2D Array Creation and Access: Practice Questions (Computer Science A 4.11)

These are original Marlbridge practice questions, not past exam questions. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. Every code fragment was compiled and run, and the values and exception names quoted come from those runs.

Question 1 (multiple choice · foundation)

Which statement about int[][] grid = new int[3][4]; is true?

(A) grid has 4 rows and 3 columns, all null
(B) grid has 3 rows and 4 columns, all 0
(C) grid has 12 rows and 1 column
(D) grid has 3 rows and 4 columns, none yet created

Answer and explanation

Answer: (B). The first size is the number of rows and the second is the number of columns. Every int element starts as 0.

(A) swaps the sizes and uses the default of a reference type.
(C) confuses the total number of elements with the number of rows.
(D) The elements are created with default values immediately.

Question 2 (multiple choice · core)

Given int[][] m = { {1, 2, 3}, {4, 5, 6} };, what is m[1][2] + m[0][1]?

(A) 7
(B) 8
(C) 6
(D) 9

Answer and explanation

Answer: (B). m[1][2] is row 1, column 2: 6. m[0][1] is row 0, column 1: 2. 6 + 2 = 8 (run and confirmed).

(A) is 3 + 4: it reads m[0][2] and m[1][0], which are the wrong cells.
(C) forgets to add the second term.
(D) uses 3 + 6, which is m[0][2] + m[1][2].

Question 3 (multiple choice · core)

For int[][] m = new int[5][2]; which expressions are the number of rows and the number of columns?

(A) rows m[0].length; columns m.length
(B) rows m.length; columns m[0].length
(C) rows m.length; columns m.length
(D) rows m.length(); columns m[0].length()

Answer and explanation

Answer: (B). m.length is the length of the outer array (rows, 5). m[0].length is the length of a row array (columns, 2).

(A) swaps them.
(C) counts rows twice.
(D) arrays use the attribute length, not a method length(); this does not compile.

Question 4 (multiple choice · core)

m is the 2-by-3 array { {1, 2, 3}, {4, 5, 6} }. Which expression throws an ArrayIndexOutOfBoundsException?

(A) m[1][2]
(B) m[0][0]
(C) m[2][0]
(D) m[1][0]

Answer and explanation

Answer: (C). There are only rows 0 and 1, so row 2 does not exist (run and confirmed).

(A) is the last element, row 1, column 2.
(B) and (D) are valid cells.

Question 5 (multiple choice · core)

What is printed?

int[][] m = { {1, 2, 3}, {4, 5, 6} };
int[] row = m[1];
row[0] = 40;
System.out.println(m[1][0]);

(A) 4
(B) 40
(C) 1
(D) The code does not compile

Answer and explanation

Answer: (B). m[1] is a reference to the second row array, so row and m[1] are two names for one array. Changing row[0] changes m[1][0] (run and confirmed: 40).

(A) assumes row is a copy.
(C) reads a cell from the wrong row.
(D) The code compiles; m[1] is an int[].

Question 6 (multiple choice · core)

What does this code do?

String[][] names = new String[2][2];
System.out.println(names[0][0].length());

(A) Prints 0
(B) Prints null
(C) Throws a NullPointerException
(D) Throws an ArrayIndexOutOfBoundsException

Answer and explanation

Answer: (C). A new array of Strings holds null in every cell. Calling length() on null throws NullPointerException (run and confirmed). (Printing names[0][0] alone would print null.)

(A) would be correct for the empty String, which is not what the cell holds.
(B) is printed only by the cell itself, not by calling a method on it.
(D) The indices 0 and 0 are valid.

Question 7 (multiple choice · core)

Which declaration creates a 2D array of double values with 2 rows and 5 columns?

(A) double[][] a = new double[2][5];
(B) double[2][5] a = new double[][];
(C) double[][] a = new double[5][2];
(D) double a[2,5];

Answer and explanation

Answer: (A). The first size is the row count, the second is the column count.

(B) puts sizes in the declaration and omits them from new; this does not compile.
(C) creates 5 rows and 2 columns.
(D) is not Java syntax.

Question 8 (multiple choice · stretch)

What is printed?

int[][] n = { {1, 2, 3}, {4, 5, 6} };
int[] tmp = n[0];
n[0] = n[1];
n[1] = tmp;
System.out.println(n[0][0] + " " + n[1][2]);

(A) 1 6
(B) 4 3
(C) 4 6
(D) 1 3

Answer and explanation

Answer: (B). The statements swap the two row references, so n becomes { {4, 5, 6}, {1, 2, 3} }. Then n[0][0] is 4 and n[1][2] is 3 (run and confirmed: 4 5 6 | 1 2 3 |).

(A) is the original array’s values.
(C) reads two elements of the same original row.
(D) mixes the original n[0][0] with the swapped n[1][2].

Question 9 (constructed response · core)

Write getColumn(int[][] m, int c), which returns a new one-dimensional array holding the elements of column c, from the top row to the bottom. Assume m is rectangular and c is valid.

Answer and explanation

public static int[] getColumn(int[][] m, int c)
{
    int[] col = new int[m.length];
    for (int r = 0; r < m.length; r++)
    {
        col[r] = m[r][c];
    }
    return col;
}

For { {1, 2, 3}, {4, 5, 6} } and c = 2 the method returns an array of length 2 holding 3 and 6 (run and confirmed).

Suggested Marlbridge rubric (4 points)

Point	Award for
1	Creates a result array whose length is the number of rows, `m.length`
2	Loops over every row index
3	Reads the element with row index first and `c` second: `m[r][c]`
4	Stores into the matching position and returns the array

Do not award point 3 for m[c][r].

Question 10 (constructed response · stretch)

(a) Write inBounds(int[][] m, int r, int c), which returns true only if m[r][c] is a valid element of the rectangular array m.

(b) State the value of inBounds for m = { {1, 2, 3}, {4, 5, 6} } with (1, 2), (2, 0), (0, 3) and (-1, 0).

Answer and explanation

(a)

public static boolean inBounds(int[][] m, int r, int c)
{
    return r >= 0 && r < m.length && c >= 0 && c < m[r].length;
}

(b) true, false, false, false (run and confirmed). (1, 2) is the last cell; (2, 0) has row 2 of 2 rows; (0, 3) has column 3 of 3 columns; (-1, 0) has a negative row.

Suggested Marlbridge rubric (5 points)

Point	Award for
1	Tests the row against 0 and `m.length` (strictly less than)
2	Tests the column against 0 and a column length (strictly less than)
3	The column bound uses `m[r].length` or `m[0].length`, not `m.length`
4	Row test comes before the column test so `m[r]` is never evaluated for a bad row
5	Part (b): all four results correct

Writing r <= m.length is a common error and costs point 1.

How did you do?

Questions 1 to 8 are one mark each. If you slipped on rows versus columns, re-read the creation and measuring sections of the Topic 4.11 study guide; then tick off the checklist.