workshop_7.md

Workshop 7 — Lists

Prerequisites

• Workshop 2: strings, string methods, len()
• Workshop 3: if/elif/else, in
• Workshop 4: for, range, the accumulator, counting with a condition
• Workshop 5: def, parameters, return
• Workshop 6: keyword arguments, returning several values, early return

1. Why lists

Three test scores, three variables:

score_1 = 90
score_2 = 75
score_3 = 82

Thirty students would need thirty variables. A list keeps many values in one variable:

scores = [90, 75, 82]
print(scores)
print(len(scores))
# [90, 75, 82]
# 3

Square brackets, commas between the items. len() — which you used on strings — tells you how many items there are. A list can hold any type, and [] is an empty list:

names = ["Nina", "Luka", "Ana"]
empty = []

2. Indexing: getting one item

Every item has a position, its index. Indexes start at 0, just like range(3) gives 0, 1, 2:

names = ["Nina", "Luka", "Ana"]
print(names[0])
print(names[2])
print(names[-1])
# Nina
# Ana
# Ana

A negative index counts from the end: -1 is the last item, -2 the one before it. The last positive index is always len(names) - 1. Ask for an index that does not exist and Python stops with an error:

print(names[3])
# IndexError: list index out of range

3. Changing an item

Unlike a string, a list can be changed after it is created. Assign to an index to replace that item:

names = ["Nina", "Luka", "Ana"]
names[1] = "Giorgi"
print(names)
# ['Nina', 'Giorgi', 'Ana']

Notice that print shows the strings with quotes when they are inside a list.

4. Adding and removing items

Lists have methods, called with a dot — like .upper() on a string. The difference: list methods change the list itself.

cart = ["bread", "milk"]
cart.append("cheese")
cart.insert(0, "water")
print(cart)
# ['water', 'bread', 'milk', 'cheese']

cart.remove("milk")
last = cart.pop()
print(last)
print(cart)
# cheese
# ['water', 'bread']

Method	What it does
append(x)	add x to the end
insert(i, x)	add x at index i, pushing the rest right
remove(x)	remove the first x (error if there is none)
pop()	remove the last item and return it
pop(i)	remove the item at index i and return it

5. Looping over a list

for walks through a list item by item, exactly like it walked through the letters of a string in Workshop 4:

prices = [4.5, 12.0, 7.5]
total = 0
for price in prices:
    total += price
print(f"Total: {total}")
print(f"Average: {total / len(prices)}")
# Total: 24.0
# Average: 8.0

If you also need the position, loop over range(len(...)) and index into the list:

names = ["Nina", "Luka", "Ana"]
for i in range(len(names)):
    print(i, names[i])
# 0 Nina
# 1 Luka
# 2 Ana

6. Building a list in a loop

Start with an empty list, append inside the loop, use it after. It is the accumulator from Workshop 4 — but you collect values instead of adding them up:

evens = []
for n in range(1, 11):
    if n % 2 == 0:
        evens.append(n)
print(evens)
# [2, 4, 6, 8, 10]

7. Finding an element

in asks "is it there?" — you used it on strings in Workshop 3. .index() gives the position of the first match, and .count() says how many there are:

fruits = ["apple", "pear", "plum", "pear"]
print("plum" in fruits)
print("kiwi" in fruits)
print(fruits.index("pear"))
print(fruits.count("pear"))
# True
# False
# 1
# 2

.index() stops with an error when the item is missing, so ask with in first:

if "kiwi" in fruits:
    print(fruits.index("kiwi"))
else:
    print("No kiwi.")
# No kiwi.

When the question is more than "is it equal?", search yourself — and return early the moment you find it (Workshop 6):

def first_over(numbers, limit):
    for n in numbers:
        if n > limit:
            return n
    return None

print(first_over([3, 8, 12, 5], 7))
print(first_over([3, 8, 12, 5], 20))
# 8
# None

8. Sorting

sorted() gives you a new, sorted list and leaves the original alone. reverse=True — a keyword argument — sorts from biggest to smallest:

scores = [82, 95, 67, 90]
print(sorted(scores))
print(sorted(scores, reverse=True))
print(scores)
# [67, 82, 90, 95]
# [95, 90, 82, 67]
# [82, 95, 67, 90]

The method .sort() sorts the list itself and returns None:

scores.sort()
print(scores)
# [67, 82, 90, 95]

So never write scores = scores.sort() — you would replace your list with None:

scores = scores.sort()
print(scores)
# None

Strings sort alphabetically: sorted(["pear", "apple", "plum"]) is ['apple', 'pear', 'plum'].

9. min, max, sum

Three built-in functions answer the most common questions about a list of numbers:

scores = [82, 95, 67, 90]
print(min(scores), max(scores), sum(scores))
print(sum(scores) / len(scores))
# 67 95 334
# 83.5

10. Putting it together: functions that take a list

A list is passed to a function like any other value. Combine it with returning several values:

def summary(scores):
    """Return the lowest, highest and average score."""
    return min(scores), max(scores), sum(scores) / len(scores)

low, high, average = summary([82, 95, 67, 90])
print(f"Lowest {low}, highest {high}, average {average}")
# Lowest 67, highest 95, average 83.5

A function can also build and return a new list:

def passed(scores, pass_mark=70):
    """Return a new list with only the passing scores."""
    result = []
    for score in scores:
        if score >= pass_mark:
            result.append(score)
    return result

print(passed([82, 95, 67, 90]))
print(passed([82, 95, 67, 90], pass_mark=85))
# [82, 95, 90]
# [95, 90]

Quick reference

Idea	Example
Create	scores = [90, 75, 82] / empty = []
Length	len(scores)
Index	scores[0], last: scores[-1]
Change an item	scores[1] = 80
Add	append(x), insert(i, x)
Remove	remove(x), pop(), pop(i)
Loop	for score in scores:
Build	result = [] then result.append(x) in a loop
Find	x in scores, scores.index(x), scores.count(x)
Sort	sorted(scores) (new list) / scores.sort() (in place)
Numbers	min(scores), max(scores), sum(scores)

What's next

See workshop_7/ for runnable examples, and your homework in python-127-homework-7.