19. Set type#

Author: Tue Nguyen

19.1. Outline#

What is a set?
Create a set
Iterate through a set
Copy a set
Operations on sets
Frozen sets

19.2. What is a set?#

A set is an mutable collection of unique immutable elements
The set itself is mutable
But its elements must be immutable and unique
Elements of a set are not ordered
- We cannot use indexing or slicing as with lists
- There’s no way to access an element of a set
- We can only check if a value is in a set or not

19.3. Create a set#

# We create a set using {}
s = {1, 2, 3}
print(s)
print(type(s))

{1, 2, 3}
<class 'set'>

# However, we cannot create an empty set using {}
# Because by default {} means an empty dict
# To create an empty set we use set()
s = set()
s

set()

19.4. Conversion to sets#

We use set() to convert an object to a set

# List to set
set([1, 2, 3])

{1, 2, 3}

# Tuple to set
set((1, 2, 3))

{1, 2, 3}

# Dict to set (keep the keys)
d = {"k1": "v1", "k2": "v2"}
set(d)

{'k1', 'k2'}

# To keep the values of the dict
set(d.values())

{'v1', 'v2'}

# String to sets (auto remove duplicates)
set("this is a string")

{' ', 'a', 'g', 'h', 'i', 'n', 'r', 's', 't'}

19.5. Iterate through set#

Same as lists and tuples

19.6. Mutablity#

A set is mutable
We can add, update, or delete elements

19.7. Copy a set#

Same as lists (there will be both shallow and deep copy)
However, note that elements of set are immubtable so there will be less confusion

19.8. Operations on sets#

As a mutable type like lists, we also have two types of operations on sets

Regular operations
Inplace operations

19.8.1. Regular operations#

# Init 2 sets
s1 = {1, 2, 3, 4, 5}
s2 = {0, 1, 2, 3}

# Count number of elements
len(s1)

# Check membership
2 in s1

True

# Set union
s1 | s2

{0, 1, 2, 3, 4, 5}

# Set intersection
s1 & s2

{1, 2, 3}

# Set difference
s1 - s2

{4, 5}

# Symmetric difference
s1 ^ s2

{0, 4, 5}

# Check subset with < or <=
print(s1 <= s2)
print({1, 2} <= s2)

False
True

Remarks

There are methods for set operations such as s1.union(s2), s1.intersection(s2), or s1.issubset()
However, using operators like |, &, or <= is shorter and more intuitive

19.8.2. Inplace operations#

# Init
s = {1, 2, 3, 4, 5}
s

{1, 2, 3, 4, 5}

# Add an element (if not yet existed)
s.add(10)
s

{1, 2, 3, 4, 5, 10}

# Remove an element 
# (will raise an error if the value is not in s)
s.remove(2)
s

{1, 3, 4, 5, 10}

# We can use .discard for safer removal
# If value is in s, then remove
# If not, then do nothing (no error)
s.discard(100)
s

{1, 3, 4, 5, 10}

# Update a set from another set
s.update({1, 2, 88, 99})
s

{1, 2, 3, 4, 5, 10, 88, 99}

# Pop a random element from a set
# (because sets are not ordered)
v = s.pop()
print(v)
print(s)

1
{2, 3, 4, 5, 99, 10, 88}

19.9. Frozen sets#

A frozen set is an immutable set
Think of set and frozen set like lists and tuple
Frozen sets have the same regular operations as sets
They don’t have inplace operations because they are immutable

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

frozenset({1, 2, 3})
<class 'frozenset'>

19.10. Summary#

What is a set?

A mutable collection of unique immutable elements
Elements of a set are not ordered

Create a set

Using {} or set()

Iterate through a set

Same as lists

Copy a set

Same as lists

Operations on sets

Regular operations: len, in, |, &, -, ^, <=, <
Inplace operations: .add(), .remove(), .discard(), .update(), .pop()

Frozen sets

Immutable versions of sets
Create a frozen set using frozenset()

19.11. Practice#

To be updated

Python tutorials

Set type

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