Sets

Overview

Teaching: 5 min
Exercises: 5 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 unsorted unique collection of things.

• Different from lists
  • Lists have a defined order of elements, sets are unordered.
  • Lists can contain the same value multiple times, sets do not.
• Sets have a number of fast operations you can use to compare or combine them.
• Sets are great to keep track of things where order an count don’t matter.
• If you put a value that is already in a set into that set again, the set will not change.

beatles = set(['John', 'Paul', 'George', 'Ringo'])
print('Beatles:', beatles)
print('length:', len(beatles))
beatles.add('Ringo')
print('Beatles:', beatles)
print('length:', len(beatles))

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

Use in to check if something is in a set

Sets allow for efficient check of members.

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

You can add values to the set by using add()

beatles.add('Pete')
print('beatles is now:', beatles)

beatles is now: {'Pete', 'John', 'Ringo', 'Paul', 'George'}

You can remove values from the set by using remove()

beatles.remove('Pete')
print('beatles is now:', beatles)

beatles is now: {'John', 'Ringo', 'Paul', 'George'}

Multiple sets can be combined using union()

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

Sets are unordered

As sets are unordered, the order of the elements can be different when you print this.

You can intersect two sets

This gives a new set containing only members existing in both sets

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

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

You can get the difference between to sets

This gives a new set containing all members of the first set not in the second set

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

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

For difference(), the order matters

Using the same sets in a different order gives a different result.

uneven_primes = primes.difference(even)
print('Primes that are not even:', uneven_primes)

Primes that are not even: {3, 5, 7}

Convert to a list and sort to get sorted output

sorted_primes = list(primes)
sorted_primes.sort()
print('Sorted primes:', sorted_primes)

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

Initialising

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

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_____.______(multiples_of______)
sorted_result2 = list(result2)
sorted_result2.sort()
print('2', sorted_result2)

1 {6}
2 [3, 9]

Key Points

• A set stores unsorted unique values.

• The set list contains no values.

• Sets may contain values of different types.