Elvis operator (?:) for default values in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a default value using the Elvis operator.

Kotlin

val name: String? = null
val displayName = name [1] "Guest"

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

A?:

B!!

C?.

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws an exception if null.

Using '?.' which is for safe calls, not default values.

The Elvis operator ?: returns the left value if it's not null; otherwise, it returns the right value.

2fill in blank

medium

Complete the code to print a default message when the nullable variable is null.

Kotlin

fun greet(user: String?) {
    println(user [1] "Anonymous")
}

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

A!!

B?:

C?.

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which causes an exception if null.

Using '?.' which is for safe calls, not default values.

The Elvis operator ?: provides a default value when the left side is null.

3fill in blank

hard

Fix the error in the code by using the correct operator to provide a default value.

Kotlin

val length = text?.length [1] 0

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

A==

B!!

C?.

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws an exception if null.

Using '?.' which is for safe calls, not default values.

The Elvis operator ?: returns 0 if text?.length is null.

4fill in blank

hard

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

Kotlin

val words = listOf("cat", null, "elephant", "dog", "lion")
val lengths = words.associateWith { it?.length [1] 0 } .filter { it.value [2] 3 }

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

A?:

B>

C>=

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws an exception if the word is null.

Using '>=' which would include words of length 3.

Use the Elvis operator ?: to return 0 if it?.length is null (when word is null), and > to filter lengths greater than 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, including only words with length greater than 3.

Kotlin

val words = listOf("apple", null, "bat", "carrot", "dog")
val result = words.associate { (it ?: "UNKNOWN")[1] to it?.length [2] 0 } .filter { it.value [3] 3 }

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

A.uppercase()

B?:

C>

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' in the second blank, which throws an exception if word is null.

Using '>=' in the third blank, which includes length 3.

Use '.uppercase()' to convert the key to uppercase (null handled as "UNKNOWN"), the Elvis operator ?: to default length to 0 if null, and '>' to filter lengths greater than 3.

Practice

1. What does the Elvis operator ?: do in Kotlin?

easy

A. Checks if two values are equal

B. Converts a value to a string

C. Throws an exception if the left value is null

D. Returns the left value if it is not null; otherwise returns the right value

Solution

1. Step 1: Understand the Elvis operator purpose

   The Elvis operator ?: is used to provide a default value when the left side is null.

2. Step 2: Analyze the behavior

   If the left value is not null, it returns that value; otherwise, it returns the right value as a fallback.

3. Final Answer:

   Returns the left value if it is not null; otherwise returns the right value -> Option D

4. Quick Check:

   Elvis operator = default value if null [OK]

Hint: Elvis operator gives a fallback value if null [OK]

Common Mistakes:

• Thinking it compares two values
• Assuming it throws an error on null
• Confusing it with string conversion

2. Which of the following is the correct syntax to use the Elvis operator in Kotlin?

easy

A. val result = value ??: defaultValue

B. val result = value ? defaultValue

C. val result = value ?: defaultValue

D. val result = value ?:? defaultValue

Solution

1. Step 1: Recall the correct Elvis operator syntax

   The Elvis operator is written as ?: without any extra characters or spaces.

2. Step 2: Match the syntax with options

   val result = value ?: defaultValue uses ?: correctly between the variable and the default value.

3. Final Answer:

   val result = value ?: defaultValue -> Option C

4. Quick Check:

   Correct Elvis syntax = value ?: defaultValue [OK]

Hint: Elvis operator is ?: exactly, no extra symbols [OK]

Common Mistakes:

• Using single question mark instead of ?:
• Adding extra question marks
• Swapping the order of symbols

3. What will be the output of this Kotlin code?

val name: String? = null
val displayName = name ?: "Guest"
println(displayName)

medium

A. null

B. Guest

C. name

D. Compilation error

Solution

1. Step 1: Check the value of name

   The variable name is explicitly set to null.

2. Step 2: Apply the Elvis operator

   Since name is null, displayName gets the default value "Guest".

3. Final Answer:

   Guest -> Option B

4. Quick Check:

   null ?: "Guest" = "Guest" [OK]

Hint: If left is null, Elvis returns right side [OK]

Common Mistakes:

• Assuming it prints null
• Thinking it prints variable name
• Expecting a compile error

4. Identify the error in this Kotlin code snippet using the Elvis operator:

val input: String? = null
val output = input ? "Default"

medium

A. Incorrect operator used; should be ?: instead of ?

B. Missing right side value after Elvis operator

C. Variable input cannot be nullable

D. No error; code is correct

Solution

1. Step 1: Examine the operator used

   The code uses ? instead of the Elvis operator ?:.

2. Step 2: Understand correct Elvis operator usage

   The Elvis operator must be ?: to provide a default value when the left side is null.

3. Final Answer:

   Incorrect operator used; should be ?: instead of ? -> Option A

4. Quick Check:

   Elvis operator must be ?: not ? [OK]

Hint: Elvis operator always uses ?: not just ? [OK]

Common Mistakes:

• Using single question mark instead of Elvis operator
• Forgetting the default value after ?:
• Assuming nullable variables cause error here

5. Given the following Kotlin code, what will be the value of finalScore?

fun calculateScore(score: Int?): Int {
    return score ?: 0
}

val userScore: Int? = 5
val finalScore = calculateScore(userScore ?: 10)

hard

A. 5

B. 10

C. 0

D. Compilation error

Solution

1. Step 1: Evaluate the expression inside the function call

   The argument is userScore ?: 10. Since userScore is 5 (not null), the expression evaluates to 5.

2. Step 2: Call calculateScore with 5

   The function returns the score if not null, so it returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   userScore ?: 10 = 5, function returns 5 [OK]

Hint: Elvis operator picks first non-null value [OK]

Common Mistakes:

• Thinking userScore ?: 10 returns 10
• Confusing function return with argument default
• Assuming null causes error here