1-D arrays (lists)

A list holds many values

• A list (the exam calls it an array) stores many values under one name.
• Write the items in square brackets, separated by commas.
• An array normally stores items of the same type.

scores = [40, 55, 72, 90]
print(scores)

Reach an item by its index

• Each item has a position number called its index.
• Python counts from 0, so the first item is scores[0].

scores = [40, 55, 72, 90]
print(scores[0])     # 40
print(scores[2])     # 72

Change an item

• Assign to an index to replace that one item.

scores = [40, 55, 72, 90]
scores[1] = 99
print(scores)        # [40, 99, 72, 90]

Length and looping

• len(scores) gives how many items there are.
• A for loop visits each item in turn.

scores = [40, 55, 72, 90]
print(len(scores))   # 4
for s in scores:
    print(s)

In the exam: arrays start at 1

• Pseudocode declares an array with a size, and usually counts from 1.

DECLARE Scores : ARRAY[1:4] OF INTEGER
Scores[1] ← 40
OUTPUT Scores[1]

Common mistakes

• Index from 0; the last item is at position length - 1.
• Going past the end gives an index error.

Now you try

• Use an index, or a loop, on each list.
• Press Check answer to test your code.