1. What is a set?

A set is an unordered collection of distinct elements. Repeated equal values are stored only once.

• A set can be changed: you can add and remove elements.
• The order of its elements is not guaranteed.
• A set does not record element positions.
• You can check membership and visit each element in a loop.

Create an empty or non-empty set

empty_set = set()
colours = {"red", "yellow", "green"}
numbers = {4, 4, 7, 2, 7}

print(empty_set) # set()
print(len(numbers)) # 3

Create a set from text

When a string is passed to set(), its distinct characters become the elements.

letters = set("banana")
print(len(letters)) # 3: the distinct letters are a, b and n

The elements of a set may have different types, such as numbers and strings.

mixed_set = {7, "seven", 2.5} # valid

Use numeric sets for the calculations below.

2. Read, check and count

Count elements and calculate statistics

numbers = {5, 3, 7, 4}

print(len(numbers)) # 4: number of distinct elements
print(sum(numbers)) # 19: sum of the numbers
print(min(numbers)) # 3: minimum
print(max(numbers)) # 7: maximum

numbers = set()
if numbers:
 print(min(numbers))
else:
 print("The set is empty")

Check whether an element is present

fruits = {"apple", "orange", "lemon"}
item = "orange"

if item in fruits:
 print("The item is in the set")
else:
 print("The item is not in the set")

print("melon" not in fruits) # True

Loop through a set

numbers = {2, 4, 7}

for number in numbers:
 print(number) # each element is visited; the order is not guaranteed

Display a set

Use print() to display the elements. Their order is not guaranteed.

numbers = {7, 2, 4}
print(numbers) # all three elements are displayed

3. Add and remove elements

Add one element: add()

members = set()
members.add(101)
members.add(104)
members.add(101) # already present: the set does not change

print(len(members)) # 2

Choose the correct deletion method

members = {101, 104, 108}
members.discard(104)
members.discard(999) # no error
print(len(members)) # 2

members = {101, 104, 108}
student_id = 104
if student_id in members:
 members.remove(student_id)
# members.remove(999) raises KeyError

pop(): keep the returned element

tokens = {10, 20, 30}
token = tokens.pop()

print(token in tokens) # False
print(len(tokens)) # 2

Do not rely on a particular value being returned. pop() removes one available element without promising an order or a random selection.

clear(): start again

members = {101, 104}
members.clear()
print(members) # set()

members.add(109)
print(len(members)) # 1

4. Set laboratory

Apply an action and compare the set before and after it. This lab uses integers only.

5. Exercises

There are 8 exercises: the embedded warm-up and 7 checked tasks about sets. Check an answer, use a hint if needed, and read the explanation.

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numbers = {4, 4, 7, 2, 7}
print(len(numbers))
print(sum(numbers))
print(min(numbers))
print(max(numbers))

members = {101, 104}
print(104 in members)
print(109 not in members)
print("101" in members)

visited = set()
visited.add(7)
print(len(visited))
visited.add(7)
print(len(visited))
visited.clear()
print(len(visited))
visited.add(3)
print(len(visited))

members = {2, 4, 6}
members.discard(4)
members.discard(9)
print(len(members))

tokens = {10, 20, 30}
token = tokens.pop()

values = {2, 2, 3, 6, 8, 8}
count = 0
total = 0
for value in values:
 if value % 2 == 0:
   count += 1
 total += value
print(count)
print(total)