Declaring and using a 2D array

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Quick Questions

Shade in the A or B circle for each question.

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

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

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

A NoughtsAndCrosses[2,3] ← 'X'

B NoughtsAndCrosses[3,2] ← 'X'

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

A 9

B 6

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1DECLARE NoughtsAndCrosses : ARRAY[1:3, 1:3] OF CHAR		ADeclares a 2D array of characters with 3 rows and 3 columns, giving 9 elements
2NoughtsAndCrosses[2,3] ← 'X'		BFOR Row ← 1 TO 3 FOR Col ← 1 TO 3 OUTPUT NoughtsAndCrosses[Row,Col] NEXT Col NEXT Row
3A 2D array is declared with a lower and upper bound for the rows and for the columns		CStores 'X' in the element in row 2, column 3
4Nested FOR loops visit every element of a 2D array, the outer loop for rows and the inner loop for columns		DOnly one index range is given, so this declares a 1D array. A 2D array needs two ranges, for example ARRAY[1:3, 1:3].
5DECLARE Board : ARRAY[1:3] OF CHAR		EDECLARE Grid : ARRAY[1:4, 1:5] OF INTEGER
6NoughtsAndCrosses[2][3] ← 'X'		FThe row and column indexes are written in separate brackets. In pseudocode they go in one pair of brackets separated by a comma: [2,3].

Definitions

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

Fill in the Blanks

Complete each line of code by filling in the missing parts.

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

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

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

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

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

Spot the Error

Shade in the letter of the line that contains an 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

Question 2

A DECLARE Board :

B ARRAY[1:3] OF CHAR

C Board[2,3] ← 'X'

Question 3

A Board[1,1] ← 'O'

B Board[2][3] ← 'X'

C OUTPUT Board[2,3]

Question 4

A Board[1,1] ← 'X'

B Board[2,2] ← 'X'

C Board[3,3] ← 'X'

D OUTPUT Board[2,4]

Question 5

A FOR Row ← 1 TO 3

B FOR Col ← 1 TO 3

C INPUT Board[Row]

D NEXT Col

E NEXT Row

Answer Key

Declaring and using a 2D array

Section A: Quick Questions

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

A — The rows, from lower index 1 to upper index 3

A — NoughtsAndCrosses[2,3] ← 'X'

A — 9

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

Section B: 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].

Section C: Definitions

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'.

Section D: Fill in the Blanks

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

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

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

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

Section E: Spot the Error

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'.

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.

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'.

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].

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].