← IGCSE Computer Science Paper 2

Declaring a 2D array: ARRAY[l1:u1, l2:u2]

Declares a 2D array called Grid. The first range is the rows, the second is the columns, split by a comma. All elements share one data type, here CHAR.

Part of: IGCSE Computer Science 0478 — 8.2 Arrays — Module 27

Understanding Declaring a 2D array: ARRAY[l1:u1, l2:u2] in IGCSE Computer Science Paper 2

What it does: The comma between 1:5 and 1:8 splits the first dimension range from the second dimension range. OF CHAR sets one data type for all nine elements, so none of them can hold a different type. A 2D declaration puts both ranges inside ONE pair of square brackets, split by a comma. Each dimension has its own range. The rows can start at 0 while the columns start at 1. Both ranges here cover 3 values. The declaration needs OF between the ranges and the data type, otherwise it is not valid pseudocode.

Common mistakes: A declaration needs a colon between the identifier and ARRAY. It should be DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR, so the first line should be DECLARE Grid :. The first range is 1:3, so the row index can only be 1, 2 or 3. Row 4 is out of range. It should be Board[3, 3] ← 'O'.

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This module's key terms are also in a printable crossword (Modules 469–478) and word search (Modules 459–478).

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Learning

Duo

Which line correctly declares a 2D array for a 3 by 3 noughts and crosses board?

DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHARDECLARE NoughtsAndCrosses : ARRAY[1:3][1:3] OF CHAR

Matches

DECLARE Grid : ARRAY[1:3][1:3] OF CHAR

↓

The two ranges are written in separate square brackets. They must be in one pair of brackets separated by a comma: ARRAY[1:3, 1:3].

Applying

Meanings

Term

DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR

Meaning

Declares a 2D array called Grid. The first range is the rows, the second is the columns, split by a comma. All elements share one data type, here CHAR.

Practice

DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR

Testing

Gaps

Board : ARRAY[1:3, 1:3] OF CHAR

Check Answer

Spots

1DECLARE Grid =

2 ARRAY[1:3, 1:3] OF CHAR

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Quick Questions

1. Which line correctly declares a 2D array for a 3 by 3 noughts and crosses board?

A. DECLARE NoughtsAndCrosses : ARRAY[1:3][1:3] OF CHAR

B. DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR ✓

A 2D declaration puts both ranges inside ONE pair of square brackets, split by a comma.

2. In ARRAY[1:4, 1:6] OF INTEGER, what separates the range for the rows from the range for the columns?

A. A comma ✓

B. A semicolon

A comma separates the two ranges. The colon is only used inside each range, between the lower and upper bound.

3. Which declaration gives an array with 5 rows and 2 columns?

A. DECLARE Scores : ARRAY[1:2, 1:5] OF INTEGER

B. DECLARE Scores : ARRAY[1:5, 1:2] OF INTEGER ✓

The first range is the rows and the second is the columns, so 5 rows and 2 columns is [1:5, 1:2].

4. True or false: ARRAY[0:2, 1:3] is a valid pair of ranges, because each dimension can have its own lower bound.

A. True ✓

B. False

Each dimension has its own range. The rows can start at 0 while the columns start at 1. Both ranges here cover 3 values.

5. Which declaration is wrong because the array is meant to hold CHAR values in some elements and INTEGER values in others?

A. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR and INTEGER ✓

B. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

All elements of an array share one data type, so you write just one type after OF. You cannot mix two.

Match the Pairs

DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR → Declares a 2D array with 3 rows and 3 columns, where every element stores one CHAR

DECLARE Marks : ARRAY[0:4, 0:2] OF INTEGER → Declares a 2D array with indexes 0 to 4 in the first dimension and 0 to 2 in the second, storing INTEGER values

The two ranges for the two dimensions are separated by a comma inside the square brackets → DECLARE Seats : ARRAY[1:5, 1:8] OF BOOLEAN

Every element in a 2D array shares one data type, written after OF → DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

DECLARE Grid : ARRAY[1:3][1:3] OF CHAR → The two ranges are written in separate square brackets. They must be in one pair of brackets separated by a comma: ARRAY[1:3, 1:3].

DECLARE Scores : ARRAY[1:4, 1:3] CHAR → The keyword OF is missing before the data type. It should be ARRAY[1:4, 1:3] OF CHAR.

Key Term

DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR

Declares a 2D array called Grid. The first range is the rows, the second is the columns, split by a comma. All elements share one data type, here CHAR.

Fill in the Blanks

1. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

Hint: Type the keyword that starts every array declaration.

2. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

Hint: Type the upper bound of the column range for a noughts and crosses board with three columns.

3. DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR

Hint: Type the keyword that links the two ranges to the data type.

4. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

Hint: Type the data type for a board where each square holds a single X or O.

5. DECLARE Table : ARRAY[0:2, 0:3] OF INTEGER

Hint: Type the lower bound of the second range so the columns are numbered from the start of a zero-based count.

Spot the Error

Question 1

A DECLARE Grid =

B ARRAY[1:3, 1:3] OF CHAR

C Grid[1, 1] ← 'X'

D OUTPUT Grid[1, 1]

Line A — A declaration needs a colon between the identifier and ARRAY. It should be DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR, so the first line should be DECLARE Grid :

Question 2

A DECLARE Board :

B ARRAY[1:3, 1:3] OF CHAR

C Board[4, 3] ← 'O'

D OUTPUT Board[3, 3]

Line C — The first range is 1:3, so the row index can only be 1, 2 or 3. Row 4 is out of range. It should be Board[3, 3] ← 'O'.

Question 3

A DECLARE Seats :

B ARRAY[0:2; 0:4] OF BOOLEAN

C Seats[0, 0] ← TRUE

Line B — The two ranges must be separated by a comma, not a semicolon. It should be ARRAY[0:2, 0:4] OF BOOLEAN.

Question 4

A DECLARE Marks :

B ARRAY[1:2, 1:3] OF INTEGER

C Marks[1, 1] ← 70

D Marks[2, 3] ← "pass"

Line D — All elements share one data type. The array is declared OF INTEGER, so a string cannot be stored in it. It should be Marks[2, 3] ← 85.

Question 5

A DECLARE Grid :

B ARRAY[1:3, 1:3] OF CHAR

C Grid[1, 2] ← 'X'

D OUTPUT Grid[2]

Line D — A 2D array needs two indexes, one for the row and one for the column. It should be OUTPUT Grid[1, 2].

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