Praudyog

Q & A – Python Set

admin

(1) What Is A Set Data Structure?

  • A Set is an unordered collection data type that is iterable, mutable and has no duplicate elements. 
  • Set is represented by { } (values enclosed in curly braces)
  • 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.
  • Since sets are unordered, we cannot access items using indexes as we do in lists.

Example-1

st = {1,4,2,5,1,3,3,4}

print(st)
{1, 2, 3, 4, 5}

(2) Does Converting An Object To A Set Maintain The Object’s Order?

  • No. A set is not an ordered data structure, so the order is not maintained.
  • Look what happens when we convert the list [3,2,1] to a set. It becomes {1,2,3}.

Example-1

a = set([3,2,1])

print(a)
{1, 2, 3}

(3) Check If A Set A Subset Of Another Set.

  • This can be done with the issubset() method.

Example-1

a = {4,5}

b = {1,2,3,4,5}

a.issubset(b)
True

(4) Check If A Set Is A Subset, Using Comparison Operators.

  • A set, s1, is a subset of s2 if all elements of s1 are in s2.

  • The operators <= will return True if all elements of the 1st set exist in the 2nd set (aka. is a subset).

Example-1

a = {'a','b'}

b = {'a','b','c'}

print(a <= b)

print(b <= a)
True
False

(5) Is A Set A Subset Of Itself?

  • Yes.

  • Because a set contains all elements in itself, it is indeed a subset of itself.

  • This is important to understand when we contrast “subset” with “proper subset” later.

Example-1

a = {10,20}

a.issubset(a)
True

(6) Check If A Specific Value Exists In A Set.

  • Like other types of tables, we can check if a value exists in a set with the in operator.

Example-1

s = {5,7,9}

5 in s
True

(7) Check If A Value Is Not In A Set.

  • We can again use the in operator, but this time prefaced by not.

Example-1

s = {'x','y''z'}

'w' not in s
False

(8) What Is The Difference Between A Subset And A Proper Subset?

  • A proper subset is a subset of a set, not equal to itself.

Example-1

{1,2,3} is a proper subset of {1,2,3,4}.

{1,2,3} is not a proper subset of {1,2,3}, but it is a subset.

(9) Check If A Set Is A Proper Subset.

  • We can check if a set is a proper subset of another set using the < operator.

Example-1

print({1,2,3} < {1,2,3,4})

print({1,2,3} < {1,2,3})

True
False

(10) Add An Element To A Set.

  • Unlike lists, we can’t use the + operator to add elements to a set.
  • Use the add method to add elements.

Example-1

{1,2,3} + {4}
TypeError: unsupported operand type(s) for +: 'set' and 'set'

Example-2

s = {'a','b','c'}

s.add('d')

print(s)
{'c', 'b', 'd', 'a'}

(11) Make A Copy Of A Set.

  • The copy() method makes a shallow copy of a set.

Example-1

s1 = {'a','b','c'}

s2 = s1.copy()

print(s2) 
{'c', 'b', 'a'}

(12) Check If A Set Is A Superset Of Another Set.

  • A set, s1 is a superset of another set, s2, if all values in s2 can be found in s1.

  • You can check if a set is a superset of another set with the issuperset() method.

Example-1

a = {10,20,30}

b = {10,20}

print(a.issuperset(b))

print(b.issuperset(a))

True
False

(13) Check If A Set Is A Superset With Comparison Operators.

  • In addition to issuperset(), we can check if a set is a superset using the >= comparison operators.

Example-1

a = {10,20,30}

b = {10,20}
     
print(a >= b)    

print(b >= a)     
True
False

(14) Is A Set A Superset Of Itself?

  • Because all the values in a set, s1, are in s1, it is a superset of itself. Though it is not a proper superset.

Example-1

a = {10,20,30}

a.issuperset(a) 
True

(15) Check If A Set Is A Proper Superset Of Another Set.

  • A set, s1 is a proper superset of another set, s2, if all the values in s2 are in s1, and s1 != s2.

  • This can be checked with the > operator.

Example-1

a = {10,20,30}
b = {10,20}
c = {10,20}

print(a > b)

print(b > c)
True
False

(16) Convert A Set To A List.

  • Calling the list constructor, list(), on a set converts a set to a list. But note that order is not guaranteed.

Example-1

a = {4,2,3}

b = list(a)

print(b)
[2, 3, 4]

(17) How Can You Iterate On Values In A Set?

  • A set can be iterated over like any other iterator with a loop. But note again, the order is not guaranteed.

Example-1

s = {'a','b','c','d','e'}

for i in s:
    print(i)
d
e
c
b
a

(18) Return The Length Of A Set.

  • The number of elements in a set can be returned with the len() function.

Example-1

s = {'a','b','c','d','e'}

len(s)
5

(19) Find The Union Of 2 Sets.

  • The union of 2 sets can be found by using the union() method.
  • It can also be found with the | operator.

Example-1

s1 = {1,2,3,4,5}

s2 = {4,5,6,7,8}

s1.union(s2)
{1, 2, 3, 4, 5, 6, 7, 8}

Example-2

s1 = {1,2,3,4,5}

s2 = {4,5,6,7,8}

s1 | s2
{1, 2, 3, 4, 5, 6, 7, 8}

(20) Find The Intersection Of 2 Sets.

  • The intersection of 2 sets can be taken with the intersection() method.
  • It can also be taken with the & operator.

Example-1

s1 = {1,2,3,4,5}

s2 = {4,5,6,7,8}

s1.intersection(s2)
{4, 5}

Example-2

s1 = {1,2,3,4,5}

s2 = {4,5,6,7,8}

s1 & s2
{4, 5}

(21) Find The Elements In s1 That Are Not In s2.

  • This can be found with the difference() method.
  • It can also be found with the - operator.

Example-1

s1 = {1,2,3,4,5}
s2 = {4,5,6,7,8}
s1.difference(s2)
{1, 2, 3}

Example-2

s1 = {1,2,3,4,5}
s2 = {4,5,6,7,8}
s1 - s2
{1, 2, 3}

(22) Remove An Element From A Set.

  • remove() removes an element from a set by value.

Example-1

s = {'x','y','z'}

s.remove('x')

print(s)
{'y', 'z'}

(23) Remove And Return An Unspecified Element From A Set.

  • pop() removes and returns an element from a set, treating the set like an unordered queue.

Example-1

s = {'z','y','x'}

removed = s.pop()

print(s)

print(removed)
{'y', 'z'}

x

(24) Check If 2 Sets Are Disjoint.

  • Sets, s1 and s2 are disjoint if they have no elements in common.

Example-1

a = {1,2,3}
b = {4,5,6}
c = {3}

print(a.isdisjoint(b))

print(a.isdisjoint(c))
True
False

(25) Add All Elements From Another Set To An Existing Set.

  • The update() method adds elements from another set.
  • This can also be done with the |= operator.

Example-1

a = {1,2,3}
b = {3,4,5}
a |= b

print(a)
{1, 2, 3, 4, 5}

(26) Remove All Elements From A Set

  • clear() removes all elements from a set.
  • The set can then be used for future operations and store other values.

Example-1

a = {1,2,3}
a.clear()
print(a)
set()

(27) Remove An Element From A Set If It Exists

  • discard() removes an element if that element exists, otherwise it does nothing.

Example-1

a = {1,2,3}
a.discard(1)
print(a)
{2, 3}

Example-2

a = {1,2,3}
a.discard(5)
print(a)
{1, 2, 3}

(28) What Is The Result Of Passing A Dictionary To A Set Constructor?

  • Only the dictionary’s keys will exist in the returned set.

Example-1

d = {'dog': 1, 'cat':2, 'fish':3}

set(d)
{'cat', 'dog', 'fish'}

(29) Can You Zip 2 Sets Together?

  • Yes.

  • But the values from each set may not be joined in order.

  • Notice how the 1st value in the integer set was combined with the 3rd value in the letter set, (1, 'c').

Example-1

set1 = {1,2,3}
set2 = {'a','b','c'}

z = zip(set1,set2)

set(z)
{(1, 'c'), (2, 'b'), (3, 'a')}

(30) Can A Set Be Accessed By Index?

  • No.
  • Trying to access a set by index will throw an error.

Example-1

s = {1,2,3}

s[0]
TypeError: 'set' object is not subscriptable

(31) What Is The Difference Between A Set And A Tuple?

  • Tuples are immutable. Sets are mutable.

  • Values in a tuple can be accessed by index.

  • Values in a set can only be accessed by value.

  • Tuples have order. Sets have no order.

  • Sets implement set theory, so they have lots of interesting functionality like union, intersect, difference, etc.

(32) What Is the Difference Between A Set And A Frozenset?

  • Frozensets behave just like sets except they are immutable.

Example-1: Normal Set

s = set([1,2,3])

s.add(4)

print(s)
{1, 2, 3, 4}
  • We can add elements to the normal Set.

Example-2: Frozen Set

fs = frozenset([1,2,3])

fs.add(4)

print(fs)
AttributeError                            Traceback (most recent call last)
Input In [56], in <cell line: 3>()
      1 fs = frozenset([1,2,3])
----> 3 fs.add(4)
      5 print(fs)

AttributeError: 'frozenset' object has no attribute 'add'
  • We can not add elements to the frozen Set.

Leave a Reply

Your email address will not be published. Required fields are marked *