Sets and Tuples

Winter 2025

1

Adrian Salguero

Announcements

• Midterms will be graded and submitted by this upcoming Wednesday (ideally)
• Coding Practice 5 due Wednesday, February 19th at 11:59pm
• Homework 4, Part 2 is due Friday, February 21st at 11:59pm
• Fill out the Mid-Quarter Feedback Form, closes tonight at 11:59pm
• 1 point extra credit for Midterm if you fill it out

2

Sets

• Mathematical Set: a collection of values, without duplicates or order
• Order does not matter
• {1, 2, 3} == {3, 2, 1}
• No duplicates
• {3, 1, 4, 1, 5} is not possible as a set because we have a duplicate 1, instead as a set this will be {3, 1, 4, 5}

3

Creating a Set

• Direct mathematical syntax
• odd = {1, 3, 5}
• prime = {2, 3, 5}
• Why can we not declare an empty set like this? → empty = { }

• Construct from a list, tuple, or string:
• odd = set([1, 3, 5]) # from a list
• prime = set((2, 3, 5)) # from a tuple
• name = set("Adrian") # from a string
• name = {'i', 'A', 'd', 'n', 'r', 'a'} (not in this order each time)

4

Set Operations

5

odd = {1, 3, 5}

prime = {2, 3, 5}

Set Iterations

• We can still iterate over sets in the traditional sense

​

for item in myset:

# do something with item

​

• We cannot index on sets to access elements
• If you need to access elements based on index position, use a list instead!

6

Modifying Sets

• Add an element to a set
• myset.add(element)
• myset = myset | {element} # adding using set union
• Remove an element from a set
• myset.remove(element)
• myset.discard(element)
• myset = myset - {element}
• What would this do? → myset = myset - element
• Remove and return an arbitrary element from a set
• myset.pop()

7

Practice with Sets

z = {5, 6, 7, 8}

y = {1, 2, 3, 1, 5}

k = z & y

j = z | y

m = y - z

n = z - y

8

More Practice with Sets

z = {5, 6, 7, 8}

y = {1, 2, 3, 1, 5}

k = z.intersection(y)

j = z.union(y)

m = y.difference(z)

n = z.difference(y)

​

9

Even More Practice with Sets

z = {5, 6, 7, 8}

y = {1, 2, 3, 1, 5}

p = z

q = set(z) # Makes a copy of set z

z.add(9)

q = q | {35}

z.discard(7)

q = q – {6, 1, 8}

​

10

List vs. Set Operations

Suppose we have two lists, list1 and list2, and we want to find the common elements in the lists.

out1 = []

for elem in list2:

if elem in list1:

out1.append(elem)

-----------------------------------------------------------------

Suppose we have two sets, set1 and set2, and we want to find the common elements in the sets.

​

set1 & set2

​

11

List vs. Set Operations (cont.)

Find elements in either list1 or list2 (or both) (without duplicates)

out2 = list(list1) # make a copy

for elem in list2:

if elem not in out2: # don’t append elements already in out2

out2.append(elem)

​

Another way:

out2 = list1 + list2 # if an item is in BOTH lists, it will appear TWICE!

for elem in out1: # out1 = common elements in both lists

out2.remove(elem) # Remove common elements, leaving just a single copy

-----------------------------------------------------------------

Find the elements in either set1 or set2 (or both)

set1 | set2

12

List vs Set Operations (cont.)

Find the elements in either list but not in both

out3 = []

out2 = list1 + list2 # if an item is in BOTH lists, it will appear TWICE!

for elem in out2:

if elem not in list1 or elem not in list2:

out3.append(elem)

-----------------------------------------------------------------

Find the elements in either set but not in both:

set1 ^ set2

​

13

Mutable vs. Immutable

• Mutable means we can add or remove elements from it, we can change it without having to declare a new value of the same type
• Lists, dictionaries, and sets
• Immutable means we cannot change or remove elements from it, we have to declare a new value of the same type
• int, float, bool, string, tuple

14

Tuples

• Like lists, tuples represents an ordered sequence of values
• Like strings, tuples are immutable
• The elements of a tuple can be anything (including mutable types)

​

Examples:

()

(4, 7, 9)

("hi", [1, 2], 5)

15

Tuple operations

• Creating a set

("Once", "upon", "a", "time", "there", "was", "a")

(3, 1) + (4, 1) => (3, 1, 4, 1) # creates a new tuple!

​

• Accessing a set
• Can index just like lists:

​

tup = ("Once", "upon", "a", "time", "there", "was", "a")

print(tup[0]) => "Once"

print(tup[-1]) => "a"

​

• Tuples are immutable, once it is created it cannot be updated

16

Tuple operations

17

• Creating a tuple

​

("Once", "upon", "a", "time", "there", "was", "a")

(3, 1) + (4, 1) => (3, 1, 4, 1) # a new tuple!

​

• Accessing a tuple

​

tup = ("Once", "upon", "a", "time", "there", "was", "a")

print(tup[0]) => "Once"

print(tup[-1]) => "a"

​

• Tuples are immutable!

​

Tuples are immutable�Lists are mutable

18

def update_record(record, position, value):

"""Change the value at the given position"""

record[position] = value

​

mylist = [1, 2, 3]

mytuple = (1, 2, 3)

update_record(mylist, 1, 10)

print(mylist)

update_record(mytuple, 1, 10)

print(mytuple)

​

See in python tutor