Accessing elements safely in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely access the first element of the list.

Kotlin

val list = listOf(1, 2, 3)
val first = list.[1]

Drag options to blanks, or click blank then click option'

Afirst()

Bget(0)

CfirstOrNull()

DelementAt(0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using get(0) which throws an exception if the list is empty.

Using first() which throws an exception if the list is empty.

The firstOrNull() function returns the first element or null if the list is empty, preventing errors.

2fill in blank

medium

Complete the code to safely access the element at index 2.

Kotlin

val list = listOf("a", "b")
val element = list.[1](2)

Drag options to blanks, or click blank then click option'

AgetOrElse

Bget

CelementAt

DelementAtOrNull

Attempts:

3 left

💡 Hint

Common Mistakes

Using get(2) which throws an exception if index is out of bounds.

Using elementAt(2) which throws an exception if index is invalid.

The elementAtOrNull() function returns the element at the given index or null if the index is out of bounds.

3fill in blank

hard

Fix the error in safely accessing the last element of the list.

Kotlin

val list = listOf<Int>()
val last = list.[1]()

Drag options to blanks, or click blank then click option'

AlastOrNull

Blast

CgetLast

DelementAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using last() which throws an exception if the list is empty.

Using elementAt() without checking index.

lastOrNull() safely returns the last element or null if the list is empty, avoiding exceptions.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, but only for words longer than 3 characters.

Kotlin

val words = listOf("cat", "elephant", "dog", "lion")
val lengths = words.associateWith { [1] }
val filtered = lengths.filter { it.value [2] 3 }

Drag options to blanks, or click blank then click option'

Ait.length

B>

C<

Dit.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the filter condition.

Using it.size which is not a property of String.

it.length gets the length of each word, and filtering with > 3 keeps only words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, filtering only words longer than 4 characters.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = words.associate { word -> [1] to [2] }
val filtered = result.filter { it.value [3] 4 }

Drag options to blanks, or click blank then click option'

Aword.uppercase()

Bword.length

C>

Dword.toUpperCase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated toUpperCase() instead of uppercase().

Using < instead of > in the filter condition.

word.uppercase() converts words to uppercase, word.length gets their length, and filtering with > 4 keeps words longer than 4 characters.

Practice

1.

What does the Kotlin expression map[key] ?: "default" do?

easy

A. Removes the key from the map if it exists.

B. Returns the value for key if it exists, otherwise returns "default".

C. Throws an error if key is not in the map.

D. Always returns "default" regardless of key.

Solution

1. Step 1: Understand map access with []

   Accessing map[key] returns the value if key exists, or null if it doesn't.

2. Step 2: Use the Elvis operator ?: for fallback

   The expression ?: "default" means if the left side is null, return "default" instead.

3. Final Answer:

   Returns the value for key if it exists, otherwise returns "default". -> Option B

4. Quick Check:

   map[key] ?: default = safe access with fallback [OK]

Hint: Use ?: to provide a default when key might be missing [OK]

Common Mistakes:

• Thinking it throws an error if key is missing
• Assuming it always returns default
• Confusing with removing keys

2.

Which of the following is the correct Kotlin syntax to safely get the element at index 2 from a list myList, returning -1 if the index is out of bounds?

easy

A. myList.getOrNull(2) ?: -1

B. myList[2] ?: -1

C. myList.get(2) ?: -1

D. myList.getOrElse(2, -1)

Solution

1. Step 1: Understand safe list access methods

   myList.getOrNull(2) returns the element at index 2 or null if out of bounds.

2. Step 2: Use Elvis operator for default

   ?: -1 returns -1 if getOrNull returns null.

3. Final Answer:

   myList.getOrNull(2) ?: -1 -> Option A

4. Quick Check:

   getOrNull + ?: = safe get with fallback [OK]

Hint: Use getOrNull(index) ?: default for safe list access [OK]

Common Mistakes:

• Using myList[2] which throws exception if index invalid
• Using get(2) which throws exception if index invalid
• Confusing getOrElse syntax

3.

What is the output of this Kotlin code?

val map = mapOf("a" to 1, "b" to 2)
println(map["c"] ?: 0)

medium

A. null

B. 2

C. Exception thrown

D. 0

Solution

1. Step 1: Check map access for key "c"

   Key "c" does not exist in the map, so map["c"] returns null.

2. Step 2: Apply Elvis operator

   ?: 0 returns 0 because the left side is null.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   map[missingKey] ?: default = default [OK]

Hint: Missing keys return null, use ?: for default [OK]

Common Mistakes:

• Expecting null to print instead of default
• Thinking exception is thrown
• Confusing existing key values

4.

Identify the error in this Kotlin code snippet and choose the corrected version:

val list = listOf(10, 20, 30)
val value = list[5] ?: -1
println(value)

medium

A. val value = list.get(5) ?: -1

B. val value = list[5]

C. val value = list.getOrNull(5) ?: -1

D. val value = list.getOrElse(5, -1)

Solution

1. Step 1: Understand list access with []

   Using list[5] throws an IndexOutOfBoundsException because index 5 is out of range.

2. Step 2: Use safe access method

   list.getOrNull(5) returns null if index is invalid, so combined with ?: -1 it safely returns -1.

3. Final Answer:

   val value = list.getOrNull(5) ?: -1 -> Option C

4. Quick Check:

   Use getOrNull + ?: to avoid exceptions [OK]

Hint: Replace [] with getOrNull + ?: for safe index access [OK]

Common Mistakes:

• Using [] which throws exception if index invalid
• Using get(5) which throws exception
• Confusing getOrElse syntax

5.

You have a map val scores = mapOf("Alice" to 90, "Bob" to 85) and a list val bonuses = listOf(5, 10). You want to safely get Bob's score and the bonus at index 2, returning 0 if missing, then sum them. Which code correctly does this?

hard

A. val total = (scores["Bob"] ?: 0) + (bonuses.getOrNull(2) ?: 0)

B. val total = (scores.getOrNull("Bob") ?: 0) + (bonuses.getOrNull(2) ?: 0)

C. val total = (scores["Bob"] ?: 0) + (bonuses[2] ?: 0)

D. val total = (scores["Bob"] ?: 0) + bonuses.get(2)

Solution

1. Step 1: Access map value safely

   scores["Bob"] ?: 0 safely gets Bob's score or 0 if missing.

2. Step 2: Access list element safely

   bonuses.getOrNull(2) ?: 0 safely gets bonus at index 2 or 0 if out of bounds.

3. Step 3: Sum the two values

   Adding these two safe values gives the total without errors.

4. Final Answer:

   val total = (scores["Bob"] ?: 0) + (bonuses.getOrNull(2) ?: 0) -> Option A

5. Quick Check:

   Safe map + safe list access with ?: for default [OK]

Hint: Use ?: with map and getOrNull for list to avoid errors [OK]

Common Mistakes:

• Using bonuses[2] which throws exception if index invalid
• Using getOrNull on map which is invalid
• Not providing default fallback