Two-dimensional array: rows and columns

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Quick Questions

Shade in the A or B circle for each question.

1. A noughts and crosses board has 3 rows and 3 columns. Which declaration suits it?

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

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

2. In Marks[2, 3], what does the first index (2) tell you?

A The column number

B The row number

3. Which statement about a 2D array is true?

A Rows run across and columns run down

B Rows run down and columns run across

4. Which element is in row 1, column 3 of the Board array?

A Board[3, 1]

B Board[1, 3]

5. How many index numbers do you need to identify ONE element in a 2D array?

A Two: a row and a column

B One: just the row

Match the Pairs

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

Left column		Right column
1DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR		AOnly one range is given, so this is a 1D list, not a table. A 2D array needs two ranges, such as ARRAY[1:4, 1:3].
2Marks[2, 3]		BA 2D array with 3 rows and 3 columns that holds characters, such as a noughts and crosses board
3The first index is the row and the second is the column, so this puts an X in the top row, third column of a noughts and crosses grid		COnly one index is given. An element of a 2D array needs two indexes, a row and a column, such as Grid[2, 1].
4A table of marks for 4 students and 3 tests needs rows and columns, so the array has two ranges		DGrid[1, 3] ← 'X'
5Grid[2] ← 'O'		EDECLARE Marks : ARRAY[1:4, 1:3] OF INTEGER
6DECLARE Marks : ARRAY[1:4] OF INTEGER		FThe element in row 2 and column 3 of the Marks table

Definitions

2D array

Fill in the Blanks

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

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

// Put X in row 2, column 3 Board[2, _] ← 'X'

// Student 4 scored 75 in test 2 Marks[_, 2] ← 75

// Show the mark in row 2, column 3 ______ Marks[2, 3]

FOR Row ← 1 TO 5 OUTPUT Marks[___, 1] NEXT Row

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A Board[1, 1] ← "X"

B Board[2, 2] ← "O"

C Board[3] ← "X"

D OUTPUT Board[2, 2]

Question 2

A Marks[1, 1] ← 72

B Marks[1, 2] ← 65

C Marks[2, 1] ← 80

D Marks(2, 2) ← 91

Question 3

A // Marks[student, test]

B Marks[3, 2] ← 58

C Total ← Marks[3, 1]

D Total ← Total + Marks[2, 3]

Question 4

A Row ← 2

B Col ← 3

C Grid[Row, Col, 1] ← "X"

D OUTPUT Grid[Row, Col]

Question 5

A Board[3, 1] ← "O"

B OUTPUT "Top right square:"

C OUTPUT Board[1, 3]

Answer Key

Two-dimensional array: rows and columns

Section A: Quick Questions

A — DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

B — The row number

A — Rows run across and columns run down

B — Board[1, 3]

A — Two: a row and a column

Section B: Match the Pairs

DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR → A 2D array with 3 rows and 3 columns that holds characters, such as a noughts and crosses board

Marks[2, 3] → The element in row 2 and column 3 of the Marks table

The first index is the row and the second is the column, so this puts an X in the top row, third column of a noughts and crosses grid → Grid[1, 3] ← 'X'

A table of marks for 4 students and 3 tests needs rows and columns, so the array has two ranges → DECLARE Marks : ARRAY[1:4, 1:3] OF INTEGER

Grid[2] ← 'O' → Only one index is given. An element of a 2D array needs two indexes, a row and a column, such as Grid[2, 1].

DECLARE Marks : ARRAY[1:4] OF INTEGER → Only one range is given, so this is a 1D list, not a table. A 2D array needs two ranges, such as ARRAY[1:4, 1:3].

Section C: Definitions

2D array — A table of rows and columns held in one variable. Each element needs two indexes: the first is the row (across), the second is the column (down), e.g. Marks[2, 3].

Section D: Fill in the Blanks

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

// Put X in row 2, column 3
Board[2, 3] ← 'X'

// Student 4 scored 75 in test 2
Marks[4, 2] ← 75

// Show the mark in row 2, column 3
OUTPUT Marks[2, 3]

FOR Row ← 1 TO 5
   OUTPUT Marks[Row, 1]
NEXT Row

Section E: Spot the Error

C — A 2D array element needs two indexes: the row and then the column. Board[3] gives only one. It should be Board[3, 1] ← "X".

D — Array indexes go inside square brackets, not round brackets. The line should be Marks[2, 2] ← 91.

D — The first index is the row (the student) and the second is the column (the test). Student 3's second test is Marks[3, 2], not Marks[2, 3].

C — A 2D array needs exactly two indexes, one for the row and one for the column. A third index is wrong. It should be Grid[Row, Col] ← "X".

A — The top right square is in row 1 (the first index) and column 3 (the second index). Board[3, 1] is the bottom left square. It should be Board[1, 3] ← "O".