# sets

###
Module

sets

###
Module Summary

Functions for set manipulation.

###
Description

Sets are collections of elements with no duplicate elements.

The data representing a set as used by this module is to be regarded as opaque by other modules. In abstract terms, the representation is a composite type of existing Erlang terms. See note on data types. Any code assuming knowledge of the format is running on thin ice.

This module provides the same interface as the ordsets(3) module but with an undefined representation. One difference is that while this module considers two elements as different if they do not match (=:=), ordsets considers two elements as different if and only if they do not compare equal (==).

Erlang/OTP 24.0 introduced a new internal representation for sets which is more performant. Developers can use this new representation by passing the {version, 2} flag to new/1 and from_list/2, such as sets:new([{version, 2}]). This new representation will become the default in future Erlang/OTP versions. Functions that work on two sets, such as union/2 and similar, will work with sets of different versions. In such cases, there is no guarantee about the version of the returned set. Explicit conversion from the old version to the new one can be done with sets:from_list(sets:to_list(Old), [{version,2}]).

###
Compatibility

The following functions in this module also exist and provide the same functionality in the gb_sets(3) and ordsets(3) modules. That is, by only changing the module name for each call, you can try out different set representations.

- add_element/2
- del_element/2
- filter/2
- fold/3
- from_list/1
- intersection/1
- intersection/2
- is_element/2
- is_empty/1
- is_set/1
- is_subset/2
- new/0
- size/1
- subtract/2
- to_list/1
- union/1
- union/2

Note

While the three set implementations offer the same functionality with respect to the aforementioned functions, their overall behavior may differ. As mentioned, this module considers elements as different if and only if they do not match (=:=), while both ordsets and gb_sets consider elements as different if and only if they do not compare equal (==).

Example:

    1> sets:is_element(1.0, sets:from_list([1])).
    false
    2> ordsets:is_element(1.0, ordsets:from_list([1])).
    true
    2> gb_sets:is_element(1.0, gb_sets:from_list([1])).
    true

###
Data Types

####
set(Element)

As returned by new/0.

####
set() = set(term())

###
Exports

####
add_element(Element, Set1) -> Set2

### Types

Set1 = Set2 = set(Element)

Returns a new set formed from Set1 with Element inserted.

####
del_element(Element, Set1) -> Set2

### Types

Set1 = Set2 = set(Element)

Returns Set1, but with Element removed.

####
filter(Pred, Set1) -> Set2

### Types

Pred = fun((Element) -> boolean())
Set1 = Set2 = set(Element)

Filters elements in Set1 with boolean function Pred.

####
fold(Function, Acc0, Set) -> Acc1

### Types

Function = fun((Element, AccIn) -> AccOut)
Set = set(Element)
Acc0 = Acc1 = AccIn = AccOut = Acc

Folds Function over every element in Set and returns the final value of the accumulator. The evaluation order is undefined.

####
from_list(List) -> Set

### Types

List = [Element]
Set = set(Element)

Returns a set of the elements in List.

####
from_list(List, Opts :: [{version, 1..2}]) -> Set
OTP 24.0

### Types

List = [Element]
Set = set(Element)

Returns a set of the elements in List at the given version.

####
intersection(SetList) -> Set

### Types

SetList = [set(Element), ...]
Set = set(Element)

Returns the intersection of the non-empty list of sets.

####
intersection(Set1, Set2) -> Set3

### Types

Set1 = Set2 = Set3 = set(Element)

Returns the intersection of Set1 and Set2.

####
is_disjoint(Set1, Set2) -> boolean()

### Types

Set1 = Set2 = set(Element)

Returns true if Set1 and Set2 are disjoint (have no elements in common), otherwise false.

####
is_element(Element, Set) -> boolean()

### Types

Set = set(Element)

Returns true if Element is an element of Set, otherwise false.

####
is_empty(Set) -> boolean()
OTP 21.0

### Types

Set = set()

Returns true if Set is an empty set, otherwise false.

####
is_set(Set) -> boolean()

### Types

Set = term()

Returns true if Set appears to be a set of elements, otherwise false. Note that the test is shallow and will return true for any term that coincides with the possible representations of a set. See also note on data types.

####
is_subset(Set1, Set2) -> boolean()

### Types

Set1 = Set2 = set(Element)

Returns true when every element of Set1 is also a member of Set2, otherwise false.

####
new() -> set(none())

Returns a new empty set.

####
new(Opts :: [{version, 1..2}]) -> set(none())
OTP 24.0

Returns a new empty set at the given version.

####
size(Set) -> integer() >= 0

### Types

Set = set()

Returns the number of elements in Set.

####
subtract(Set1, Set2) -> Set3

### Types

Set1 = Set2 = Set3 = set(Element)

Returns only the elements of Set1 that are not also elements of Set2.

####
to_list(Set) -> List

### Types

Set = set(Element)
List = [Element]

Returns the elements of Set as a list. The order of the returned elements is undefined.

####
union(SetList) -> Set

### Types

SetList = [set(Element)]
Set = set(Element)

Returns the merged (union) set of the list of sets.

####
union(Set1, Set2) -> Set3

### Types

Set1 = Set2 = Set3 = set(Element)

Returns the merged (union) set of Set1 and Set2.

###
See Also

gb_sets(3), ordsets(3)