Selection and Iteration: Unit Diagnostic (Computer Science A Unit 2)

Use this diagnostic to find which Unit 2 topics to revisit: one short question per topic, with two for Topic 2.6.

These are original Marlbridge practice questions, not past exam questions. The diagnostic is not calibrated and gives no predicted score.

How to take it. Allow 30 minutes and answer every question before you open any answer. No calculator. Assume every code segment compiles unless a question says otherwise. Use only the String methods length, substring, indexOf, equals and compareTo.

Question 1 (multiple choice · 2.1)

An algorithm says: “Repeat for each of six doors: if the door is unlocked, lock it.” Which statement describes how the building blocks are arranged?

(A) A repetition inside a selection
(B) A selection inside a repetition
(C) Sequencing only
(D) Two separate selections

Answer and explanation

Answer: (B). The “repeat for each door” is the repetition, and the decision “if the door is unlocked” is asked on every pass, so the selection is inside the repetition.

(A) would mean one decision is made first and the repeat only happens if it is true.
(C) ignores the repeat and the decision.
(D) ignores the repeat.

If you missed this: Topic 2.1 study guide.

Question 2 (multiple choice · 2.2)

Let a = 14 and b = 4. Which expression is true?

(A) a / b > 3.5
(B) a % b == 2
(C) a / b == 3.5
(D) a - b * 3 > 3

Answer and explanation

Answer: (B). 14 % 4 is 2, so 2 == 2 is true.

(A) a / b is int division: 14 / 4 = 3, and 3 > 3.5 is false.
(C) 3 == 3.5 is false.
(D) 14 - 4 * 3 = 2, and 2 > 3 is false.

If you missed this: Topic 2.2 study guide.

Question 3 (multiple choice · 2.3)

What is printed?

int x = 5;
if (x > 3) {
    x = x * 2;
}
if (x > 8) {
    x = x - 7;
}
System.out.println(x);

(A) 3
(B) 5
(C) 10
(D) 8

Answer and explanation

Answer: (A). The first if is true, so x becomes 10. The second condition is tested on the new value: 10 > 8 is true, so x becomes 3.

(B) assumes neither body runs.
(C) forgets the second if.
(D) treats the second if as an else.

If you missed this: Topic 2.3 study guide.

Question 4 (multiple choice · 2.4)

What is printed?

int n = 10;
String r;
if (n > 20) {
    r = "A";
} else if (n > 10) {
    r = "B";
} else if (n > 5) {
    r = "C";
} else {
    r = "D";
}
System.out.println(r);

(A) B
(B) C
(C) D
(D) A

Answer and explanation

Answer: (B). 10 > 20 false; 10 > 10 false (the boundary is not included); 10 > 5 true, so r is “C”.

(A) treats n > 10 as n >= 10.
(C) skips the third test.
(D) assumes the first condition is true.

If you missed this: Topic 2.4 study guide.

Question 5 (multiple choice · 2.5)

What is the value of r?

int a = 3;
int b = 0;
boolean r = b != 0 && a / b > 1 || a > 2;
System.out.println(r);

(A) false
(B) true
(C) The program throws an ArithmeticException.
(D) The program does not compile.

Answer and explanation

Answer: (B). && is evaluated before ||. b != 0 is false, so the division is skipped by short-circuit evaluation. That leaves false || a > 2, which is false || true: true.

(A) ignores the right side of the ||.
(C) assumes the division is evaluated.
(D) is wrong: the code is valid.

If you missed this: Topic 2.5 study guide.

Question 6 (multiple choice · 2.6)

Which expression is equivalent to !(x < 5 || y >= 8)?

(A) x >= 5 || y < 8
(B) x > 5 && y <= 8
(C) x >= 5 && y < 8
(D) x < 5 && y >= 8

Answer and explanation

Answer: (C). De Morgan’s law turns || into && and negates each part: !(x < 5) is x >= 5, and !(y >= 8) is y < 8.

(A) keeps the ||.
(B) uses the wrong complements (> and <= lose the boundary values).
(D) negates nothing.

If you missed this: Topic 2.6 study guide.

Question 7 (multiple choice · 2.6)

What is printed?

String a = new String("hi");
String b = a;
String c = new String("hi");
System.out.println((a == b) + " " + (a == c) + " " + a.equals(c));

(A) true true true
(B) true false true
(C) false false true
(D) true false false

Answer and explanation

Answer: (B). b = a copies the reference, so a == b is true. c is a separate object, so a == c is false, but the characters are the same, so a.equals(c) is true.

(A) treats a == c as a content comparison.
(C) says that a copied reference is a different object.
(D) says that equals is false for identical text.

If you missed this: Topic 2.6 study guide, section “Comparing object references”.

Question 8 (multiple choice · 2.7)

What is printed?

int n = 1;
int steps = 0;
while (n < 50) {
    n = n * 3;
    steps++;
}
System.out.println(n + " " + steps);

(A) 27 3
(B) 81 4
(C) 81 5
(D) 243 5

Answer and explanation

Answer: (B). n goes 1, 3, 9, 27, 81. The body runs while n is 1, 3, 9 and 27, which is 4 times, and then 81 < 50 is false.

(A) stops before the pass that produces 81.
(C) counts the final test as an iteration.
(D) continues too far.

If you missed this: Topic 2.7 study guide.

Question 9 (multiple choice · 2.8)

How many times does the body of this loop run?

for (int i = 4; i <= 30; i += 5) {
    System.out.print("*");
}

(A) 5
(B) 6
(C) 7
(D) 26

Answer and explanation

Answer: (B). i takes the values 4, 9, 14, 19, 24 and 29; the next value, 34, fails the test. That is 6 iterations.

(A) stops at 24.
(C) counts the failing test.
(D) is the difference 30 - 4.

If you missed this: Topic 2.8 study guide.

Question 10 (multiple choice · 2.9)

What is printed?

int n = 5280;
int count = 0;
while (n > 0) {
    if (n % 10 > 1) {
        count++;
    }
    n = n / 10;
}
System.out.println(count);

(A) 2
(B) 3
(C) 4
(D) 15

Answer and explanation

Answer: (B). The digits are 0, 8, 2 and 5 (last first). Those greater than 1 are 8, 2 and 5: 3 digits.

(A) misses one of them.
(C) counts every digit, including 0.
(D) is the sum of the digits 8 + 2 + 5, not the count.

If you missed this: Topic 2.9 study guide.

Question 11 (multiple choice · 2.10)

What is printed?

String s = "mississippi";
int count = 0;
for (int i = 0; i <= s.length() - 2; i++) {
    if (s.substring(i, i + 2).equals("ss")) {
        count++;
    }
}
System.out.println(count);

(A) 2
(B) 4
(C) 1
(D) The code throws an exception.

Answer and explanation

Answer: (A). “mississippi” has “ss” starting at index 2 and at index 5, so 2 windows match. The loop stops at i = length - 2, so no window runs past the end.

(B) is the number of letter s in the word, not the number of “ss” windows.
(C) misses one occurrence.
(D) would happen with i < s.length(), but the bound here is correct.

If you missed this: Topic 2.10 study guide.

Question 12 (multiple choice · 2.11)

What is the value of count?

int count = 0;
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 4; j++) {
        if (i != j) {
            count++;
        }
    }
}

(A) 12
(B) 9
(C) 7
(D) 3

Answer and explanation

Answer: (B). There are 3 x 4 = 12 pairs (i, j). The pairs with i == j are (0,0), (1,1) and (2,2): 3 pairs. So 12 - 3 = 9 pairs pass the test.

(A) forgets the if.
(C) subtracts 5.
(D) counts only the pairs that fail the test.

If you missed this: Topic 2.11 study guide.

Question 13 (multiple choice · 2.12)

For n = 6, how many times does c++ execute?

int c = 0;
for (int i = 0; i < n; i++) {
    for (int j = i; j < n; j++) {
        c++;
    }
}

(A) 15
(B) 21
(C) 30
(D) 36

Answer and explanation

Answer: (B). The inner loop runs 6, 5, 4, 3, 2 and 1 times as i goes from 0 to 5: 6 + 5 + 4 + 3 + 2 + 1 = 21 = 6 x 7 / 2.

(A) is 5 + 4 + 3 + 2 + 1, which would be the count if the inner loop started at i + 1.
(C) is 6 x 5, which treats the inner loop as running 5 times each.
(D) is 6 x 6, which ignores the dependence on i.

If you missed this: Topic 2.12 study guide, with Topic 2.11 for nested bounds.

Question 14 (short answer · 2.3 and 2.4)

Write a method category(int age) that returns the String “child” for ages below 13, “teen” for ages 13 to 19 inclusive, and “adult” for any other age. Use an if-else-if chain. (4 marks)

Answer and explanation

Model answer.

public static String category(int age) {
    if (age < 13) {
        return "child";
    } else if (age <= 19) {
        return "teen";
    } else {
        return "adult";
    }
}

The second condition does not need to repeat age >= 13, because it is only reached when age < 13 was false.

Suggested Marlbridge rubric (4 marks).

Marks	Awarded for
1	Correct header with a `String` return type
1	“child” for `age < 13` (boundary 13 excluded)
1	“teen” up to and including 19
1	“adult” in the final `else` and every path returns a value

If you missed this: Topic 2.4 study guide and Topic 2.3 study guide.

Question 15 (short answer · 2.7, 2.8 and 2.9)

(a) Trace the loop and state what is printed. (2 marks)

int total = 0;
for (int i = 1; i <= 5; i++) {
    if (i % 2 == 1) {
        total = total + i;
    } else {
        total = total - 1;
    }
}
System.out.println(total);

(b) Rewrite the loop as an equivalent while loop. (2 marks)

Answer and explanation

(a)

`i`	`i % 2 == 1`?	`total` after
1	true	1
2	false	0
3	true	3
4	false	2
5	true	7

The output is 7.

(b)

int total = 0;
int i = 1;
while (i <= 5) {
    if (i % 2 == 1) {
        total = total + i;
    } else {
        total = total - 1;
    }
    i++;
}
System.out.println(total);

Suggested Marlbridge rubric (4 marks). 1 mark for a correct trace table (the decisions and the total column); 1 mark for 7; 1 mark for the initialization before the while and the same condition; 1 mark for the update i++ as the last statement in the body.

If you missed this: Topic 2.8 study guide and Topic 2.7 study guide.

What next?

Make a list of the topics for which you missed a question. Read those study guides, work through the matching practice questions, and then try the Unit 2 review, which mixes topics together.