Lesson 6 – Arrays (Lists)

In this lesson you will:

• Explain what an array is (a data structure for storing multiple values).
• Use indexes to access and update elements.
• Use len() to find how many elements are in a list.
• Write loops to process arrays (sum / max / search).
• Predict the output of simple array code.
• Build a list from user input using append().

By the end you should be able to:

• Create a 1D array (list) and access elements using 0-based indexing.
• Update an element correctly and avoid "index out of range" errors.
• Loop through a list to calculate totals and find the maximum value.
• Use a list to store repeated inputs and then process them.
• Explain how a 2D array represents rows and columns (at GCSE level).

1. Quick recap

From the previous lessons, remember:

• Variables store values.
• We can display output using print().
• We can read input using input() and convert to int() or float().
• Indentation matters: Python uses indentation to show which lines belong together.
• Loops repeat code (for loops repeat a fixed number of times; while loops repeat while a condition is True).

Mini-check (predict):

What will this print?

x = 3
x = x + 1
print(x)

2. What is an array?

An array stores multiple related values under one name.

At GCSE, you'll use these key terms:

• element = one item in the array
• index = the position of an element
• value = what is stored in the element
• 0-based indexing = the first element is at index 0

In Python, we usually use a list to represent an array.

Example list:

scores = [8, 10, 21, 3]

Why arrays matter:

Instead of score1, score2, score3… you store them in one structure and use loops to process them.

3. Example 1 – Accessing elements (0-based indexing)

Predict first: What will this print?

scores = [8, 10, 21, 3]
print(scores[0])
print(scores[2])

What happens:

• scores[0] is the first value (8)
• scores[2] is the third value (21)

Try it yourself

Run it, then:

• Print the last value
• Print the second value

scores = [8, 10, 21, 3]
print(scores[0])      # First value
print(scores[2])      # Third value
print(scores[3])      # Last value (index 3)
print(scores[1])     # Second value (index 1)

4. Understanding indexes and len()

len() tells you how many elements are in a list.

Example:

scores = [8, 10, 21, 3]
print(len(scores))

Important rule:

If there are len(scores) elements, the last index is len(scores) - 1

Example:

scores = [8, 10, 21, 3]
last_index = len(scores) - 1
print(scores[last_index])

Key points to remember:

• Indexes start at 0
• len(list) is the count, not the last index
• Last index is always len − 1

Mini-check (predict):

If scores = [5, 9, 2], what is:

• len(scores) ?
• the last index?

5. Example 2 – Updating an element

Predict first: What will the list look like after the change?

scores = [8, 10, 21, 3]
scores[1] = 99
print(scores)

What happens:

This replaces the element at index 1 (10) with 99.

Try it yourself

Change it so:

• the first element becomes 0
• the last element becomes 100

scores = [8, 10, 21, 3]
scores[0] = 0        # First element
scores[3] = 100      # Last element (index 3)
print(scores)       # [0, 10, 21, 100]

6. Example 3 – Looping through a list and printing items

One of the simplest ways to use a loop with a list is to print each item.

Predict first: What will this print?

names = ["Alice", "Bob", "Charlie", "Diana"]

for i in range(len(names)):
    print(names[i])

What happens:

• len(names) gives us 4 (the number of items)
• range(len(names)) creates 0, 1, 2, 3
• For each index i, we print names[i]
• This prints each name on a separate line

Alternative way (simpler):

You can also loop directly through the values:

names = ["Alice", "Bob", "Charlie", "Diana"]

for name in names:
    print(name)

Both methods work! The first method (for i in range(len(names))) is useful when you need the index number.

Try it yourself

Create a list of your favorite colors and print each one using a loop.

# Example with colors
colors = ["red", "blue", "green", "yellow"]

for i in range(len(colors)):
    print(colors[i])

# Output:
# red
# blue
# green
# yellow

7. Example 4 – Looping through a list (total / accumulator)

This is a common pattern: accumulator (running total).

Predict first: What will the total be?

scores = [8, 10, 21, 3]
total = 0

for s in scores:
    total = total + s

print(total)

What happens:

• total starts at 0
• each element is added to total
• final total is printed

Try it yourself

Change the list values and predict the new total before running.

# Example with different values
scores = [5, 15, 25]
total = 0

for s in scores:
    total = total + s

print(total)  # 45

8. Example 5 – Finding the highest value (max)

This pattern finds the maximum value in a list by comparing each element to the "best so far".

Predict first: What will highest be at the end?

scores = [8, 10, 21, 3]
highest = scores[0]

for s in scores:
    if s > highest:
        highest = s

print(highest)

How it works step-by-step:

1. Start with the first element: highest = scores[0] sets highest to 8 (the first value). This is our starting "best so far".
2. Loop through each score: The loop checks each value in the list.
3. Compare and update: If the current score is greater than highest, we update highest to that new value.
4. After the loop: highest contains the maximum value.

Trace through the example:

• Start: highest = 8 (first element)
• Check 8: 8 > 8? No → highest stays 8
• Check 10: 10 > 8? Yes → highest = 10
• Check 21: 21 > 10? Yes → highest = 21
• Check 3: 3 > 21? No → highest stays 21
• Result: highest = 21

Why we start with scores[0]:

We need a real value from the list as our starting point. If we started with 0, and all scores were negative, we'd get the wrong answer! Using scores[0] ensures we're comparing against an actual value in the list.

Try it yourself

Try these challenges:

• Change the list to [15, 5, 25, 20] and predict what the highest will be
• What happens if all values are the same, like [7, 7, 7, 7]?
• Test with negative numbers: [-5, -2, -10, -1]

# Example 1: Different values
scores = [15, 5, 25, 20]
highest = scores[0]  # Start with 15

for s in scores:
    if s > highest:
        highest = s

print(highest)  # 25

# Example 2: All same values
scores = [7, 7, 7, 7]
highest = scores[0]  # Start with 7

for s in scores:
    if s > highest:  # 7 > 7? No, so highest stays 7
        highest = s

print(highest)  # 7 (correct!)

# Example 3: Negative numbers
scores = [-5, -2, -10, -1]
highest = scores[0]  # Start with -5

for s in scores:
    if s > highest:
        highest = s

print(highest)  # -1 (the least negative, which is the highest)

9. Example 6 – Building a list using append()

Predict first: How many numbers will be stored?

nums = []

for i in range(3):
    n = int(input("Enter a number: "))
    nums.append(n)

print(nums)

What happens:

• Start with an empty list []
• Each input is added to the end of the list

Try it yourself

Change it to ask for 5 numbers instead of 3.

nums = []

for i in range(5):  # Changed from 3 to 5
    n = int(input("Enter a number: "))
    nums.append(n)

print(nums)

10. Repeating input with a loop (then processing the list)

A very common pattern is:

• collect several inputs into a list
• then process the list (total / average / max)

Watch the video first

This video demonstrates collecting several inputs into a list and then processing the list (calculating total, average, and maximum). Watch the video below, and if you can't access it or need more explanation, read the detailed explanation that follows.

Detailed explanation

If you couldn't watch the video or need a written explanation, here's how this pattern works:

The pattern has two main steps:

1. Collect inputs: Use a loop to ask the user for multiple values and store them in a list using append().
2. Process the list: After collecting all inputs, loop through the list to calculate totals, averages, or find maximum/minimum values.

Example – ask for 5 test scores, then print the average

Predict first: Which line calculates the average?

scores = [] 
total = 0

for i in range(5):
    s = int(input("Enter a score: "))
    scores.append(s)
    total = total + s

average = total / len(scores)
print("Average:", average)

How this code works:

1. Initialize: scores = [] creates an empty list. total = 0 starts our accumulator at zero.
2. Collect inputs (loop): The for i in range(5) loop runs 5 times:
   • Each time, it asks the user for a score with input()
   • Converts it to an integer with int()
   • Adds it to the list with scores.append(s)
   • Adds it to the running total with total = total + s
3. Calculate average: After the loop, average = total / len(scores) divides the total by the number of scores (which is 5).
4. Display result: Prints the average.

Why calculate total during the loop?

We could calculate the total after collecting all scores, but doing it during the loop is more efficient. We're already going through each value once, so we can add to the total at the same time!

Try it yourself

Extend it so it also prints the highest score.

scores = []
total = 0

for i in range(5):
    s = int(input("Enter a score: "))
    scores.append(s)
    total = total + s

average = total / len(scores)
print("Average:", average)

# Find highest
highest = scores[0]
for s in scores:
    if s > highest:
        highest = s
print("Highest:", highest)

11. Indentation and loop blocks (important)

When you process arrays using loops, indentation still matters.

Example – one line inside the loop:

scores = [8, 10, 21, 3]
for s in scores:
    print(s)

Example – two lines inside the loop:

scores = [8, 10, 21, 3]
total = 0

for s in scores:
    total = total + s
    print("Running total:", total)

print("Final total:", total)

What happens:

• The indented lines repeat for each element
• The final print() runs once at the end

Try it yourself

Move print("Running total:", total) back to the left. Predict what changes, then run it.

# Correct version:
scores = [8, 10, 21, 3]
total = 0

for s in scores:
    total = total + s
    print("Running total:", total)  # Must be indented!

print("Final total:", total)

12. Common misconceptions

Read these carefully — they are very common in tests.

Mistake 1: Thinking the first element is index 1

Fix: first element is [0]

Mistake 2: Thinking the last index is len(list)

Wrong:

scores[len(scores)]

Fix:

scores[len(scores) - 1]

Mistake 3: Mixing up index and value

Wrong:

for x in scores:
    print(scores[x])

Fix:

for x in scores:
    print(x)

Mistake 4: Trying to assign beyond the list length

Example list:

scores = [10, 20, 30, 40, 50]  # List has 5 elements (indices 0-4)

Wrong:

scores[10] = 99

Fix: use append() to add, or use a valid index to replace.

Mistake 5: Printing the loop index instead of the element

Wrong:

for i in range(len(scores)):
    print(i)

Fix:

for i in range(len(scores)):
    print(scores[i])

Mistake 6: Off-by-one in list loops

Use i < len(list), not i <= len(list).

Mistake 7: Confusing "append" with "replace"

Replace needs an index: scores[2] = 50
Append adds a new element: scores.append(50)

14. Understanding Quiz

Answer all questions below, then click "Check Answers" at the end to see your score.

scores = [8, 10, 21, 3]
print(scores[0])

scores = [10, 20, 30]
scores[1] = 99

nums = [2, 4, 6]
total = 0
for n in nums:
    total = total + n
print(total)

scores = [8, 10, 21, 3]
scores[0] = scores[0] + 5
print(scores[0])

nums = [1, 2, 3]
nums.append(9)
print(nums)

nums = []
for i in range(4):
    n = int(input())
    nums.append(n)

nums = [4, 7, 2, 9, 7, 1]
print(nums[1])

13. Coding tasks – Arrays (lists) practice

Have a go at these tasks! Work through them one at a time, run your code, and check your output matches what you expect.

scores = [8, 10, 21, 3]

# your code here

scores = [8, 10, 21, 3]

# your code here

scores = [8, 10, 21, 3]

# your code here
print(scores)

nums = []

# your code here
print(nums)

names = ["Alice", "Bob", "Charlie", "Diana"]

# your code here

names = ["Alice", "Bob", "Charlie", "Diana"]

# your code here

scores = [8, 10, 21, 3]
total = 0

# your code here
print(total)

scores = [8, 10, 21, 3]

# your code here

scores = [15, 5, 25, 20]
highest = scores[0]

# your code here
print(highest)

scores = [8, 10, 21, 3, 10, 9, 14]
count = 0

# your code here
print(count)

nums = [4, 7, 2, 9, 7, 1]
target = int(input("Enter a number: "))

# your code here

nums = [4, 7, 2, 9, 7, 1, 7]
target = int(input("Enter a number: "))
count = 0

# your code here
print(count)

scores = []
total = 0

# your code here

items = ["pen", "pencil", "ruler", "pen"]
prices = [1.20, 0.80, 1.50, 1.20]

items = ["pen", "pencil", "ruler", "pen"]
prices = [1.20, 0.80, 1.50, 1.20]

target = input("Item name: ")

# your code here

times = []

# your code here

Answer all the above tasks here

Use the editor below to complete the coding tasks above.