# Set

In Python, a **set** is an unordered collection of unique, immutable objects. It supports adjunction, membership testing, union, and intersection, among other set operations.

## Contents

## Creation

A set is created by enclosing comma-separated values in braces:

>>> {1, 2, 2, 3} {1, 2, 3}

Alternatively, a set can also be constructed by `set(s)`

for any sequence `s`

. An empty set is constructed by `set()`

(not `{}`

). Example:

>>> set(['one', 'one', 'two']) {'one', 'two'}

## Length

The number of elements in set `s`

is given by `len(s)`

.

## Adjunction

Adjunction means to add an element to a set if it is not already present. An object `o`

is adjoined to set `s`

by `s.add(o)`

. Example:

>>> s = set() # create empty set >>> s.add(1) >>> s.add(2) >>> s.add(2) >>> s.add(3) >>> s {1, 2, 3}

## Removal

To remove an object `o`

from set `s`

, use `s.remove(o)`

. Example:

>>> s = {1, 2, 3} >>> s.remove(2) >>> s {1, 3}

## Membership test

To test if object `o`

is an element of set `s`

, use `o in s`

. Example:

>>> s = {1, 2, 3} >>> 3 in s True

## Intersection and union

Any element which is in both `s1`

and `s2`

will appear in their intersection. In Python, the intersection of `s1`

and `s2`

is given by `s1 & s2`

or `s1.intersection(s2)`

. Example:

>>> s1 = {1, 2, 3} >>> s2 = {1, 3, 5} >>> s1 & s2 {1, 3} >>> s1.intersection(s2) {1, 3}

The union is the merger of two sets. Any element in `s1`

or `s2`

will appear in their union. In Python, the union of `s1`

and `s2`

is given by `s1 | s2`

or `s1.union(s2)`

. Example:

>>> s1 = {0, 2, 4} >>> s2 = {1, 3, 5} >>> s1 | s2 {0, 1, 2, 3, 4, 5} >>> s1.union(s2) {0, 1, 2, 3, 4, 5}

## Iteration

A set is an iterable, so its elements can be iterated through in a for loop:

>>> s = {1, 2, 3} >>> for elem in s: ... print(elem) ... 1 2 3