ligo/gitlab-pages/docs/reference/set.md

---
id: set-reference
title: Sets — Unordered unique collection of a type
---

Sets are unordered collections of values of the same type, like lists
are ordered collections. Like the mathematical sets and lists, sets
can be empty and, if not, elements of sets in LIGO are *unique*,
whereas they can be repeated in a *list*.

# Empty Sets

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
```pascaligo group=sets
const my_set : set (int) = set []
```

<!--CameLIGO-->
```cameligo group=sets
let my_set : int set = Set.empty
```

<!--ReasonLIGO-->
```reasonligo group=sets
let my_set : set (int) = Set.empty;
```
<!--END_DOCUSAURUS_CODE_TABS-->

# Non-empty Sets

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
```pascaligo group=sets
const my_set : set (int) = set [3; 2; 2; 1]
```

<!--CameLIGO-->
```cameligo group=sets
let my_set : int set =
  Set.add 3 (Set.add 2 (Set.add 2 (Set.add 1 (Set.empty : int set))))
```

<!--ReasonLIGO-->
```reasonligo group=sets
let my_set : set (int) =
  Set.add (3, Set.add (2, Set.add (2, Set.add (1, Set.empty : set (int)))));
```
<!--END_DOCUSAURUS_CODE_TABS-->

# Set Membership

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
```pascaligo group=sets
const contains_3 : bool = my_set contains 3
```

<!--CameLIGO-->
```cameligo group=sets
let contains_3 : bool = Set.mem 3 my_set
```

<!--ReasonLIGO-->
```reasonligo group=sets
let contains_3 : bool = Set.mem (3, my_set);
```
<!--END_DOCUSAURUS_CODE_TABS-->

# Cardinal of Sets

The predefined function `Set.size` returns the number of
elements in a given set as follows.

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
```pascaligo group=sets
const cardinal : nat = Set.size (my_set)
```

> Note that `size` is *deprecated*.

<!--CameLIGO-->
```cameligo group=sets
let cardinal : nat = Set.size my_set
```
<!--ReasonLIGO-->
```reasonligo group=sets
let cardinal : nat = Set.size (my_set);
```
<!--END_DOCUSAURUS_CODE_TABS-->

# Updating Sets

There are two ways to update a set, that is to add or remove from it.

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
In PascaLIGO, either we create a new set from the given one, or we
modify it in-place. First, let us consider the former way:
```pascaligo group=sets
const larger_set  : set (int) = Set.add (4, my_set)
const smaller_set : set (int) = Set.remove (3, my_set)
```

> Note that `set_add` and `set_remove` are *deprecated*.

If we are in a block, we can use an instruction to modify the set
bound to a given variable. This is called a *patch*. It is only
possible to add elements by means of a patch, not remove any: it is
the union of two sets.

```pascaligo group=sets
function update (var s : set (int)) : set (int) is block {
  patch s with set [4; 7]
} with s

const new_set : set (int) = update (my_set)
```

<!--CameLIGO-->
```cameligo group=sets
let larger_set  : int set = Set.add 4 my_set
let smaller_set : int set = Set.remove 3 my_set
```

<!--ReasonLIGO-->
```reasonligo group=sets
let larger_set  : set (int) = Set.add (4, my_set);
let smaller_set : set (int) = Set.remove (3, my_set);
```
<!--END_DOCUSAURUS_CODE_TABS-->

# Functional Iteration over Sets

A *functional iterator* is a function that traverses a data structure
and calls in turn a given function over the elements of that structure
to compute some value. Another approach is possible in PascaLIGO:
*loops* (see the relevant section).

There are three kinds of functional iterations over LIGO maps: the
*iterated operation*, the *mapped operation* (not to be confused with
the *map data structure*) and the *folded operation*.

## Iterated Operation

The first, the *iterated operation*, is an iteration over the map with
no return value: its only use is to produce side-effects. This can be
useful if for example you would like to check that each value inside
of a map is within a certain range, and fail with an error otherwise.

<!--DOCUSAURUS_CODE_TABS-->
<!--PascaLIGO-->
```pascaligo group=sets
function iter_op (const s : set (int)) : unit is
  block {
    function iterated (const i : int) : unit is
      if i > 2 then Unit else (failwith ("Below range.") : unit)
  } with Set.iter (iterated, s)
```

> Note that `set_iter` is *deprecated*.

<!--CameLIGO-->
```cameligo group=sets
let iter_op (s : int set) : unit =
  let predicate = fun (i : int) -> assert (i > 3)
  in Set.iter predicate s
```

<!--ReasonLIGO-->
```reasonligo group=sets
let iter_op = (s : set (int)) : unit => {
  let predicate = (i : int) => assert (i > 3);
  Set.iter (predicate, s);
};
```
<!--END_DOCUSAURUS_CODE_TABS-->

## Folded Operation

A *folded operation* is the most general of iterations. The folded
function takes two arguments: an *accumulator* and the structure
*element* at hand, with which it then produces a new accumulator. This
enables having a partial result that becomes complete when the
traversal of the data structure is over.

<!--DOCUSAURUS_CODE_TABS-->

<!--PascaLIGO-->
```pascaligo group=sets
function sum (const acc : int; const i : int): int is acc + i
const sum_of_elements : int = Set.fold (sum, my_set, 0)
```

> Note that `set_fold` is *deprecated*.

It is possible to use a *loop* over a set as well.

```pascaligo group=sets
function loop (const s : set (int)) : int is block {
  var sum : int := 0;
  for element in set s block {
    sum := sum + element
  }
} with sum
```

<!--CameLIGO-->
```cameligo group=sets
let sum (acc, i : int * int) : int = acc + i
let sum_of_elements : int = Set.fold sum my_set 0
```

<!--ReasonLIGO-->
```reasonligo group=sets
let sum = ((acc, i) : (int, int)) : int => acc + i;
let sum_of_elements : int = Set.fold (sum, my_set, 0);
```
<!--END_DOCUSAURUS_CODE_TABS-->