Array Creation and Access: Practice Questions (Computer Science A 4.3)

These are original Marlbridge practice questions, not past exam questions. All data is fictional. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. No calculator is allowed. Every code fragment was compiled and run, and the stated outputs are the real ones.

Question 1 (multiple choice · foundation)

Which statement creates an array that can hold 5 int values, all initially 0?

(A) int[] a = new int(5);
(B) int[] a = new int[5];
(C) int a = new int[5];
(D) int[5] a;

Answer and explanation

Answer: (B). new int[5] creates five int elements with the default value 0, and int[] is the correct variable type.

(A) uses parentheses, which would call a constructor, and int has none.
(C) assigns an array to a plain int variable.
(D) is not valid Java syntax for declaring an array.

Question 2 (multiple choice · foundation)

An array data has length 8. Which set contains exactly the valid indices?

(A) 1 to 8
(B) 0 to 8
(C) 0 to 7
(D) 1 to 7

Answer and explanation

Answer: (C). Valid indices run from 0 through length - 1, which is 7.

(A) starts at 1 and includes 8, which is out of range.
(B) includes 8, which throws ArrayIndexOutOfBoundsException.
(D) omits both the first element (index 0) and the last (index 7).

Question 3 (multiple choice · core)

int[] counts = new int[4];
double[] prices = new double[3];
boolean[] flags = new boolean[3];
String[] names = new String[2];
System.out.println(counts.length + " " + counts[0] + " " + prices[1] + " " + flags[2] + " " + names[1]);

What is printed?

(A) 4 0 0 false ""
(B) 4 0 0.0 false null
(C) 3 0 0.0 true null
(D) 4 null 0.0 false null

Answer and explanation

Answer: (B). counts.length is 4; the defaults are 0, 0.0, false and null. The compiled program printed 4 0 0.0 false null.

(A) prints 0 for a double and an empty string for a reference.
(C) uses the wrong length and makes the boolean true.
(D) gives an int element the value null.

Question 4 (multiple choice · core)

int[] w = new int[5];
w[0] = 3;
w[1] = w[0] * 2;
w[w[0]] = 8;
System.out.println(w[0] + " " + w[1] + " " + w[2] + " " + w[3] + " " + w[4]);

What is printed?

(A) 3 6 8 0 0
(B) 3 6 0 8 0
(C) 8 6 0 0 0
(D) The code throws ArrayIndexOutOfBoundsException.

Answer and explanation

Answer: (B). w[w[0]] is w[3], since w[0] is 3, so 8 goes at index 3. The result is [3, 6, 0, 8, 0].

(A) puts the 8 at index 2.
(C) puts the 8 at index 0.
(D) is wrong: index 3 is within 0 to 4.

Question 5 (multiple choice · core)

int[] q = {5, 4, 3, 2, 1};
q[q[4]] = q[q[3]] + 10;
System.out.println(q[0] + " " + q[1] + " " + q[2] + " " + q[3] + " " + q[4]);

What is printed?

(A) 5 4 3 2 1
(B) 5 12 3 2 1
(C) 5 13 3 2 1
(D) 15 4 3 2 1

Answer and explanation

Answer: (C). On the left, q[4] is 1, so the target is q[1]. On the right, q[3] is 2, q[2] is 3, and 3 + 10 = 13. So q[1] becomes 13: [5, 13, 3, 2, 1]. This was confirmed by running the code.

(A) assumes the assignment changes nothing.
(B) uses q[3] (2) plus 10 and forgets the inner index lookup.
(D) puts the result in q[0].

Question 6 (multiple choice · core)

An array int[] s = new int[4]; is declared. Which statement throws ArrayIndexOutOfBoundsException when executed?

(A) s[0] = 1;
(B) s[s.length - 1] = 1;
(C) s[s.length] = 1;
(D) s[2] = s[1] + 1;

Answer and explanation

Answer: (C). s.length is 4, and the valid indices are 0 to 3, so s[4] is one past the end.

(A) is the first element.
(B) is the last element, index 3.
(D) uses indices 2 and 1, both valid.

Question 7 (constructed response · core)

int[] v = {4, 9, 1};
int temp = v[0];
v[0] = v[2];
v[2] = temp;
v[1] = v[1] + v[0];
System.out.println(v[0] + " " + v[1] + " " + v[2]);

(a) Complete a trace table showing temp and the three elements after each statement.

(b) State the output.

Answer and explanation

(a)

Statement	`temp`	v[0]	v[1]	v[2]
start		4	9	1
`int temp = v[0];`	4	4	9	1
`v[0] = v[2];`	4	1	9	1
`v[2] = temp;`	4	1	9	4
`v[1] = v[1] + v[0];`	4	1	10	4

(b) 1 10 4.

(c) After v[0] = v[2]; the original v[0] (4) would be lost. temp holds it until it is stored in v[2].

Suggested Marlbridge rubric (5 marks).

Mark	Requirement
1	Part (a): `temp` is 4 from the second row
1	Part (a): rows 3 and 4 correct for `v[0]` and `v[2]`
1	Part (a): last row `v[1]` is 10
1	Part (b): output `1 10 4`
1	Part (c): temp saves the value that would otherwise be overwritten

Question 8 (constructed response · core)

(a) Write one statement that creates a double array prices holding 10.0, 12.5, 11.0, 9.5, 13.0 and 12.0, using an initializer list.

(b) Write a statement that sets element 3 to 2.5 more than element 0.

(c) Write a statement that prints the last element without using the number 5, and give the output of (b) followed by (c).

Answer and explanation

(a) double[] prices = {10.0, 12.5, 11.0, 9.5, 13.0, 12.0};

(b) prices[3] = prices[0] + 2.5; (element 3 becomes 12.5).

(c) System.out.println(prices[prices.length - 1]); prints 12.0.

Running these, prices[3] is 12.5 and the last element is 12.0 (output of a combined print: 12.5 12.0).

Suggested Marlbridge rubric (5 marks).

Mark	Requirement
1	Part (a): `double[]` type with an initializer list
1	Part (a): all six values in order
1	Part (b): assigns to `prices[3]`
1	Part (b): uses `prices[0] + 2.5`
1	Part (c): last element via `prices.length - 1`, output 12.0

Question 9 (constructed response · stretch)

Each statement below does not compile. State the compiler’s complaint in your own words and give a fix.

int[] s = {1, 2, 3};
System.out.println(s.length());
String[] n = new String[2];
n[0] = 5;
int k = s[1.0];
int[] z = new int[3.0];

Answer and explanation

s.length(): an array has a length attribute, not a length() method (cannot find symbol). Fix: s.length.
n[0] = 5;: an int cannot be stored in a String element (int cannot be converted to String). Fix: n[0] = "5";.
s[1.0]: an index must be an int (possible lossy conversion from double to int). Fix: s[1].
new int[3.0]: the length must be an int. Fix: new int[3].

All four messages were confirmed by compiling.

Suggested Marlbridge rubric (8 marks). One mark for the correct complaint and one for a valid fix, for each of the four statements.

How did you do?

Questions 1 to 6 right and the rubrics mostly met: move on to the Array Traversals study guide.
Missed Questions 1, 2 or 6: reread “Creating an array” and “Out-of-range indexes” in the study guide.
Missed Questions 3, 4 or 5: reread “Accessing and changing elements” and Worked example 1.
Missed Questions 7, 8 or 9: reread Worked example 2, then tick the topic checklist.