Set

< Object include Enumerable 61 instance methods 3 class methods

set/subclass_compatible.rb - Provides compatibility for set subclasses

Documentation by Akinori MUSHA and Gavin Sinclair.

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 <[email protected]> (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:

Example

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:

Example

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.

Class methods 3

.[] .json_create .new

Instance methods 61

#& #+ #- #< #<< #<= #<=> #== #=== #> #>= #^ #| #add #add? #as_json #classify #clear #collect! #compare_by_identity #compare_by_identity? #delete #delete? #delete_if #difference #disjoint? #divide #each #empty? #encode_with #eql? #filter! #flatten #flatten! #hash #include? #init_with #inspect #intersect? #intersection #join #keep_if #length #map! #member? #merge #proper_subset? #proper_superset? #reject! #replace #reset #select! #size #subset? #subtract #superset? #to_a #to_json #to_s #to_set #union

Ruby version changes 4

4.0
- Set is now a core class, instead of an autoloaded stdlib class. [Feature #21216]
- Set#inspect now uses a simpler display, similar to literal arrays. (e.g., Set[1, 2, 3] instead of #<Set: {1, 2, 3}>). [Feature #21389]
- Passing arguments to Set#to_set and Enumerable#to_set is now deprecated. [Feature #21390]
3.2
- Set is now available as a built-in class without the need for require "set". [Feature #16989] It is currently autoloaded via the Set constant or a call to Enumerable#to_set.