Tuples and sets

What you'll learn

Use tuples as unchangeable pairs of values
Remove duplicate values with sets
Form intersections, unions and differences

Tuples: values that belong together

A tuple looks like a list, just with round brackets. The big difference: you can’t change it. That suits things that belong together for good – a coordinate, a date, a colour:

point = (3, 7)
print(point[0], point[1])

# Unpacking: give every value a name
x, y = point
print("x =", x, "y =", y)

colour = (255, 128, 0)
red, green, blue = colour
print(red + green + blue)

Python uses tuples a lot behind the scenes – for example when a function returns two values:

def min_max(numbers):
    return min(numbers), max(numbers)


smallest, biggest = min_max([4, 9, 1, 7])
print(smallest, biggest)

Sets: every value only once

A set is written with curly braces. It has no fixed order and contains every value only once:

visitors = ["Ana", "Ben", "Ana", "Cem", "Ben"]
unique = set(visitors)
print(unique)
print(len(unique), "different visitors")
print("Cem" in unique)

Calculating with sets

These signs only work with sets. If you have lists, turn them into sets with set(...) first.

Sign	Name	Meaning
`a & b`	intersection	in both
`a \| b`	union	in at least one
`a - b`	difference	in `a` but not in `b`

football = {"Ana", "Ben", "Cem"}
chess = {"Cem", "Dora", "Ana"}
print("both:", football & chess)
print("either:", football | chess)
print("only football:", football - chess)

Quick check

Before you start the task: what does this program print?

numbers = [5, 3, 5, 1, 3]
print(len(numbers), len(set(numbers)))

Your task

Your turn

Write two functions. different(lst) returns how many different values are in the list. common(a, b) gets two lists and returns a sorted list of the values that appear in both (each only once). Use sets for this. Careful: & only works with sets, not with lists – a & b gives an error here. So turn each list into a set first.

def different(lst):
    pass


def common(a, b):
    pass


print(different([1, 2, 2, 3, 3, 3]))
print(common(["Ana", "Ben", "Cem"], ["Cem", "Dora", "Ana"]))

Something to think about
set(lst) removes all duplicates – len() of that is the number you want.
Which command helps?
Turn both lists into sets and intersect them with &. sorted(...) turns that into a sorted list.

Gap code – fill in the ___

                        def different(lst):
    return ___


def common(a, b):
    return ___


print(different([1, 2, 2, 3, 3, 3]))
print(common(["Ana", "Ben", "Cem"], ["Cem", "Dora", "Ana"]))

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

def different(lst):
    return len(set(lst))


def common(a, b):
    return sorted(set(a) & set(b))


print(different([1, 2, 2, 3, 3, 3]))
print(common(["Ana", "Ben", "Cem"], ["Cem", "Dora", "Ana"]))