1-D arrays

DECLARE an array

• An array stores many values under one name.
• DECLARE Score : ARRAY[1:5] OF INTEGER makes room for 5 integers.
• The numbers 1:5 are the first and last index.

DECLARE Score : ARRAY[1:5] OF INTEGER
Score[1] ← 40
OUTPUT Score[1]   // → 40

The declaration sets the index bounds

• In Score : ARRAY[1:5], the first item is Score[1] and the last is Score[5]. Cambridge usually uses a lower bound of 1, but other bounds are valid.
• ARRAY[0:2] instead has three cells, indexed 0, 1 and 2. Follow the declaration rather than assuming every pseudocode array starts at 1. Python and JavaScript lists/arrays start at 0.

DECLARE Name : ARRAY[1:3] OF STRING
Name[1] ← "Ada"
Name[2] ← "Sam"
OUTPUT Name[2]    // → Sam

Loop through an array

• A FOR loop visits each index in turn.

DECLARE Score : ARRAY[1:3] OF INTEGER
Score[1] ← 10
Score[2] ← 20
Score[3] ← 30
DECLARE I : INTEGER
FOR I ← 1 TO 3
    OUTPUT Score[I]   // 10, 20, 30
NEXT I

Watch out

• The index must stay inside the declared range: Score[6] is out of [1:5].
• ARRAY[1:5] has 5 cells, numbered 1 to 5 — not 0 to 4.

Common mistakes

• Declare an array with its bounds: DECLARE Values : ARRAY[1:N] OF INTEGER.
• Index only within the declared range.

Now you try

• Declare an array, fill it, then read or loop over it.
• Press Check answer to test it.