# sets.3erl - Man Page

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

- 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()

- 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())

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

## Referenced By

erl_syntax_lib.3erl(3), gb_sets.3erl(3), ordsets.3erl(3), sofs.3erl(3).