Enum entries iteration in Kotlin - Time & Space Complexity

Time Complexity: Enum entries iteration

O(n)

Understanding Time Complexity

When we loop through all entries of an enum, we want to know how the time needed changes as the number of entries grows.

We ask: How does the work grow when the enum has more items?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

enum class Color {
    RED, GREEN, BLUE, YELLOW
}

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

This code loops through all enum entries and prints each one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through all enum entries using Color.values().
• How many times: Once for each enum entry, so as many times as there are entries.

How Execution Grows With Input

As the number of enum entries grows, the loop runs once per entry, so the work grows directly with the number of entries.

Input Size (n)	Approx. Operations
10	About 10 print operations
100	About 100 print operations
1000	About 1000 print operations

Pattern observation: The work increases evenly as the number of enum entries increases.

Final Time Complexity

Time Complexity: O(n)

This means the time grows in a straight line with the number of enum entries.

Common Mistake

[X] Wrong: "Looping over enum entries is constant time because enums are fixed."

[OK] Correct: Even though enums have fixed entries, the loop still runs once per entry, so time grows with the number of entries.

Interview Connect

Understanding how looping over enum entries scales helps you explain performance clearly and shows you know how to analyze simple loops well.

Self-Check

"What if we nested another loop inside to compare each enum entry with every other? How would the time complexity change?"

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