06: Liste

Store and manipulate collections of values with Python lists.

python lists indexing slice list-methods

Lesson 06 · Lists

What you'll learn

  • How to create and access a list
  • Indexing and slicing
  • The main list methods
  • Iterating over lists with for

What is a list?

A list stores multiple values in a single variable, in order:

grades = [8, 9, 7, 10, 6]
fruits = ["apple", "pear", "cherry"]
mixed = [42, "hello", True, 3.14]   # can contain different types
empty = []                          # empty list

Indexing

Each element has an index (position), starting from 0:

fruits = ["apple", "pear", "cherry", "currant"]
#           0        1       2          3

print(fruits[0])   # apple
print(fruits[2])   # cherry
print(fruits[-1])  # currant — last element
print(fruits[-2])  # cherry — second-to-last

Negative indexing

-1 is the last element, -2 the second-to-last, etc.

Slicing — sub-lists

grades = [5, 7, 8, 9, 10]
#         0  1  2  3   4

print(grades[1:3])    # [7, 8]      — from index 1 to 3 (exclusive)
print(grades[:3])     # [5, 7, 8]   — from start to 3
print(grades[2:])     # [8, 9, 10]  — from 2 to the end
print(grades[::2])    # [5, 8, 10]  — every other element
print(grades[::-1])   # [10, 9, 8, 7, 5] — reversed list

Modifying lists

colors = ["red", "green", "blue"]

colors[1] = "yellow"
print(colors)   # ["red", "yellow", "blue"]

The main methods

grades = [8, 6, 9, 7]

grades.append(10)         # add to the end: [8, 6, 9, 7, 10]
grades.insert(0, 5)       # insert at position 0: [5, 8, 6, 9, 7, 10]
grades.remove(6)          # remove the first occurrence of 6
removed_grade = grades.pop()     # remove and return the last element
removed_grade = grades.pop(0)    # remove and return the element at index 0
grades.sort()             # sort ascending (modifies the original list)
grades.sort(reverse=True) # sort descending
grades.reverse()          # reverse the list
print(len(grades))        # number of elements
print(grades.count(9))    # how many times 9 appears
print(grades.index(9))    # index of the first occurrence of 9

Iterating with for

fruits = ["apple", "pear", "cherry"]

for fruit in fruits:
    print(fruit)

enumerate() — index and value

for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple
# 1: pear
# 2: cherry

Useful functions

grades = [8, 6, 9, 7, 10]

print(len(grades))    # 5
print(sum(grades))    # 40
print(max(grades))    # 10
print(min(grades))    # 6
print(sorted(grades)) # [6, 7, 8, 9, 10] — does not modify the original

Membership check with in

fruits = ["apple", "pear", "cherry"]

print("pear" in fruits)        # True
print("pineapple" in fruits)   # False
print("pineapple" not in fruits) # True

Exercises

Exercise 1 — Access

You have the list planets = ["Mercury", "Venus", "Earth", "Mars"]. Display the first and last planet.

Solution 
planets = ["Mercury", "Venus", "Earth", "Mars"]
print(planets[0])    # Mercury
print(planets[-1])   # Mars

Exercise 2 — Sum of grades

Ask the user for 5 grades, store them in a list, and display the sum and the average.

Solution 
grades = []
for i in range(5):
    grade = float(input(f"Grade {i + 1}: "))
    grades.append(grade)
print(f"Sum: {sum(grades)}")
print(f"Average: {sum(grades) / len(grades):.2f}")

Exercise 3 — Filter

From the list [3, 7, 2, 9, 4, 6, 1, 8, 5], build a new list containing only the numbers greater than 5.

Solution 
numbers = [3, 7, 2, 9, 4, 6, 1, 8, 5]
large = []
for n in numbers:
    if n > 5:
        large.append(n)
print(large)   # [7, 9, 6, 8]

Mini-project: Top 5 scores

Ask the user for 5 scores, sort them descending, and display the leaderboard.

Example: 

Score 1: 850
Score 2: 1200
Score 3: 450
Score 4: 990
Score 5: 750

=== TOP 5 SCORES ===
#1: 1200
#2: 990
#3: 850
#4: 750
#5: 450

Solution 
scores = []
for i in range(5):
    score = int(input(f"Score {i + 1}: "))
    scores.append(score)

scores.sort(reverse=True)

print()
print("=== TOP 5 SCORES ===")
for i, score in enumerate(scores):
    print(f"#{i + 1}: {score}")

Summary

  • Lists store multiple values: [val1, val2, val3]
  • Indexing starts at 0; negative indexes count from the end
  • Slicing: list[start:stop:step]
  • Key methods: append(), remove(), sort(), pop(), len()
  • for elem in list: iterates over all elements
  • x in list checks whether x exists in the list

Next step: → Lesson 07: Dictionaries

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