?(2)                    #=> nil

as_json (*)

(*untyped) → Hash[String, String | Array[A]]

Source

# File vendor/bundle/ruby/4.0.0/gems/json-2.21.0/lib/json/add/set.rb, line 28
def as_json(*)
  {
    JSON.create_id => self.class.name,
    'a'            => to_a,
  }
end

Methods Set#as_json and Set.json_create may be used to serialize and deserialize a Set object; see Marshal.

Method Set#as_json serializes self, returning a 2-element hash representing self:

require 'json/add/set'
x = Set.new(%w/foo bar baz/).as_json
# => {"json_class"=>"Set", "a"=>["foo", "bar", "baz"]}

Method JSON.create deserializes such a hash, returning a Set object:

Set.json_create(x) # => #<Set: {"foo", "bar", "baz"}>

Methods Set#as_json and Set.json_create may be used to serialize and deserialize a Set object; see Marshal.

Method Set#as_json serializes self, returning a 2-element hash representing self:

require 'json/add/set'
x = Set.new(%w/foo bar baz/).as_json
# => {"json_class"=>"Set", "a"=>["foo", "bar", "baz"]}

Method JSON.create deserializes such a hash, returning a Set object:

Set.json_create(x) # => #<Set: {"foo", "bar", "baz"}>

classify

[X] () { (A) → X } → Hash[X, self]

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 414
def classify: [X] () { (A) -> X } -> Hash[X, self]

Classifies the set by the return value of the given block and returns a hash of {value => set of elements} pairs. The block is called once for each element of the set, passing the element as parameter.

files = Set.new(Dir.glob("*.rb"))
hash = files.classify { |f| File.mtime(f).year }
hash       #=> {2000 => Set["a.rb", "b.rb"],
           #    2001 => Set["c.rb", "d.rb", "e.rb"],
           #    2002 => Set["f.rb"]}

Returns an enumerator if no block is given.

clear

() → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 426
def clear: () -> self

Removes all elements and returns self.

set = Set[1, 'c', :s]             #=> Set[1, "c", :s]
set.clear                         #=> Set[]
set                               #=> Set[]

collect!

() { (A) → A } → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 436
def collect!: () { (A) -> A } -> self

Replaces the elements with ones returned by collect. Returns an enumerator if no block is given.

Also aliased as: map!

compare_by_identity

() → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 488
def compare_by_identity: () -> self

Makes the set compare its elements by their identity and returns self.

compare_by_identity?

() → bool

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 497
def compare_by_identity?: () -> bool

Returns true if the set will compare its elements by their identity. Also see Set#compare_by_identity.

delete

(A) → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 451
def delete: (A) -> self

Deletes the given object from the set and returns self. Use subtract to delete many items at once.

delete?

(A) → self?

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 460
def delete?: (A) -> self?

Deletes the given object from the set and returns self. If the object is not in the set, returns nil.

delete_if

() { (A) → untyped } → self

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 470
def delete_if: () { (A) -> untyped } -> self

Deletes every element of the set for which block evaluates to true, and returns self. Returns an enumerator if no block is given.

difference

(_Each[A]) → self

Returns a new set built by duplicating the set, removing every element that appears in the given enumerable object.

Set[1, 3, 5] - Set[1, 5]                #=> Set[3]
Set['a', 'b', 'z'] - ['a', 'c']         #=> Set["b", "z"]

Alias for: -

disjoint?

(Set[A] | Enumerable[A]) → bool

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 511
def disjoint?: (Set[A] | Enumerable[A]) -> bool

Returns true if the set and the given enumerable have no element in common. This method is the opposite of intersect?.

Set[1, 2, 3].disjoint? Set[3, 4]   #=> false
Set[1, 2, 3].disjoint? Set[4, 5]   #=> true
Set[1, 2, 3].disjoint? [3, 4]      #=> false
Set[1, 2, 3].disjoint? 4..5        #=> true

divide

() { (A, A) → Hash::_Key } → Set[self]
() { (A) → Hash::_Key } → Set[self]

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 535
def divide: () { (A, A) -> Hash::_Key } -> Set[self]
            | () { (A) -> Hash::_Key } -> Set[self]

Divides the set into a set of subsets according to the commonality defined by the given block.

If the arity of the block is 2, elements o1 and o2 are in common if both block.call(o1, o2) and block.call(o2, o1) are true. Otherwise, elements o1 and o2 are in common if block.call(o1) == block.call(o2).

numbers = Set[1, 3, 4, 6, 9, 10, 11]
set = numbers.divide { |i,j| (i - j).abs == 1 }
set        #=> Set[Set[1],
           #       Set[3, 4],
           #       Set[6],
           #       Set[9, 10, 11]]

Returns an enumerator if no block is given.

each

() { (A) → void } → self
() → Enumerator[A, self]

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 546
def each: () { (A) -> void } -> self
          | () -> Enumerator[A, self]

Calls the given block once for each element in the set, passing the element as parameter. Returns an enumerator if no block is given.

empty?

() → bool

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 555
def empty?: () -> bool

Returns true if the set contains no elements.

filter!

() { (A) → untyped } → self?

Equivalent to Set#keep_if, but returns nil if no changes were made. Returns an enumerator if no block is given.

Alias for: select!

flatten

() → Set[untyped]

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 564
def flatten: () -> Set[untyped]

Returns a new set that is a copy of the set, flattening each containing set recursively.

flatten!

() → self?

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 573
def flatten!: () -> self?

Equivalent to Set#flatten, but replaces the receiver with the result in place. Returns nil if no modifications were made.

include?

(Hash::_Key) → bool

Source

# File vendor/bundle/ruby/4.0.0/gems/rbs-4.0.3/core/set.rbs, line 359
def include?: (Hash::_Key) -> bool

Returns true if the set contains the given object:

Set[1, 2, 3].include? 2   #=> true
Set[1, 2, 3].include? 4   #=> false

Note that include? and member? do not test member equality using == as do other Enumerables.

This is aliased to #===, so it is usable in case expressions:

case :apple
when Set[:potato, :carrot]
  "vegetable"
when Set[:apple, :banana]
  "fruit"
end
# => "fruit"

class Set

Methods for Set Operations

Methods for `Set` Operations