← IGCSE Computer Science Paper 2

Declaring and using a 2D array

Declares a 2D array of characters with 3 rows and 3 columns. Reach one element with a row index then a column index, e.g. NoughtsAndCrosses[2,3] ← 'X'.

Part of: IGCSE Computer Science 0478 — 7c Pseudocode and Flowcharts — Module 41

Understanding Declaring and using a 2D array in IGCSE Computer Science Paper 2

What it does: The two index ranges 1:4 and 1:5 give 4 rows and 5 columns, which is how a 2D array is declared. For each row the inner loop runs through all the columns, so every element is reached by its row index and column index. The first range gives the row indices (lower:upper). The second range gives the column indices. The data type comes after OF. 3 rows x 3 columns = 9 elements. Do not add the two sizes together. The row index must be between 1 and 3. Row 4 is outside the declared range, so [4,1] does not exist.

Common mistakes: The inner loop is controlled by Col, so it must end with NEXT Col. NEXT Row belongs to the outer loop on the last line. The line should be ' NEXT Col'. A 2D array needs two index ranges, one for rows and one for columns. ARRAY[1:3] only declares a 1D array. It should be ARRAY[1:3, 1:3] OF CHAR.

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Learning

Duo

Which line correctly declares a 2D array of characters with 3 rows and 3 columns?

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

Matches

DECLARE Board : ARRAY[1:3] OF CHAR

↓

Only one index range is given, so this declares a 1D array. A 2D array needs two ranges, for example ARRAY[1:3, 1:3].

Applying

Meanings

Term

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

Meaning

Declares a 2D array of characters with 3 rows and 3 columns. Reach one element with a row index then a column index, e.g. NoughtsAndCrosses[2,3] ← 'X'.

Practice

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

Testing

Gaps

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

Check Answer

Spots

3 Board[Row,Col] ← '-'

4 NEXT Row

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Quick Questions

1. Which line correctly declares a 2D array of characters with 3 rows and 3 columns?

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

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

Cambridge pseudocode puts both index ranges inside ONE pair of square brackets, separated by a comma: ARRAY[l1:u1, l2:u2].

2. In ARRAY[1:3, 1:3], what does the first range 1:3 describe?

A. The rows, from lower index 1 to upper index 3 ✓

B. The data type of every element

The first range gives the row indices (lower:upper). The second range gives the column indices. The data type comes after OF.

3. Which statement stores 'X' in row 2, column 3 of the grid?

A. NoughtsAndCrosses[2,3] ← 'X' ✓

B. NoughtsAndCrosses[3,2] ← 'X'

The row index comes first and the column index second, so row 2, column 3 is written [2,3].

4. How many elements does DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR hold?

A. 9 ✓

B. 6

3 rows x 3 columns = 9 elements. Do not add the two sizes together.

5. Which code visits every element of the 3 by 3 grid?

A. A single FOR loop from 1 TO 3 using one index

B. A FOR loop for Row with a nested FOR loop for Column inside it ✓

A 2D array needs two indices, so use nested FOR loops: the outer loop steps through the rows and the inner loop steps through the columns.

Match the Pairs

DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR → Declares a 2D array of characters with 3 rows and 3 columns, giving 9 elements

NoughtsAndCrosses[2,3] ← 'X' → Stores 'X' in the element in row 2, column 3

A 2D array is declared with a lower and upper bound for the rows and for the columns → DECLARE Grid : ARRAY[1:4, 1:5] OF INTEGER

Nested FOR loops visit every element of a 2D array, the outer loop for rows and the inner loop for columns → FOR Row ← 1 TO 3 FOR Col ← 1 TO 3 OUTPUT NoughtsAndCrosses[Row,Col] NEXT Col NEXT Row

DECLARE Board : ARRAY[1:3] OF CHAR → Only one index range is given, so this declares a 1D array. A 2D array needs two ranges, for example ARRAY[1:3, 1:3].

NoughtsAndCrosses[2][3] ← 'X' → The row and column indexes are written in separate brackets. In pseudocode they go in one pair of brackets separated by a comma: [2,3].

Key Term

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

Declares a 2D array of characters with 3 rows and 3 columns. Reach one element with a row index then a column index, e.g. NoughtsAndCrosses[2,3] ← 'X'.

Fill in the Blanks

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

Hint: Type the keyword that declares a table of values with two index ranges.

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

Hint: Type the data type used to store single characters such as X and O.

3. // 3 rows and 4 columns DECLARE Board : ARRAY[1:3, 1:4] OF CHAR

Hint: Type the upper bound of the second range, which sets the number of columns.

4. // row 2, column 3 NoughtsAndCrosses[2,3] ← 'X'

Hint: Type the first index, which picks the row.

5. FOR Row ← 1 TO 3 FOR Col ← 1 TO 3 Grid[Row, Col] ← '-'

Hint: Type the inner loop variable that is used as the column index.

Spot the Error

Question 1

A FOR Row ← 1 TO 3

B FOR Col ← 1 TO 3

C Board[Row,Col] ← '-'

D NEXT Row

E NEXT Row

Line D — The inner loop is controlled by Col, so it must end with NEXT Col. NEXT Row belongs to the outer loop on the last line. The line should be ' NEXT Col'.

Question 2

A DECLARE Board :

B ARRAY[1:3] OF CHAR

C Board[2,3] ← 'X'

Line B — A 2D array needs two index ranges, one for rows and one for columns. ARRAY[1:3] only declares a 1D array. It should be ARRAY[1:3, 1:3] OF CHAR.

Question 3

A Board[1,1] ← 'O'

B Board[2][3] ← 'X'

C OUTPUT Board[2,3]

Line B — In Cambridge pseudocode both indices go inside one pair of square brackets, separated by a comma: row then column. It should be Board[2,3] ← 'X'.

Question 4

A Board[1,1] ← 'X'

B Board[2,2] ← 'X'

C Board[3,3] ← 'X'

D OUTPUT Board[2,4]

Line D — Board is declared as ARRAY[1:3, 1:3], so the column index can only be 1, 2 or 3. Column 4 does not exist. Use a valid index such as Board[2,2].

Question 5

A FOR Row ← 1 TO 3

B FOR Col ← 1 TO 3

C INPUT Board[Row]

D NEXT Col

E NEXT Row

Line C — An element of a 2D array needs both a row index and a column index. Board[Row] has no column. It should be INPUT Board[Row,Col].

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