Assignment Statements and Input: Practice Questions (Computer Science A 1.4)

These are original Marlbridge practice questions, not past exam questions. All code is invented for these questions and was compiled and run before publication. The rubrics are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No calculator is allowed on the exam, so work everything by hand. Code fragments are assumed to be inside a main method.

Question 1 (multiple choice · foundation)

Which statement increases the value of the int variable count by 1?

(A) count = count + 1;
(B) count + 1 = count;
(C) count == count + 1;
(D) count + 1;

Answer and explanation

Answer: (A). The right side is evaluated using the current value of count, and the result is stored back in count.

(B) has the expression on the left. The left side of an assignment must be a variable.
(C) uses ==, which compares values. As a statement on its own it does not compile and it never changes count.
(D) calculates a value but stores it nowhere; as a statement on its own it does not compile.

Question 2 (multiple choice · foundation)

What is printed by the following code segment?

int a = 5;
int b = 2;
a = a + b;
b = a - b;
System.out.println(a + " " + b);

(A) 7 2
(B) 7 5
(C) 5 2
(D) 7 7

Answer and explanation

Answer: (B).

After statement	a	b
`int a = 5;`	5	-
`int b = 2;`	5	2
`a = a + b;`	7	2
`b = a - b;`	7	5

The last statement uses the new a (7): 7 - 2 = 5.

(A) forgets that b was reassigned.
(C) prints the starting values, as if no assignment happened.
(D) copies a into b, but b = a - b subtracts the old b.

Question 3 (multiple choice · core)

The variables int n = 4; and double d = 2.5; have been declared. Which statement compiles?

(A) n = d;
(B) n = 2.5;
(C) d = "3.5";
(D) d = n;

Answer and explanation

Answer: (D). An int value can be stored in a double variable, so d becomes 4.0.

(A) stores a double value in an int variable, which the compiler rejects.
(B) stores the double literal 2.5 in an int, also rejected.
(C) stores a String in a double variable; text in quotes is not a number.

Question 4 (multiple choice · core)

What happens when this code segment is compiled?

int total;
total = total + 5;
System.out.println(total);

(A) It compiles and prints 5
(B) It compiles and prints 0
(C) It does not compile, because total is used before it has been assigned a value
(D) It compiles but throws an exception when it runs

Answer and explanation

Answer: (C). In total = total + 5; the right side uses total, which has not been initialized. Every variable must be assigned a value before it is used in an expression, and the compiler reports it as an error.

(A) and (B) assume a default value for a local variable. Java does not give one for a variable declared in a method.
(D) is wrong because the problem is caught at compile time, so the program never runs.

Question 5 (multiple choice · core)

What is the value of result after the following statements?

int x = 7;
double result = x / 2 + 1.0;
x = 1;

(A) 4.0
(B) 4.5
(C) 3.0
(D) 2.0

Answer and explanation

Answer: (A). x / 2 is integer division: 3. Then 3 + 1.0 is 4.0, stored in result. The later assignment x = 1; does not change result, which holds its own value.

(B) treats x / 2 as 3.5.
(C) leaves out the 1.0.
(D) uses the final value of x (1), as if result were recalculated when x changed. A variable is not a formula.

Question 6 (multiple choice · stretch)

The following program is run, and the user types the two lines 8 and apple, each followed by the Enter key.

import java.util.Scanner;

public class Echo {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        String s = in.nextLine();
        System.out.println(n + "[" + s + "]");
    }
}

What is printed?

(A) 8[]
(B) 8[apple]
(C) 8[ ] with a space between the brackets
(D) An InputMismatchException is thrown

Answer and explanation

Answer: (A). nextInt() reads the 8 and leaves the end of that line unread. nextLine() then reads the rest of the same line, which is empty, so s holds the empty string. The word apple is still waiting to be read.

(B) assumes that nextLine() moves on to the next line first. It does not.
(C) has nothing to put a space there: the remainder of the line is empty.
(D) would happen only if nextInt() met text that is not an integer. The first item is 8.

Question 7 (tracing · core)

Trace the following code segment and state what it prints.

int a = 6;
int b = 4;
int c = a / b;
double d = a / b;
double e = a / 4.0;
a = c + b;
b = a * 2;
System.out.println(a + " " + b + " " + c + " " + d + " " + e);

Worked solution

After statement	a	b	c	d	e
`int a = 6;`	6	-	-	-	-
`int b = 4;`	6	4	-	-	-
`int c = a / b;`	6	4	1	-	-
`double d = a / b;`	6	4	1	1.0	-
`double e = a / 4.0;`	6	4	1	1.0	1.5
`a = c + b;`	5	4	1	1.0	1.5
`b = a * 2;`	5	10	1	1.0	1.5

The output is 5 10 1 1.0 1.5. Note that d is 1.0, not 1.5: the division a / b is an integer division, and only afterwards is the result stored as a double.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	`c` is 1 (integer division)
1	`d` is 1.0 and the reason is stated or shown
1	`e` is 1.5
1	`a` becomes 5 and `b` becomes 10 (uses the new `a`)
1	Output line exactly `5 10 1 1.0 1.5`

Question 8 (writing code · core)

Write a program fragment that reads two whole numbers from the keyboard using a Scanner named in, stores them in int variables first and second, and prints their sum on one line and their product on the next. Show the output when the user enters 6 and 7.

Worked solution

import java.util.Scanner;

public class SumProduct {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int first = in.nextInt();
        int second = in.nextInt();
        int sum = first + second;
        int product = first * second;
        System.out.println(sum);
        System.out.println(product);
    }
}

13
42

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	Creates a `Scanner` that reads from the keyboard (accept the form used in your course)
1	Reads both values with `nextInt()` and stores them in `int` variables
1	Sum calculated correctly and stored or printed
1	Product calculated correctly and stored or printed
1	Prints 13 then 42 on separate lines (`println`)

Question 9 (null and compatibility · stretch)

(a) Which of the following two lines compile? Give a reason for each: String name = null; and int count = null;.

(b) Explain in one or two sentences what null means for a String variable.

(c) The statement double average = total / count; appears in a method, where total and count are int variables with the values 7 and 2. State the value stored in average and explain why it is not 3.5.

Worked solution

(a) String name = null; compiles: a variable of a reference type can hold an object reference or null. int count = null; does not compile: int is a primitive type and holds only int values, never null.

(b) null means the variable is not associated with any object: it refers to no String at all. (This is different from an empty string "", which is an object with no characters.)

(c) average holds 3.0. total / count is evaluated first, with two int operands, so it is integer division and gives 3; only then is 3 stored in a double, becoming 3.0. To keep the fraction, at least one operand would have to be a double, for example total / 2.0.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	`String name = null;` compiles, with a reason about reference types
1	`int count = null;` does not compile, with a reason about primitives
1	`null` means no object (not 0, not empty text)
1	`average` is 3.0
1	Explains that the division is done on `int` values first

How did you do?

Questions 1 and 2: re-read “Assignment: evaluate on the right, store on the left” in the study guide.
Questions 3 to 5 and 7: practise tracing with a table, one row per statement, and check the type of each value.
Questions 6 and 8: revisit “Input: getting values from a user” and run the examples if you can.
Question 9: revisit “Reference types and null”.

Then try the checklist.