# Sets in Python

A Set is an unordered collection data type that is iterable, mutable, and has no duplicate elements. Python’s set class represents the mathematical notion of a set. The major advantage of using a set, as opposed to a list, is that it has a highly optimized method for checking whether a specific element is contained in the set. This is based on a data structure known as a hash table.

Frozen Sets Frozen sets are immutable objects that only support methods and operators that produce a result without a?ecting the frozen set or sets to which they are applied.

 `# Python program to demonstrate differences ` `# between normal and frozen set ` ` `  `# Same as {"a", "b","c"} ` `normal_set ``=` `set``([``"a"``, ``"b"``,``"c"``]) ` ` `  `# Adding an element to normal set is fine ` `normal_set.add(``"d"``) ` ` `  `print``(``"Normal Set"``) ` `print``(normal_set) ` ` `  `# A frozen set ` `frozen_set ``=` `frozenset``([``"e"``, ``"f"``, ``"g"``]) ` ` `  `print``(``"Frozen Set"``) ` `print``(frozen_set) ` ` `  `# Uncommenting below line would cause error as ` `# we are trying to add element to a frozen set ` `# frozen_set.add("h") `

Output:

```Normal Set
set(['a', 'c', 'b', 'd'])
Frozen Set
frozenset(['e', 'g', 'f'])
```

Methods for Sets

1. add(x) Method: Adds the item x to set if it is not already present in the set.

```people = {"Jay", "Idrish", "Archil"}
```

-> This will add Daxit in people set.

2. union(s) Method: Returns a union of two set.Using the ‘|’ operator between 2 sets is the same as writing set1.union(set2)

```people = {"Jay", "Idrish", "Archil"}
vampires = {"Karan", "Arjun"}
population = people.union(vampires)
```

OR

```population = people|vampires
```

-> Set population set will have components of both people and vampire

3. intersect(s) Method: Returns an intersection of two sets.The ‘&’ operator comes can also be used in this case.

```victims = people.intersection(vampires)
```

-> Set victims will contain the common element of people and vampire

4. difference(s) Method: Returns a set containing all the elements of invoking set but not of the second set. We can use ‘-‘ operator here.

```safe = people.difference(vampires)
```

OR

```safe = people – vampires
```

-> Set safe  will have all the elements that are in people but not vampire
5. clear() Method: Empties the whole set.

```victims.clear()
```

-> Clears victim set

However there are two major pitfalls in Python sets:

1. The set doesn’t maintain elements in any particular order.
2. Only instances of immutable types can be added to a Python set.

Operators for Sets

Sets and frozen sets support the following operators:

key in s       # containment check

key not in s   # non-containment check

s1 == s2       # s1 is equivalent to s2

s1 != s2       # s1 is not equivalent to s2

s1 <= s2    # s1is subset of s2 s1 < s2     # s1 is proper subset of s2 s1 >= s2    # s1is superset of s2

s1 > s2     # s1 is proper superset of s2

s1 | s2        # the union of s1 and s2

s1 & s2 # the intersection of s1 and s2

s1 – s2        # the set of elements in s1 but not s2

s1 ˆ s2        # the set of elements in precisely one of s1 or s2

Code Snippet to illustrate all Set operations in Python

 `# Python program to demonstrate working# of ` `# Set in Python ` ` `  `# Creating two sets ` `set1 ``=` `set``() ` `set2 ``=` `set``() ` ` `  `# Adding elements to set1 ` `for` `i ``in` `range``(``1``, ``6``): ` `    ``set1.add(i) ` ` `  `# Adding elements to set2 ` `for` `i ``in` `range``(``3``, ``8``): ` `    ``set2.add(i) ` ` `  `print``(``"Set1 = "``, set1) ` `print``(``"Set2 = "``, set2) ` `print``(``"\n"``) ` ` `  `# Union of set1 and set2 ` `set3 ``=` `set1 | set2``# set1.union(set2) ` `print``(``"Union of Set1 & Set2: Set3 = "``, set3) ` ` `  `# Intersection of set1 and set2 ` `set4 ``=` `set1 & set2``# set1.intersection(set2) ` `print``(``"Intersection of Set1 & Set2: Set4 = "``, set4) ` `print``(``"\n"``) ` ` `  `# Checking relation between set3 and set4 ` `if` `set3 > set4: ``# set3.issuperset(set4) ` `    ``print``(``"Set3 is superset of Set4"``) ` `elif` `set3 < set4: ``# set3.issubset(set4) ` `    ``print``(``"Set3 is subset of Set4"``) ` `else` `: ``# set3 == set4 ` `    ``print``(``"Set3 is same as Set4"``) ` ` `  `# displaying relation between set4 and set3 ` `if` `set4 < set3: ``# set4.issubset(set3) ` `    ``print``(``"Set4 is subset of Set3"``) ` `    ``print``(``"\n"``) ` ` `  `# difference between set3 and set4 ` `set5 ``=` `set3 ``-` `set4 ` `print``(``"Elements in Set3 and not in Set4: Set5 = "``, set5) ` `print``(``"\n"``) ` ` `  `# checkv if set4 and set5 are disjoint sets ` `if` `set4.isdisjoint(set5): ` `    ``print``(``"Set4 and Set5 have nothing in common\n"``) ` ` `  `# Removing all the values of set5 ` `set5.clear() ` ` `  `print``(``"After applying clear on sets Set5: "``) ` `print``(``"Set5 = "``, set5) `

Output:

```('Set1 = ', set([1, 2, 3, 4, 5]))
('Set2 = ', set([3, 4, 5, 6, 7]))

('Union of Set1 & Set2: Set3 = ', set([1, 2, 3, 4, 5, 6, 7]))
('Intersection of Set1 & Set2: Set4 = ', set([3, 4, 5]))

Set3 is superset of Set4
Set4 is subset of Set3

('Elements in Set3 and not in Set4: Set5 = ', set([1, 2, 6, 7]))

Set4 and Set5 have nothing in common

After applying clear on sets Set5:
('Set5 = ', set([]))
```

Recent articles on Python Set.

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