Expressions and Output: Practice Questions (Computer Science A 1.3)

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)

What is displayed by this code segment?

System.out.print("A");
System.out.println("B");
System.out.print("C");
System.out.println("D");

(A) ABCD on one line
(B) AB on one line and CD on the next line
(C) A, B, C and D each on its own line
(D) A on one line, BC on the next line and D on the last line

Answer and explanation

Answer: (B). print("A") leaves the cursor after the A, so println("B") continues on the same line and then moves to a new line. Then print("C") and println("D") do the same on the next line.

(A) forgets that println moves the cursor to a new line after displaying.
(C) treats every statement as if it were println.
(D) groups the output as if println moved to a new line before displaying instead of after.

Question 2 (multiple choice · foundation)

What is displayed by the following statement?

System.out.println("Say \"go\" to C:\\now");

(A) Say \"go\" to C:\\now
(B) Say go to C:now
(C) The statement does not compile
(D) Say "go" to C:\now

Answer and explanation

Answer: (D). \" displays a double quote and \\ displays a single backslash.

(A) displays the escape sequences as typed, as if the backslashes were ordinary characters.
(B) drops the quotes and the backslash, as if the backslashes simply hid the character after them.
(C) is wrong because both escape sequences are valid and the string is properly closed.

Question 3 (multiple choice · core)

What is the value of the expression 25 / 4 + 25 % 4?

(A) 7.25
(B) 6
(C) 7
(D) 1

Answer and explanation

Answer: (C). 25 / 4 is integer division: 6. 25 % 4 is the remainder: 25 - 24 = 1. Then 6 + 1 = 7.

(A) would come from using real division, but both operands are int.
(B) is the quotient alone and forgets to add the remainder.
(D) is the remainder alone.

Question 4 (multiple choice · core)

What is printed by System.out.println(5 + 3 * 2 - 8 / 3);?

(A) 2
(B) 9
(C) 8
(D) 11

Answer and explanation

Answer: (B). 3 * 2 is 6 and 8 / 3 is 2 (integer division). The expression becomes 5 + 6 - 2, which is 11 - 2, which is 9.

(A) comes from ignoring precedence and working strictly left to right: ((5 + 3) * 2 - 8) / 3 is 8 / 3, which is 2.
(C) rounds 8 / 3 up to 3 instead of truncating to 2, giving 5 + 6 - 3.
(D) forgets to subtract the division term at all: 5 + 6.

Question 5 (multiple choice · core)

What value is stored in r after the following statement?

double r = 10 / 4 * 2.0;

(A) 5.0
(B) 2.0
(C) 4.0
(D) 6.0

Answer and explanation

Answer: (C). Equal precedence goes left to right, so 10 / 4 comes first. Both operands are int, so the result is 2. Then 2 * 2.0 is the double 4.0.

(A) would come from 10 / 4 = 2.5 and then 2.5 * 2.0, which is wrong because the first division is an integer division.
(B) stops after the division, forgetting that 2 is then multiplied by 2.0.
(D) comes from rounding 2.5 up to 3 and then multiplying by 2.0. Integer division truncates; it never rounds.

Question 6 (multiple choice · core)

Which of the following statements compiles but causes an ArithmeticException when it executes? Assume int z = 0; has been declared.

(A) System.out.println(8 / 2.0);
(B) System.out.println(8 % 3);
(C) System.out.println(0 / 8);
(D) System.out.println(8 % z);

Answer and explanation

Answer: (D). The remainder operation with an int divisor of zero cannot be performed, so an ArithmeticException is thrown at run time. The compiler does not check the value of z.

(A) is fine: it divides by 2.0 and prints 4.0.
(B) prints 2.
(C) is dividing zero by 8, which is allowed and prints 0. The problem arises when zero is the divisor, not the dividend.

Question 7 (writing code · core)

A school has 1,234 pupils to seat at tables of 8. Write code that stores the number of pupils in an int variable, then displays (a) the number of full tables and (b) the number of pupils left over for a partly filled table. State the values displayed.

Worked solution

int pupils = 1234;
int fullTables = pupils / 8;
int leftOver = pupils % 8;
System.out.println(fullTables);
System.out.println(leftOver);

1234 / 8: 8 * 154 = 1232, so the integer quotient is 154 and the remainder is 1234 - 1232 = 2. The output is two lines: 154 and 2. Check: 154 * 8 + 2 = 1234.

Suggested Marlbridge rubric (4 marks):

Mark	Awarded for
1	Declares and initialises `pupils` as an `int`
1	Uses `/ 8` for full tables
1	Uses `% 8` for the leftover pupils
1	Correct values 154 and 2 (and displays them)

Question 8 (tracing output · core)

State exactly what the following program displays, showing each line of output.

public class Trace {
    public static void main(String[] args) {
        int a = 14;
        int b = 4;
        System.out.print("Q: " + a / b);
        System.out.println(" R: " + a % b);
        System.out.println("Half: " + a / 2.0);
        System.out.println("Mix: " + (a + b) / 3 * 1.5);
        System.out.println("Tab\\\"x\"");
    }
}

Worked solution

Q: 3 R: 2
Half: 7.0
Mix: 9.0
Tab\"x"

a / b is 14 / 4 = 3 and a % b is 14 - 12 = 2. The first statement is print, so the second continues on the same line: Q: 3 R: 2.
a / 2.0 has a double operand, so it is 7.0.
(a + b) / 3 * 1.5: the bracket is 18; 18 / 3 is 6 (equal precedence, left to right); 6 * 1.5 is 9.0.
The last string is Tab, then \\ (one backslash), then \" (a double quote), then x, then \" (a double quote): displayed as Tab\"x".

Suggested Marlbridge rubric (4 marks): 1 mark for each correct line. Line 1 must show Q: 3 R: 2 on the same line.

Question 9 (digits · stretch)

A program stores the three-digit whole number n = 407.

(a) Write expressions, using only / and %, for the hundreds digit, the tens digit and the ones digit of n.

(b) Use them to find the sum of the digits of 407.

Worked solution

(a) hundreds = n / 100 (integer division removes the last two digits: 4). tens = n / 10 % 10 (first n / 10 is 40, then 40 % 10 is 0). ones = n % 10 (7).

(b) The sum is 4 + 0 + 7 = 11.

(c) Dividing by 10 groups the number into tens with a remainder. The remainder is what is left after removing every complete group of ten, which is exactly the digit in the ones place.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	Hundreds expression `n / 100`
1	Tens expression `n / 10 % 10` (or equivalent such as `n % 100 / 10`)
1	Ones expression `n % 10`
1	Sum 11
1	Explanation that the remainder after complete groups of ten is the ones digit

Question 10 (finding the bug · stretch)

A program should display the average of three int test scores, with a fractional part if there is one. For the scores 70, 80 and 90 it should display 80.0.

int a = 70;
int b = 80;
int c = 90;
System.out.println(a + b + c / 3.0);

(a) State what the code displays. What kind of error is this, and how would it be detected?

(b) Rewrite the last statement so that it displays the correct average.

Worked solution

(a) c / 3.0 is evaluated first: 90 / 3.0 = 30.0. Then 70 + 80 = 150 and 150 + 30.0 = 180.0. The code compiles and runs, so this is a logic error, detected by testing with data whose answer you know (80.0).

(b) System.out.println((a + b + c) / 3.0);

(c) (70 + 80 + 85) = 235 and 235 / 3.0 = 78.33333333333333 (a repeating decimal, shown to the precision a double can hold). Using 3.0 makes the division a double division. With 3 the sum 235 would be divided by 3 using integer division and give 78, losing the fraction.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	Displays 180.0 (division first, then additions)
1	Identifies a logic error found by testing
1	Brackets around the sum in the corrected statement
1	Keeps a `double` division (`3.0`) in the corrected statement
1	Explains that `3` would truncate the fractional part (integer division)

How did you do?

Questions 1 and 2: re-read “Showing results” and “String literals and escape sequences” in the study guide.
Questions 3 to 5 and 8: evaluate on paper, rewriting the expression after each step as in Figure 1.
Questions 6 and 10: revisit “Dividing by zero” and the idea that the type of each operation is decided separately.
Questions 7 and 9: practise using / and % together on other numbers.

Then try the checklist.