Python: Sets

Set is an unordered collection of elements with no duplicates. We can perform union, intersection, difference, and symmetric difference operations on sets.

How to create set

You can create set using {} (or) set() function. ‘set()’ creates empty set.

>>> evenNumbers={2, 4, 6, 8, 8, 4, 10} >>> evenNumbers {8, 10, 2, 4, 6}

Observe above snippet, evenNumbers is a set that contains even numbers. Observe the output, set doesn’t contain duplicate elements.

Following operations are supported by set.

len(s) : cardinality of set

Returns cardinality(Number of distinct elements) of the set.

>>> evenNumbers={2, 4, 6, 8, 8, 4, 10}
>>> len(evenNumbers)
5

x in s : Check whether element is in set or not

‘in’ operator is used to check whether element is in set or not, return true if the element is set, else false.

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> 
>>> 100 in evenNumbers
False
>>> 
>>> 2 in evenNumbers
True

x not in s : Check whether element is in set or not

‘not in’ operator is opposite of ‘in’ operator, return true if the element is not in set, else false.

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> 
>>> 10 not in evenNumbers
False
>>> 
>>> 100 not in evenNumbers
True

isdisjoint(other)

Return true if two sets are disjoint, else false. Two sets are said to be disjoint if they have no element in common.    

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> 
>>> evenNumbers.isdisjoint({1, 3, 5, 7})
True
>>> 
>>> evenNumbers.isdisjoint({1, 3, 5, 7, 8})
False

issubset(other)

Return true, if this set is subset of other, else false.    

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> 
>>> evenNumbers.issubset({2, 4})
False
>>> 
>>> evenNumbers.issubset({2, 4, 6, 8, 10, 12})
True

set <= other

Return true if every element in the set is in other.

set < other

Return true, if the set is proper subset of other, that is, set >= other and set != other.    

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> 
>>> evenNumbers <= {2, 4, 6, 8, 10}
True
>>> evenNumbers <= {2, 4, 6, 8, 10, 12}
True
>>> 
>>> evenNumbers < {2, 4, 6, 8, 10}
False
>>> evenNumbers < {2, 4, 6, 8, 10, 12}
True

Union of two sets

union(other, ...)

‘set | other | ...’ 

>>> evenNumbers={2, 4, 6, 8, 10}
>>> oddNumbers={1, 3, 5, 7, 9}
>>> result=evenNumbers|oddNumbers
>>> result
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

Intersection of two sets

intersection(other, ...)

‘set & other & ...’ 

>>> evenNumbers={2, 4, 6, 8, 10}
>>> powersOf2={1, 2, 4, 8, 16}
>>> result=evenNumbers&powersOf2
>>> result
{8, 2, 4}

Difference between two sets

difference(other, ...)

‘set - other - ...’

Return a new set with elements in the set that are not in the others.    

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> evenNumbers-powersOf2
{10, 6}
>>> powersOf2-evenNumbers
{16, 1}

Symmetric difference between two sets

symmetric_difference(other)

set ^ other

If A and B are two sets, then Simmetric difference between A and B is A^B = (A-B) union (B-A)

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> evenNumbers-powersOf2
{10, 6}
>>> powersOf2-evenNumbers
{16, 1}
>>> 
>>> evenNumbers^powersOf2
{1, 6, 10, 16}

Copy elements of set

‘copy’ function return a new set with a shallow copy of s.

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> temp=evenNumbers.copy()
>>> temp
{8, 10, 2, 4, 6}

Update the set

update(other, ...)

set |= other | ...

Update the set by adding elements from other sets.

>>> evenNumbers
{8, 10, 2, 4, 6}
>>> oddNumbers
{9, 3, 5, 1, 7}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> 
>>> evenNumbers.update(oddNumbers, powersOf2)
>>> evenNumbers
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 16}

Intersection of all sets

intersection_update(other, ...)

set &= other & ...

Update the set, keeping only elements found in it and all others.    

>>> numbers
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 16}
>>> oddNumbers
{9, 3, 5, 1, 7}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> numbers.intersection_update(oddNumbers, powersOf2)
>>> numbers
{1}

Difference update

difference_update(other, ...)

set -= other | ...

Update the set, removing elements found in others.    

>>> oddNumbers
{9, 3, 5, 1, 7}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> oddNumbers.difference_update(powersOf2)
>>> oddNumbers
{9, 3, 5, 7}

Symmetric difference update

symmetric_difference_update

set ^= other

Update the set, keeping only elements found in either set, but not in both.

>>> oddNumbers
{9, 3, 5, 7}
>>> powersOf2
{16, 8, 2, 4, 1}
>>> oddNumbers.symmetric_difference_update(powersOf2)
>>> oddNumbers
{1, 2, 3, 4, 5, 7, 8, 9, 16}

Add element to the set

‘add’ method is used to add element to set.

>>> temp
{2, 3, 5, 7}
>>> 
>>> temp.add(11)
>>> temp.add(13)
>>> 
>>> temp
{2, 3, 5, 7, 11, 13}

Remove an element from set

‘remove’ method is used to remove element from set.

>>> temp
{2, 3, 5, 7, 11, 13}
>>> 
>>> temp.remove(2)
>>> temp
{3, 5, 7, 11, 13}
>>> 
>>> temp.remove(11)
>>> temp
{3, 5, 7, 13}

Throws KeyError, if element is not in the set.

>>> temp.remove(100)

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

KeyError: 100

Remove arbitrary element from set

‘pop()’ is used to remove and return an arbitrary element from the set. Throws KeyError, if the set is empty.

>>> temp
{5, 7, 13}
>>> temp.pop()
5
>>> 
>>> temp.pop()
7
>>> temp.pop()
13
>>> temp.pop()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'pop from an empty set'

Remove all elements from set

‘clear’ method is used to remove all elements from set.

>>> powersOf2
{16, 8, 2, 4, 1}
>>> 
>>> powersOf2.clear()
>>> powersOf2
set()

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