Enum entries iteration in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of iterating enum entries with for loop

What is the output of this Kotlin code that iterates over enum entries using a for loop?

Kotlin

enum class Color { RED, GREEN, BLUE }

fun main() {
    for (color in Color.values()) {
        println(color)
    }
}

A red\ngreen\nblue

B RED\nGREEN\nBLUE

C Color.RED\nColor.GREEN\nColor.BLUE

D RED GREEN BLUE

💡 Hint

Remember that printing enum entries calls their toString() method which returns the name by default.

The values() function returns all enum entries. Printing each entry prints its name in uppercase as declared.

❓ Predict Output

intermediate

Output of iterating enum entries with forEach and lambda

What will this Kotlin code print when iterating enum entries with forEach and a lambda?

Kotlin

enum class Direction { NORTH, EAST, SOUTH, WEST }

fun main() {
    Direction.values().forEach { println(it.name.lowercase()) }
}

A north\neast\nsouth\nwest

B NORTH\nEAST\nSOUTH\nWEST

C Direction.NORTH\nDirection.EAST\nDirection.SOUTH\nDirection.WEST

D north east south west

💡 Hint

The name property returns the enum constant name as declared. The lowercase() function converts it to lowercase.

Each enum entry's name is converted to lowercase and printed on its own line.

🧠 Conceptual

advanced

Understanding enum entries iteration with indices

Given this Kotlin code, what is the output when iterating enum entries with their indices?

Kotlin

enum class Level { LOW, MEDIUM, HIGH }

fun main() {
    Level.values().forEachIndexed { index, level ->
        println("$index: $level")
    }
}

A Level.LOW: 0\nLevel.MEDIUM: 1\nLevel.HIGH: 2

B LOW: 0\nMEDIUM: 1\nHIGH: 2

C 0 LOW\n1 MEDIUM\n2 HIGH

D 0: LOW\n1: MEDIUM\n2: HIGH

💡 Hint

forEachIndexed provides the index and the element in order.

The index starts at 0 and increments by 1 for each enum entry. The output prints index and enum name separated by colon.

🔧 Debug

advanced

Identify the error in enum iteration code

What error does this Kotlin code produce when trying to iterate enum entries?

Kotlin

enum class Status { STARTED, RUNNING, FINISHED }

fun main() {
    for (s in Status) {
        println(s)
    }
}

A No error, prints STARTED RUNNING FINISHED

B Error: Missing parentheses after Status

C Error: 'Status' is not iterable

D Error: Cannot use enum class in for loop

💡 Hint

You must call values() to get an array of enum entries to iterate.

Enum classes are not directly iterable. You must use Status.values() to get the array of entries.

🚀 Application

expert

Count enum entries with a specific property

Given this enum with a property, what is the output of counting entries with isActive = true?

Kotlin

enum class TaskStatus(val isActive: Boolean) {
    TODO(true), IN_PROGRESS(true), DONE(false), CANCELLED(false)
}

fun main() {
    val activeCount = TaskStatus.values().count { it.isActive }
    println(activeCount)
}

A 2

B 4

C 3

D 1

💡 Hint

Check which enum entries have isActive set to true.

TODO and IN_PROGRESS have isActive = true, so count is 2.

Practice

1. What does EnumName.values() return in Kotlin?

easy

A. An array of all enum entries in the order they are declared

B. A list of enum names as strings

C. The first enum entry only

D. A map of enum entries to their ordinal values

Solution

1. Step 1: Understand the purpose of values()

   The values() function returns all enum entries as an array in the order they are declared.

2. Step 2: Compare options with this behavior

   Only An array of all enum entries in the order they are declared correctly describes this behavior; others describe different data types or partial data.

3. Final Answer:

   An array of all enum entries in the order they are declared -> Option A

4. Quick Check:

   EnumName.values() returns all entries [OK]

Hint: Remember: values() gives all enum entries as an array [OK]

Common Mistakes:

• Thinking values() returns strings instead of enum objects
• Assuming values() returns only one entry
• Confusing values() with valueOf()

2. Which of the following is the correct syntax to iterate over all entries of an enum Color in Kotlin?

easy

A. for (c in Color.values()) { println(c) }

B. for (c in Color) { println(c) }

C. for (c in Color.entries()) { println(c) }

D. for (c in Color.all()) { println(c) }

Solution

1. Step 1: Recall the correct method to get enum entries

   In Kotlin, values() returns all enum entries as an array.

2. Step 2: Check each option's syntax

   The correct syntax is for (c in Color.values()) { println(c) }, which uses Color.values() correctly in a for loop. The other options use invalid syntax: iterating directly over Color, or non-existent methods like entries() or all().

3. Final Answer:

   for (c in Color.values()) { println(c) } -> Option A

4. Quick Check:

   Use values() to loop enums [OK]

Hint: Use EnumName.values() to loop enums [OK]

Common Mistakes:

• Trying to loop directly over enum class without values()
• Using non-existent methods like entries() or all()
• Missing parentheses after values()

3. Given the enum:

enum class Direction { NORTH, EAST, SOUTH, WEST }

What is the output of this code?

for (d in Direction.values()) {
    print(d.name.first())
}

medium

A. Direction.NORTHDirection.EASTDirection.SOUTHDirection.WEST

B. north east south west

C. NESW

D. N E S W

Solution

1. Step 1: Understand the loop and print statement

   The loop goes through all enum entries: NORTH, EAST, SOUTH, WEST. For each, it prints the first character of the name.

2. Step 2: Determine printed characters

   First letters are 'N', 'E', 'S', 'W'. They are printed without spaces or newlines.

3. Final Answer:

   NESW -> Option C

4. Quick Check:

   First letters combined = NESW [OK]

Hint: Check what exactly is printed inside the loop [OK]

Common Mistakes:

• Assuming spaces or newlines are printed
• Printing full names instead of first letters
• Confusing enum name with enum value

4. What is wrong with this Kotlin code snippet?

enum class Status { STARTED, RUNNING, FINISHED }

for (s in Status) {
    println(s)
}

medium

A. The loop variable must be declared as val

B. Enums cannot be iterated in Kotlin

C. The enum class must be declared inside a function

D. Missing call to values() to get enum entries

Solution

1. Step 1: Identify how to iterate enums in Kotlin

   Enums cannot be iterated directly; you must use Status.values() to get all entries.

2. Step 2: Check the code for missing values()

   The code tries to loop over Status directly, which is invalid syntax.

3. Final Answer:

   Missing call to values() to get enum entries -> Option D

4. Quick Check:

   Use Enum.values() to iterate enums [OK]

Hint: Always use values() to loop enums [OK]

Common Mistakes:

• Trying to loop enum class directly without values()
• Thinking enums are iterable by default
• Adding unnecessary val in loop declaration

5. You have this enum:

enum class Level(val difficulty: Int) {
    EASY(1), MEDIUM(2), HARD(3)
}

How do you create a map from each enum entry to its difficulty using iteration?

hard

A. val map = Level.values().map { it.difficulty }

B. val map = Level.values().associateWith { it.difficulty }

C. val map = Level.associate { it to it.difficulty }

D. val map = Level.values().associateBy { it.difficulty }

Solution

1. Step 1: Understand the goal

   We want a map where keys are enum entries and values are their difficulty.

2. Step 2: Use values() and associateWith

   Level.values() gives all entries; associateWith maps each entry to a value.

3. Step 3: Check options

   val map = Level.values().associateWith { it.difficulty } correctly uses associateWith { it.difficulty }. val map = Level.associate { it to it.difficulty } is invalid because Level is a class, not a collection. val map = Level.values().map { it.difficulty } returns a list, not a map. val map = Level.values().associateBy { it.difficulty } maps difficulty to enum, reversing keys and values.

4. Final Answer:

   val map = Level.values().associateWith { it.difficulty } -> Option B

5. Quick Check:

   Use values() + associateWith for map [OK]

Hint: Use values().associateWith { } to map enums [OK]

Common Mistakes:

• Trying to call associate on enum class directly
• Using map() instead of associateWith() for key-value pairs
• Confusing associateBy() which reverses keys and values