▣ Foundation Exercises

---

▣ How to Use These Exercises

These exercises are practical. Reading about debugging is useful. Doing it is where the skill is built.

Work through each exercise before checking the answer. If you get it wrong, do not simply read the correct answer and move on — trace back through the workflow and understand why your classification or reading was incorrect. That reasoning is the exercise.

No IDE required for most exercises

The classification and error-reading exercises are designed to be completed on paper or in a plain text editor. You are practising the reading and reasoning skills, not the mechanical act of compiling code. Some exercises in Set C involve running code — those are marked accordingly.

---

▣ Set A — Classify the Error Type

For each code sample and its associated output, identify:

• The error type (syntax, runtime, logical, or semantic)
• A one-sentence explanation of what is wrong

---

A1 — Java

public class Main {
    public static void main(String[] args) {
        int x = 10
        int y = 20;
        System.out.println(x + y);
    }
}

Output:

Main.java:3: error: ';' expected
        int x = 10
                  ^
1 error

Show answer

Error type: Syntax error

Explanation: A semicolon is missing at the end of int x = 10. Java requires every statement to end with ;. The compiler detected the missing terminator at line 3 and reported the position with a ^ caret.

---

A2 — C

using System;

class Program {
    static void Main() {
        int[] scores = { 85, 92, 78 };
        Console.WriteLine(scores[5]);
    }
}

Output:

Unhandled exception. System.IndexOutOfRangeException:
Index was outside the bounds of the array.
   at Program.Main() in Program.cs:line 6

Show answer

Error type: Runtime error

Explanation: The array scores has three elements at indices 0, 1, and 2. Index 5 does not exist. The program compiled and started running, but crashed when it attempted to access a position beyond the array boundary. This is a classic IndexOutOfRangeException.

---

A3 — Kotlin

fun main() {
    val numbers = listOf(10, 20, 30, 40, 50)
    var total = 0
    for (n in numbers) {
        total = n
    }
    println("Total: $total")
}

Output:

Total: 50

Expected output: Total: 150

Show answer

Error type: Logical error

Explanation: The loop assigns total = n on each iteration instead of accumulating with total += n. The program runs without error, but produces the last value in the list (50) instead of the sum of all values (150). The correct line is total += n.

---

A4 — JavaScript

function isEven(n) {
    return n % 2 = 0;
}

console.log(isEven(4));

Output:

SyntaxError: Invalid left-hand side in assignment

Show answer

Error type: Syntax error (with a semantic intent behind it)

Explanation: = is the assignment operator. == or === is the comparison operator. n % 2 = 0 attempts to assign 0 to the result of n % 2, which is not a valid assignment target. The correct expression is return n % 2 === 0;.

---

A5 — Java

public class Greeting {
    public static void main(String[] args) {
        String name = null;
        if (name.equals("Alice")) {
            System.out.println("Hello, Alice!");
        }
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException:
Cannot invoke "String.equals(Object)" because "name" is null
    at Greeting.main(Greeting.java:4)

Show answer

Error type: Runtime error

Explanation: name is explicitly set to null, then .equals() is called on it. Calling any method on a null reference throws a NullPointerException at runtime. The program compiled successfully — the null assignment is syntactically valid. The fix is either to initialise name with a value, or to reverse the comparison: "Alice".equals(name), which is null-safe in Java.

---

A6 — TypeScript

function celsiusToFahrenheit(c: number): number {
    return c * 9 / 5 + 32;
}

// Convert 100°C — expected result: 212°F
console.log(celsiusToFahrenheit(100)); // prints 212 ✓

// Convert 0°C — expected result: 32°F
console.log(celsiusToFahrenheit(0));   // prints 32 ✓

// Convert -40°C — expected result: -40°F
console.log(celsiusToFahrenheit(-40)); // prints -40 ✓

Now consider this version:

function celsiusToFahrenheit(c: number): number {
    return c * (9 / 5 + 32); // (1)
}

1. Parentheses changed — does this affect the result?

Show answer

Error type: Logical / Semantic error

Explanation: The parentheses change the order of operations. The original formula is (c × 9 ÷ 5) + 32. The modified version calculates 9 ÷ 5 + 32 = 33.8 first, then multiplies by c: c × 33.8. For c = 100, the result is 3380 instead of 212. No syntax error, no crash — but the formula is now mathematically wrong. This illustrates how operator precedence errors produce plausible-looking code that is silently incorrect.

---

▣ Set B — Read the Error Message

For each error output below, answer the four reading questions without looking at any code.

Reading questions: 1. What is the error type? 2. What does the description tell you happened? 3. In which file and at which line did it occur? 4. What would you look at first?

---

B1

error: cannot find symbol
        System.out.println(mesage);
                           ^
  symbol:   variable mesage
  location: class Main
Main.java:8: error

Show answer

1. Error type: Compile-time error — cannot find symbol
2. Description: The variable mesage does not exist. The compiler cannot find any declaration with that name.
3. File and line: Main.java, line 8
4. Look at first: Line 8 in Main.java. The variable name is likely a spelling error — mesage instead of message. Check the declaration of the variable and compare spelling exactly.

---

B2

Unhandled exception. System.DivideByZeroException:
Attempted to divide by zero.
   at Calculator.Divide(Calculator.cs:line 15)
   at Program.Main(Program.cs:line 7)

Show answer

1. Error type: Runtime error — DivideByZeroException
2. Description: A division by zero was attempted. The divisor was 0 at the time of execution.
3. File and line: Crash occurred in Calculator.cs at line 15. Called from Program.cs at line 7.
4. Look at first: Line 15 in Calculator.cs — find the division operation. Then check how the divisor value arrives at that line. Either it was set to 0 directly, or it came from user input or a calculation that produced 0. A guard condition (if (divisor == 0)) should be added.

---

B3

FATAL EXCEPTION: main
Process: com.example.shoppingapp, PID: 9842
java.lang.NullPointerException: Attempt to invoke virtual method
'void android.widget.TextView.setText(java.lang.CharSequence)'
on a null object reference
    at com.example.shoppingapp.ProductActivity.displayProduct(ProductActivity.kt:63)
    at com.example.shoppingapp.ProductActivity.onCreate(ProductActivity.kt:31)
    at android.app.Activity.performCreate(Activity.java:8051)

Show answer

1. Error type: Runtime error — NullPointerException
2. Description: setText() was called on a TextView that is null. The view reference was not initialised before use.
3. File and line (your code): ProductActivity.kt line 63, inside displayProduct(). It was called from onCreate() at line 31.
4. Look at first: Line 63 in ProductActivity.kt — find the TextView whose setText() is being called. Check how it was initialised. It was likely declared but never assigned via findViewById(), or displayProduct() was called before the layout was inflated.

---

▣ Set C — Apply the Workflow

These exercises ask you to apply the full six-step workflow to a broken program. For each one, document your process using the six steps as a structure.

---

C1 — Java: The Wrong Average

public class Average {
    public static void main(String[] args) {
        int[] scores = {70, 85, 90, 60, 75};
        int total = 0;

        for (int score : scores) {
            total = total + score;
        }

        int average = total / scores.length;
        System.out.println("Average: " + average);
    }
}

Expected output: Average: 76

Actual output: Average: 76

Before you move on

Wait — the expected and actual output match. Does that mean there are no errors?

Re-examine the expected output. The scores are 70, 85, 90, 60, 75. Their sum is 380. Divided by 5 that is 76. So Average: 76 is correct.

But what if the scores were 70, 85, 90, 60, 76? The sum would be 381. Divided by 5 that is 76.2. But integer division in Java truncates — the output would still be Average: 76.

This is a hidden logical issue: integer division silently discards the decimal portion. The program appears correct for these inputs but produces inaccurate results for inputs where the average is not a whole number.

Show the fix and explanation

Issue: Integer division truncates. 381 / 5 in Java integer arithmetic = 76, not 76.2.

Fix: Use floating-point division.

double average = (double) total / scores.length;
System.out.printf("Average: %.1f%n", average);

Explanation: Casting total to double before dividing forces floating-point arithmetic. The result is now accurate to decimal places.

Lesson: Always test with inputs where the expected result is not a round number. Edge cases reveal assumptions.

---

C2 — Kotlin: The Loop That Does Not Loop

fun main() {
    val items = listOf("apple", "banana", "cherry")
    var result = ""

    for (item in items) {
        result = item
    }

    println("Items: $result")
}

Expected output: Items: apple, banana, cherry

Actual output: Items: cherry

Apply the six steps. Identify the error type, locate the problem, and write the corrected version.

Show the workflow and fix

Step I — Read: No error message. The program runs but produces wrong output.

Step II — Identify: Logical error — the output is the last item only.

Step III — Locate: The for loop body — specifically the assignment result = item.

Step IV — Compare: Expected all items concatenated. Actual is only the last item.

Step V — One change: The loop replaces result each iteration instead of appending. Change result = item to build a concatenated string.

Step VI — Test:

fun main() {
    val items = listOf("apple", "banana", "cherry")
    val result = items.joinToString(", ")
    println("Items: $result")
}

Or with explicit loop:

for ((index, item) in items.withIndex()) {
    result += if (index < items.size - 1) "$item, " else item
}

Output: Items: apple, banana, cherry

---

C3 — JavaScript: The Function That Returns Nothing

function multiply(a, b) {
    const product = a * b;
}

const result = multiply(6, 7);
console.log("Result: " + result);

Expected output: Result: 42

Actual output: Result: undefined

Apply the six steps. Identify the error type and correct it.

Show the workflow and fix

Step I — Read: No error thrown. Output is undefined instead of 42.

Step II — Identify: Logical error (or semantic error — the function does not do what was intended).

Step III — Locate: The multiply function — specifically, the missing return statement.

Step IV — Compare: Expected 42. Actual undefined. In JavaScript, a function that does not explicitly return a value returns undefined.

Step V — One change: Add return product; before the closing brace.

Step VI — Test:

function multiply(a, b) {
    const product = a * b;
    return product;
}

const result = multiply(6, 7);
console.log("Result: " + result); // Result: 42

---

▣ Reflection

After completing these exercises, consider:

Reflect on your process

• Which error type was easiest to identify? Which was hardest?
• Did you find yourself wanting to look at the answer before completing your own reading?
• In Set C, did writing out the six steps change how you approached the problem compared to just reading the code?
• What would you do differently the next time you encounter an error in your own code?

There are no correct answers to these reflection questions. They are for your own development.

---

You have completed 01 · Foundations.

Continue to: 02 · Investigation →

← Back to: Section Index