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

Set[*objects] → new_set click to toggle source

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

static VALUE
set_s_create(int argc, VALUE *argv, VALUE klass)
{
    VALUE set = set_alloc_with_size(klass, argc);
    set_table *table = RSET_TABLE(set);
    int i;

    for (i=0; i < argc; i++) {
        set_table_insert_wb(table, set, argv[i]);
    }

    return set;
}

json_create(object) click to toggle source

See as_json.

# File ext/json/lib/json/add/set.rb, line 9
def self.json_create(object)
  new object['a']
end

new(object = nil) → new_set click to toggle source

new(object = nil) {|element| ... } → new_set

Returns a new Set object based on the given object, which must be an Enumerable or nil.

With argument object given as nil, returns a new empty Set object:

Set.new                          # => Set[]
Set.new { fail 'Cannot happen' } # => Set[]  # Block not called.

With no block given and object given as an Enumerable, populates the new set with the elements of object:

Set.new(%w[ a b c ])     # => Set["a", "b", "c"]
Set.new({foo: 0, bar: 1})     # => Set[[:foo, 0], [:bar, 1]]
Set.new(4..10)     # => Set[4, 5, 6, 7, 8, 9, 10]
Set.new(Dir.new('lib')).take(5)
# => [".", "..", "bundled_gems.rb", "bundler", "bundler.rb"]
Set.new(File.new('doc/NEWS/NEWS-4.0.0.md')).take(3)
# => ["# NEWS for Ruby 4.0.0\n", "\n", "This document is a list of user-visible feature changes\n"]

With a block given and object given as an Enumerable, calls the block with each element of object; adds the block’s return value to the new set:

Set.new(4..10) {|i| i * 2 } # => Set[8, 10, 12, 14, 16, 18, 20]

static VALUE
set_i_initialize(int argc, VALUE *argv, VALUE set)
{
    if (RBASIC(set)->flags & RSET_INITIALIZED) {
        rb_raise(rb_eRuntimeError, "cannot reinitialize set");
    }
    RBASIC(set)->flags |= RSET_INITIALIZED;

    VALUE other;
    rb_check_arity(argc, 0, 1);

    if (argc > 0 && (other = argv[0]) != Qnil) {
        if (RB_TYPE_P(other, T_ARRAY)) {
            long i;
            int block_given = rb_block_given_p();
            set_table *into = RSET_TABLE(set);
            for (i=0; i<RARRAY_LEN(other); i++) {
                VALUE key = RARRAY_AREF(other, i);
                if (block_given) key = rb_yield(key);
                set_table_insert_wb(into, set, key);
            }
        }
        else {
            rb_block_call(other, enum_method_id(other), 0, 0,
                rb_block_given_p() ? set_initialize_with_block : set_initialize_without_block,
                set);
        }
    }

    return set;
}

Public Instance Methods

set & enum → new_set click to toggle source

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"]

static VALUE
set_i_intersection(VALUE set, VALUE other)
{
    VALUE new_set = set_s_alloc(rb_obj_class(set));
    set_table *stable = RSET_TABLE(set);
    set_table *ntable = RSET_TABLE(new_set);

    if (rb_obj_is_kind_of(other, rb_cSet)) {
        set_table *otable = RSET_TABLE(other);
        if (set_table_size(stable) >= set_table_size(otable)) {
            /* Swap so we iterate over the smaller set */
            otable = stable;
            set = other;
        }

        struct set_intersection_data data = {
            .set = new_set,
            .into = ntable,
            .other = otable
        };
        set_iter(set, set_intersection_i, (st_data_t)&data);
    }
    else {
        struct set_intersection_data data = {
            .set = new_set,
            .into = ntable,
            .other = stable
        };
        rb_block_call(other, enum_method_id(other), 0, 0, set_intersection_block, (VALUE)&data);
    }

    return new_set;
}

Also aliased as: intersection

+(p1)

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: |

set - enum → new_set click to toggle source

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"]

static VALUE
set_i_difference(VALUE set, VALUE other)
{
    return set_i_subtract(rb_obj_dup(set), other);
}

Also aliased as: difference

<(p1)

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

Alias for: proper_subset?

<<(p1)

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

<=(p1)

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

Alias for: subset?

set <=> other → -1, 0, 1, or nil click to toggle source

Returns 0 if the set are equal, -1 / 1 if the set is a proper subset / superset of the given set, or nil if they both have unique elements.

static VALUE
set_i_compare(VALUE set, VALUE other)
{
    if (rb_obj_is_kind_of(other, rb_cSet)) {
        size_t set_size = RSET_SIZE(set);
        size_t other_size = RSET_SIZE(other);

        if (set_size < other_size) {
            if (set_le(set, other) == Qtrue) {
                return INT2NUM(-1);
            }
        }
        else if (set_size > other_size) {
            if (set_le(other, set) == Qtrue) {
                return INT2NUM(1);
            }
        }
        else if (set_le(set, other) == Qtrue) {
            return INT2NUM(0);
        }
    }

    return Qnil;
}

set == other → true or false click to toggle source

Returns true if two sets are equal.

static VALUE
set_i_eq(VALUE set, VALUE other)
{
    if (!rb_obj_is_kind_of(other, rb_cSet)) return Qfalse;
    if (set == other) return Qtrue;

    set_table *stable = RSET_TABLE(set);
    set_table *otable = RSET_TABLE(other);
    size_t ssize = set_table_size(stable);
    size_t osize = set_table_size(otable);

    if (ssize != osize) return Qfalse;
    if (ssize == 0 && osize == 0) return Qtrue;
    if (stable->type != otable->type) return Qfalse;

    struct set_equal_data data;
    data.set = other;
    return rb_exec_recursive_paired(set_recursive_eql, set, other, (VALUE)&data);
}

Also aliased as: eql?

===(p1)

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"

See also Enumerable#include?

Alias for: include?

>(p1)

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

Alias for: proper_superset?

>=(p1)

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

Alias for: superset?

set ^ enum → new_set click to toggle source

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"]

static VALUE
set_i_xor(VALUE set, VALUE other)
{
    VALUE new_set = rb_obj_dup(set);

    if (rb_obj_is_kind_of(other, rb_cSet)) {
        set_iter(other, set_xor_i, (st_data_t)new_set);
    }
    else {
        VALUE tmp = set_s_alloc(rb_cSet);
        set_merge_enum_into(tmp, other);
        set_iter(tmp, set_xor_i, (st_data_t)new_set);
    }

    return new_set;
}

add(obj) → self click to toggle source

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]

static VALUE
set_i_add(VALUE set, VALUE item)
{
    rb_check_frozen(set);
    if (set_iterating_p(set)) {
        if (!set_table_lookup(RSET_TABLE(set), (st_data_t)item)) {
            no_new_item();
        }
    }
    else {
        set_insert_wb(set, item);
    }
    return set;
}

Also aliased as: <<

add?(obj) → self or nil click to toggle source

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

static VALUE
set_i_add_p(VALUE set, VALUE item)
{
    rb_check_frozen(set);
    if (set_iterating_p(set)) {
        if (!set_table_lookup(RSET_TABLE(set), (st_data_t)item)) {
            no_new_item();
        }
        return Qnil;
    }
    else {
        return set_insert_wb(set, item) ? Qnil : set;
    }
}

as_json(*) click to toggle source

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"}>

# File ext/json/lib/json/add/set.rb, line 28
def as_json(*)
  {
    JSON.create_id => self.class.name,
    'a'            => to_a,
  }
end

classify { |o| ... } → hash click to toggle source

classify → enumerator

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.

static VALUE
set_i_classify(VALUE set)
{
    RETURN_SIZED_ENUMERATOR(set, 0, 0, set_enum_size);
    VALUE args[2];
    args[0] = rb_hash_new();
    args[1] = rb_obj_class(set);
    set_iter(set, set_classify_i, (st_data_t)args);
    return args[0];
}

clear → self click to toggle source

Removes all elements and returns self.

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

static VALUE
set_i_clear(VALUE set)
{
    rb_check_frozen(set);
    if (RSET_SIZE(set) == 0) return set;
    if (set_iterating_p(set)) {
        set_iter(set, set_clear_i, 0);
    }
    else {
        set_table_clear(RSET_TABLE(set));
        set_compact_after_delete(set);
    }
    return set;
}

collect! { |o| ... } → self click to toggle source

collect! → enumerator

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

static VALUE
set_i_collect(VALUE set)
{
    RETURN_SIZED_ENUMERATOR(set, 0, 0, set_enum_size);
    rb_check_frozen(set);

    VALUE new_set = set_s_alloc(rb_obj_class(set));
    set_iter(set, set_collect_i, (st_data_t)new_set);
    set_i_initialize_copy(set, new_set);

    return set;
}

Also aliased as: map!

compare_by_identity → self click to toggle source

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

static VALUE
set_i_compare_by_identity(VALUE set)
{
    if (RSET_COMPARE_BY_IDENTITY(set)) return set;

    if (set_iterating_p(set)) {
        rb_raise(rb_eRuntimeError, "compare_by_identity during iteration");
    }

    return set_reset_table_with_type(set, &identhash);
}

compare_by_identity? → true or false click to toggle source

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

static VALUE
set_i_compare_by_identity_p(VALUE set)
{
    return RBOOL(RSET_COMPARE_BY_IDENTITY(set));
}

delete(obj) → self click to toggle source

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

static VALUE
set_i_delete(VALUE set, VALUE item)
{
    rb_check_frozen(set);
    if (set_table_delete(RSET_TABLE(set), (st_data_t *)&item)) {
        set_compact_after_delete(set);
    }
    return set;
}

delete?(obj) → self or nil click to toggle source

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

static VALUE
set_i_delete_p(VALUE set, VALUE item)
{
    rb_check_frozen(set);
    if (set_table_delete(RSET_TABLE(set), (st_data_t *)&item)) {
        set_compact_after_delete(set);
        return set;
    }
    return Qnil;
}

delete_if { |o| ... } → self click to toggle source

delete_if → enumerator

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

static VALUE
set_i_delete_if(VALUE set)
{
    RETURN_SIZED_ENUMERATOR(set, 0, 0, set_enum_size);
    rb_check_frozen(set);
    set_iter(set, set_delete_if_i, 0);
    set_compact_after_delete(set);
    return set;
}

difference(p1)

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) → true or false click to toggle source

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

static VALUE
set_i_disjoint(VALUE set, VALUE other)
{
    return RBOOL(!RTEST(set_i_intersect(set, other)));
}

divide { |o1, o2| ... } → set click to toggle source

divide { |o| ... } → set

divide → enumerator

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.

static VALUE
set_i_divide(VALUE set)
{
    RETURN_SIZED_ENUMERATOR(set, 0, 0, set_enum_size);

    if (rb_block_arity() == 2) {
        return set_divide_arity2(set);
    }

    VALUE values = rb_hash_values(set_i_classify(set));
    set = set_alloc_with_size(rb_cSet, RARRAY_LEN(values));
    set_merge_enum_into(set, values);
    return set;
}

each { |o| ... } → self click to toggle source

each → enumerator

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

static VALUE
set_i_each(VALUE set)
{
    RETURN_SIZED_ENUMERATOR(set, 0, 0, set_enum_size);
    set_iter(set, set_each_i, 0);
    return set;
}

empty? → true or false click to toggle source

Returns true if the set contains no elements.

static VALUE
set_i_empty(VALUE set)
{
    return RBOOL(RSET_EMPTY(set));
}

encode_with(coder) click to toggle source

# File ext/psych/lib/psych/core_ext.rb, line 23
def encode_with(coder)
  hash = {}
  each do |m|
    hash[m] = true
  end
  coder["hash"] = hash
  coder
end

eql?(p1)

Returns true if two sets are equal.

Alias for: ==

filter!()

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 click to toggle source

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

static VALUE
set_i_flatten(VALUE set)
{
    VALUE new_set = set_s_alloc(rb_obj_class(set));
    set_flatten_merge(new_set, set, rb_hash_new());
    return new_set;
}

flatten! → self click to toggle source

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

static VALUE
set_i_flatten_bang(VALUE set)
{
    bool contains_set = false;
    set_iter(set, set_contains_set_i, (st_data_t)&contains_set);
    if (!contains_set) return Qnil;
    rb_check_frozen(set);
    return set_i_replace(set, set_i_flatten(set));
}

hash → integer click to toggle source

Returns hash code for set.

static VALUE
set_i_hash(VALUE set)
{
    st_index_t size = RSET_SIZE(set);
    st_index_t hval = rb_st_hash_start(size);
    hval = rb_hash_uint(hval, (st_index_t)set_i_hash);
    if (size) {
        set_iter(set, set_hash_i, (VALUE)&hval);
    }
    hval = rb_st_hash_end(hval);
    return ST2FIX(hval);
}

include?(item) → true or false click to toggle source

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"