Tuples & sets

A tuple is like a list that can never change. It uses round brackets and is great for fixed groups like coordinates. Unpacking splits it into variables:

point = (3, 4)
x, y = point

A set keeps only unique items, in no particular order:

set(["a", "b", "a"])   # {'a', 'b'}
{1, 2} & {2, 3}        # {2}  in both
{1, 2} | {2, 3}        # {1, 2, 3}  either
{1, 2} - {2, 3}        # {1}  in the first but not the second

Use set() for an empty set ({} is an empty dictionary). cities.add("Oslo") adds one item to a set; adding the same item again has no effect. "Oslo" in cities tests membership. cities.discard("Oslo") removes an item if it is there and quietly does nothing if not; cities.remove("Oslo") raises a KeyError for a missing item.

Tuples are immutable: point[0] = 5 raises a TypeError. To change one, copy it into a list, change the list, and convert back:

cells = list(point)    # [3, 4]
cells[0] = 5
point = tuple(cells)   # (5, 4)

See it run

colors = {"red", "blue"}
colors.add("red")
colors.add("green")
a, b = 1, 2
a, b = b, a
print(len(colors), a, b)

A set never keeps duplicates, and a tuple can be unpacked into several names at once.

Practice tuples & sets

Exercise 1

Use point and tags in the editor below.

Unpack point into x and y, then make dist = √(x² + y²) using ** 0.5 (5.0).
Make unique the set of tags and count its size.

Exercise 2

Use the chess and drama club sets in the editor below. Using set operators:

both = students in both clubs
any_club = everyone in at least one club
only_chess = students in chess but not drama (use -)

Exercise 3

Use routes in the editor below. Each route is a tuple (start, end, km). Loop with for start, end, km in routes: to:

add up total_km
build cities, a set of every city mentioned (use .add)

Exercise 4

The start tuple in the editor below holds a robot's (x, y, heading). Its second line tries to change x in place and crashes with a TypeError, because tuples are immutable. Replace that line: copy start into a list, set index 0 to 5, and convert back so moved is the tuple (5, 2, "N"). Leave start unchanged.

Exercise 5

A gym logs every check-in in visits (names repeat).

members = the set of distinct names, made with set(visits)
came_ana = whether "Ana" is in members (use in)
Bo cancelled: remove "Bo" with .discard, then also discard "Zed", who never visited
unique_count = how many members remain

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Practice

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Memory · names → values

Variables appear here as Python creates them.

Output & plots