Lists (1-D arrays)

A list holds many values

• A list keeps many values in one variable.
• Write the items inside square brackets: [10, 20, 30].
• The items can be numbers, strings, or a mix.

Get and change items

• Use an index, just like a string: nums[0] is the first item.
• nums[-1] is the last item.
• You can also change an item: nums[0] = 99.

fruits = ["apple", "pear", "plum"]
print(fruits[0])
print(fruits[-1])

Add items and count them

• nums.append(x) adds x to the end of the list.
• len(nums) tells you how many items there are.
• A list can grow as your program runs.

fruits = ["apple", "pear"]
fruits.append("plum")
print(len(fruits))
print(fruits)

Loop over a list

• for item in nums: visits each item in turn.
• The loop variable holds one item per pass.
• This is the easy way to process every value.

scores = [10, 20, 30]
total = 0
for s in scores:
    total = total + s
print(total)

In Cambridge pseudocode

• A list is the exam's 1-D array. Note: exam arrays often start at index 1, but Python lists start at 0.

DECLARE scores : ARRAY[1:3] OF INTEGER
scores[1] ← 10
scores[2] ← 20
scores[3] ← 30
FOR i ← 1 TO 3
    OUTPUT scores[i]
NEXT i

Common mistakes

• The first item is a[0] and the last is a[-1]; a[len(a)] is out of range.
• b = a does not copy the list — both names share it. Use a.copy() to make a real copy.
• append adds one item to the end.

Now you try

• Build or loop over a list in each task.
• Press Check answer to test your code.