Sets

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• What is a set, and how do I use it?

Objectives

• Explain how sets work.

• Learn about set operations.

A set keeps an unordered collection of unique items.

• Different from lists
  • Lists have a defined order and can contain duplicate values, whereas sets are unordered and contain only unique items.
• Efficient operations
  • Sets offer fast membership testing and support operations like union, intersection, and difference.
• Mutability
  • You can add or remove items from a set, but duplicate entries are ignored.

beatles = set(['John', 'Paul', 'George', 'Ringo'])
print('Beatles:', beatles)
print('Length:', len(beatles))
beatles.add('Ringo')  # Adding 'Ringo' again does nothing
print('Beatles:', beatles)
print('Length:', len(beatles))

Beatles: {'John', 'Paul', 'George', 'Ringo'}
Length: 4
Beatles: {'John', 'Paul', 'George', 'Ringo'}
Length: 4

Check Membership with in

Use the in keyword to test if an element exists in a set.

print('Ringo is one of the Beatles:', 'Ringo' in beatles)
print('Keith is one of the Beatles:', 'Keith' in beatles)

Ringo is one of the Beatles: True
Keith is one of the Beatles: False

Adding and Removing Items

Add Items with add()

beatles.add('Pete')
print('After adding Pete:', beatles)

After adding Pete: {'John', 'Paul', 'George', 'Ringo', 'Pete'}

Remove Items with remove()

beatles.remove('Pete')
print('After removing Pete:', beatles)

After removing Pete: {'John', 'Paul', 'George', 'Ringo'}

Set Operations

Union of Sets

Combine two sets using union() (or the | operator):

odd = set([1, 3, 5, 7, 9])
even = set([2, 4, 6, 8, 10])
all_numbers = odd.union(even)
print('Odd numbers:', odd)
print('Even numbers:', even)
print('All numbers:', all_numbers)

Odd numbers: {1, 3, 5, 7, 9}
Even numbers: {2, 4, 6, 8, 10}
All numbers: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

Note

Since sets are unordered, the printed order of elements may vary.

Intersection of Sets

Retrieve common elements with intersection():

primes = set([2, 3, 5, 7])
odd_primes = primes.intersection(odd)
print('Primes that are odd:', odd_primes)

Primes that are odd: {3, 5, 7}

Difference of Sets

Get elements in one set but not in another with difference():

even_non_primes = even.difference(primes)
print('Even numbers that are not prime:', even_non_primes)

Even numbers that are not prime: {4, 6, 8, 10}

Order Matters in difference()

The result of difference() depends on the order of the sets.

Converting a Set to a Sorted List

To display a set in order, convert it to a list and sort it:

sorted_primes = sorted(primes)
print('Sorted primes:', sorted_primes)

Sorted primes: [2, 3, 5, 7]

Initialising Challenge

What does the following program print?

letters = set('Hello world!')
sorted_letters = list(letters)
sorted_letters.sort()
print('Letters in greeting:', sorted_letters)

Fill in the Blanks Challenge

Fill in the blanks “__” so that the program below produces the output shown.

multiples_of_two = set([2, 4, 6, 8, 10])
multiples_of_three = set([3, 6, 9])
result1 = multiples_of_two.__(multiples_of_three)
print('1', result1)
result2 = multiples_of_three.__(multiples_of_two)
sorted_result2 = sorted(result2)
print('2', sorted_result2)

1 {6}
2 [3, 9]

Comparing Bacterial Isolates

You have two sets representing bacteria isolated from two different sources:

clinical_isolates = {"Staphylococcus aureus", "Escherichia coli", "Pseudomonas aeruginosa", "Klebsiella pneumoniae"}
environmental_isolates = {"Bacillus subtilis", "Escherichia coli", "Staphylococcus epidermidis", "Pseudomonas aeruginosa"}

Write code to:

1. Print the bacteria common to both sets (intersection).
2. Print the bacteria unique to the clinical sample (difference).
3. Print all unique bacteria from both sets (union) as a sorted list.

Solution

~~~python # 1. Intersection: bacteria present in both samples common_bacteria = clinical_isolates.intersection(environmental_isolates) print("Common bacteria:", common_bacteria) # 2. Difference: bacteria unique to the clinical sample unique_clinical = clinical_isolates.difference(environmental_isolates) print("Unique to clinical sample:", unique_clinical) # 3. Union: all unique bacteria from both sets, sorted alphabetically all_bacteria = sorted(clinical_isolates.union(environmental_isolates)) print("All unique bacteria:", all_bacteria) ~~~

Key Points

• A set stores an unordered collection of unique values.

• Sets automatically remove duplicate entries.

• Sets support operations like union, intersection, and difference.