Two-dimensional arrays

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Quick Questions

Shade in the A or B circle for each question.

1. A 2D array stores data in...

A A single line of values only

B Rows and columns, like a table

2. Which gets the element in row 2, column 3 of a seating plan?

A seats[3][2]

B seats[2][3]

3. In grid[1][4], what does the 4 represent?

A The column index

B The row index

4. Which Python code creates a 2D array with 2 rows and 3 columns?

A grid = [[0, 0, 0], [0, 0, 0]]

B grid = [0, 0, 0, 0, 0, 0]

5. A game board has 3 rows and 3 columns. How many elements does it hold?

A 9

B 6

Match the Pairs

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

Left column		Right column
1seats[2][0]		ABoth indexes are put in one set of square brackets. Each index needs its own brackets, so use seats[0][1].
2board = [["X", "O"], ["O", "X"]]		Brows = len(timetable)
3Each row of a timetable is an inner list, so len() of the outer list gives the number of rows		CThe row index 2 is out of range because there are only 2 rows, numbered 0 and 1. Use timetable[1][0] to get Art.
4Change one cell of a game board by giving its row index first, then its column index		Dboard[1][2] = "X"
5seats = [["Ava", "Liam"], ["Mia", "Noah"]] print(seats[0, 1])		EGets the element in row 2, column 0 of the seating plan: row index first, then column index
6timetable = [["Maths", "English"], ["Art", "PE"]] print(timetable[2][0])		FCreates a 2D array for a game board with 2 rows and 2 columns, as a list of row lists

Definitions

grid[row][col]

Fill in the Blanks

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

grid = [[1, 2, 3], [4, 5, 6]] print(grid[_][2]) # second row, third column

board = [["X", "O"], ["O", "X"]] print(board[0][_ ]) # first row, second column

board = [["-", "-"], ["-", "-"]] _____[1][0] = "X" print(board)

for r in range(___(grid)): for c in range(len(grid[r])): print(grid[r][c])

board = [["X", "O"], ["O", "X"]] if board[0][0] __ "X": print("Top left is X")

Spot the Error

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

Question 1

A grid = [[1, 2, 3], [4, 5, 6]]

B row = 1

C col = 3

D print(grid[row][col])

Question 2

A board = [["X","O"],["O","X"]

B row = 0

C col = 1

D print(board[row][col])

Question 3

A plan = [[0, 0, 0], [0, 0, 0]]

B plan[1, 2] = 5

C print(plan[1][2])

Question 4

A grid = [[1, 2], [3, 4]]

B total = 0

C for r in range(2):

D for c in range(3):

E total += grid[r][c]

Question 5

A board = [["X","O"],["O","X"]]

B board[0][1] = X

C print(board[0][1])

Answer Key

Two-dimensional arrays

Section A: Quick Questions

B — Rows and columns, like a table

B — seats[2][3]

A — The column index

A — grid = [[0, 0, 0], [0, 0, 0]]

A — 9

Section B: Match the Pairs

seats[2][0] → Gets the element in row 2, column 0 of the seating plan: row index first, then column index

board = [["X", "O"], ["O", "X"]] → Creates a 2D array for a game board with 2 rows and 2 columns, as a list of row lists

Each row of a timetable is an inner list, so len() of the outer list gives the number of rows → rows = len(timetable)

Change one cell of a game board by giving its row index first, then its column index → board[1][2] = "X"

seats = [["Ava", "Liam"], ["Mia", "Noah"]] print(seats[0, 1]) → Both indexes are put in one set of square brackets. Each index needs its own brackets, so use seats[0][1].

timetable = [["Maths", "English"], ["Art", "PE"]] print(timetable[2][0]) → The row index 2 is out of range because there are only 2 rows, numbered 0 and 1. Use timetable[1][0] to get Art.

Section C: Definitions

grid[row][col] — A 2D array stores data in rows and columns like a table. Use the row index first, then the column index, to reach one element, such as a seat in a seating plan.

Section D: Fill in the Blanks

grid = [[1, 2, 3], [4, 5, 6]]
print(grid[1][2])  # second row, third column

board = [["X", "O"], ["O", "X"]]
print(board[0][1
])  # first row, second column

board = [["-", "-"], ["-", "-"]]
board[1][0] = "X"
print(board)

for r in range(len(grid)):
    for c in range(len(grid[r])):
        print(grid[r][c])

board = [["X", "O"], ["O", "X"]]
if board[0][0] == "X":
    print("Top left is X")

Section E: Spot the Error

C — Each row has 3 columns, so valid column indexes are 0, 1 and 2. Column 3 gives an IndexError. It should be col = 2.

A — The outer list is missing its closing bracket, so the board is a syntax error. The line should end with ]] so both rows are inside the outer list.

B — Python lists need one pair of brackets per index: the row first, then the column. plan[1, 2] causes a TypeError. It should be plan[1][2] = 5.

D — Each row has only 2 columns, so the inner loop must use range(2). range(3) makes c reach 2 and causes an IndexError.

B — The text X needs quotes, otherwise Python looks for a variable called X and raises a NameError. It should be board[0][1] = "X".