Data Structures · Unit 3

Sets

Unordered collections without duplicates

Unit 3 · Data Structures

Choose the right container for your data

A data structure organizes values so a program can find and work with them. Explore four built-in Python structures and use each lab to see how its rules affect real data.

Lists

Ordered and changeable

Tuples

Ordered and fixed

Sets

Unique values

Dictionaries

Key → value lookup

Unique values · changeable · no index

Sets: collections without duplicates

A set keeps one copy of each distinct, hashable value. It is useful for membership checks and mathematical comparisons. Sets do not provide a reliable positional order, so use a list when sequence matters.

Think of event check-in: a guest is either on the attendee list or not. Checking membership is easy, and scanning the same name twice does not create a second person.

Set operations lab

Edit two class subject lists, then compare their unique values.

Set A · comma-separated valuesSet B · comma-separated values

Set A (3 unique)

PythonOSNetworks

Set B (3 unique)

OSData StructuresPython

Result · A | B (4 unique)

PythonOSNetworksData Structures

Everything in either set. Duplicate entries in the text fields collapse into one set value.

The chips are arranged for readability only; Python sets do not guarantee this display order. The example sorts values when printing for stable output.

Run the set comparison in Python

Starting up Python…

group_a = set(["Python","OS","Networks"])
group_b = set(["OS","Data Structures","Python"])
result = group_a | group_b
print("Unique in A:", sorted(group_a))
print("Unique in B:", sorted(group_b))
print("Result:", sorted(result))

The editor is ready while Python loads in the background.

Output

Your program's output will appear here.

Common Python mistakes

NameError: name 'mark' is not defined

NameError: A name was used before Python could find a variable or function with that name.

IndexError: list index out of range

IndexError: The requested position is outside the items in this list or sequence.

KeyError: 'email'

KeyError: That key does not exist in this dictionary.

TypeError: can only concatenate str (not "int") to str

TypeError: This operation does not work with the kind of value it received.

ZeroDivisionError: division by zero

ZeroDivisionError: A number cannot be divided by zero.

SyntaxError: invalid syntax

SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.

IndentationError: expected an indented block

IndentationError: The spaces at the start of a line do not match Python's block structure.

Avoid common set mistakes

An empty set is set(): {} creates an empty dictionary.
No positional access: subjects[0] is not valid for a set.
Set members must be hashable: strings and numbers work, but a mutable list cannot be a set member.

Sets check

1. What happens to repeated values when building a set?

2. Which operator finds values shared by A and B?

Compare the four structures

Ask two questions: does order matter, and should the collection change after creation?

Structure	Ordered?	Changeable?	Access values by
list	Yes	Yes	Position (index)
tuple	Yes	No	Position (index)
set	No guaranteed order	Yes	Membership, not index
dict	Insertion order is preserved	Yes	Key

Use a list for a sequence that changes, a tuple for a fixed record, a set for unique membership, and a dictionary for named lookups.

Tuples Dictionaries