When as expression returning value in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of when expression returning a value

What is the output of this Kotlin code using when as an expression?

Kotlin

val x = 3
val result = when (x) {
    1 -> "One"
    2 -> "Two"
    3 -> "Three"
    else -> "Unknown"
}
println(result)

A Three

B Two

C One

D Unknown

💡 Hint

Check which case matches the value of x.

The variable x is 3, so the when expression returns "Three".

❓ Predict Output

intermediate

When expression with multiple matching conditions

What will this Kotlin code print?

Kotlin

val y = 5
val output = when (y) {
    in 1..4 -> "Low"
    in 5..10 -> "Medium"
    else -> "High"
}
println(output)

A Low

B Syntax Error

C High

D Medium

💡 Hint

Look at the range that includes 5.

The value 5 falls in the range 5..10, so the when expression returns "Medium".

❓ Predict Output

advanced

When expression with smart casting and return value

What is the output of this Kotlin code?

Kotlin

fun describe(obj: Any): String {
    return when (obj) {
        is String -> "String of length ${obj.length}"
        is Int -> "Integer value $obj"
        else -> "Unknown type"
    }
}

println(describe("Hello"))

A Integer value Hello

B String of length 5

C Unknown type

D Compilation error

💡 Hint

Check the type of the argument passed to describe.

The argument is a String "Hello". The when expression smart casts it to String and returns its length.

❓ Predict Output

advanced

When expression with no matching branch and else clause

What will this Kotlin code print?

Kotlin

val z = 100
val result = when (z) {
    in 1..10 -> "Small"
    in 11..50 -> "Medium"
    else -> "Large"
}
println(result)

A Medium

B Small

C Large

D Runtime exception

💡 Hint

Check which condition matches the value 100.

100 does not fall in 1..10 or 11..50, so the else branch returns "Large".

❓ Predict Output

expert

When expression returning different types and type inference

What is the type and value of result after running this Kotlin code?

Kotlin

val input: Any = 42
val result = when (input) {
    is Int -> input * 2
    is String -> input.length
    else -> 0
}
println(result)

A Type: Int, Value: 84

B Type: String, Value: "42"

C Type: Any?, Value: null

D Type: Int, Value: 42

💡 Hint

Look at the type of input and what each branch returns.

The input is an Int 42, so the when expression returns 42 * 2 = 84. The inferred type is Int.

Practice

1. What does the when expression in Kotlin return when used as an expression?

easy

A. It returns a value based on the matched case.

B. It always returns Unit (no value).

C. It returns a Boolean indicating if a case matched.

D. It throws an exception if no case matches.

Solution

1. Step 1: Understand when as expression

   In Kotlin, when can be used as an expression that returns a value depending on which case matches.

2. Step 2: Compare with other options

   It does not always return Unit, nor a Boolean, and it requires an else branch to avoid exceptions, so it won't throw by default.

3. Final Answer:

   It returns a value based on the matched case. -> Option A

4. Quick Check:

   when returns value [OK]

Hint: Remember: when returns the matched case value [OK]

Common Mistakes:

• Thinking when always returns Unit
• Confusing when with if returning Boolean
• Forgetting to add an else branch

2. Which of the following is the correct syntax to use when as an expression returning a value in Kotlin?

easy

A. val result = when x { 1 -> "One"; else -> "Other" }

B. val result = when (x) { x > 0 -> "Positive" else -> "Non-positive" }

C. val result = when { x > 0 -> "Positive"; else -> "Non-positive" }

D. val result = when (x) { in 1..10 -> "Small"; else -> "Large" }

Solution

1. Step 1: Check correct when syntax with argument

   val result = when (x) { in 1..10 -> "Small"; else -> "Large" } uses when (x) with cases separated by semicolons and an else branch, which is correct.

2. Step 2: Identify syntax errors in other options

   val result = when { x > 0 -> "Positive"; else -> "Non-positive" } uses semicolons inside braces correctly; val result = when (x) { x > 0 -> "Positive" else -> "Non-positive" } misses semicolon before else; val result = when x { 1 -> "One"; else -> "Other" } misses parentheses around x.

3. Final Answer:

   val result = when (x) { in 1..10 -> "Small"; else -> "Large" } -> Option D

4. Quick Check:

   Correct when syntax uses parentheses and semicolons [OK]

Hint: Use parentheses around variable and semicolons between cases [OK]

Common Mistakes:

• Omitting parentheses around the variable
• Using commas instead of semicolons
• Missing semicolon before else branch

3. What is the output of the following Kotlin code?

val x = 5
val result = when {
  x < 0 -> "Negative"
  x == 0 -> "Zero"
  x in 1..10 -> "Small"
  else -> "Large"
}
println(result)

medium

A. Small

B. Zero

C. Negative

D. Large

Solution

1. Step 1: Evaluate conditions in order

   x = 5 is not less than 0, not equal to 0, but is in 1..10 range, so the third case matches.

2. Step 2: Determine returned value

   The matched case returns "Small", so result is "Small".

3. Final Answer:

   Small -> Option A

4. Quick Check:

   5 in 1..10 = true, returns "Small" [OK]

Hint: Check conditions top to bottom; first true case returns value [OK]

Common Mistakes:

• Assuming else branch runs for 5
• Confusing equality and range checks
• Ignoring order of conditions

4. Identify the error in this Kotlin code using when as an expression:

val x = 3
val result = when (x) {
  1 -> "One"
  2 -> "Two"
}
println(result)

medium

A. Incorrect variable name in when.

B. Missing else branch causes compilation error.

C. Semicolons missing between cases.

D. Cannot use when as expression.

Solution

1. Step 1: Check completeness of when expression

   The when expression covers only 1 and 2, but x = 3 is not handled and no else branch is provided.

2. Step 2: Understand Kotlin requirement for exhaustive when

   When used as an expression, when must be exhaustive, so missing else causes a compilation error.

3. Final Answer:

   Missing else branch causes compilation error. -> Option B

4. Quick Check:

   Exhaustive when needs else [OK]

Hint: Always add else if not all cases covered [OK]

Common Mistakes:

• Ignoring missing else branch
• Thinking semicolons are mandatory
• Assuming when always compiles without else

5. You want to assign a grade based on a score using when as an expression. Which code correctly returns "A" for scores 90 and above, "B" for 80-89, "C" for 70-79, and "F" otherwise?

hard

A. val grade = when (score) {
score >= 90 -> "A"
score >= 80 -> "B"
score >= 70 -> "C"
else -> "F"
}

B. val grade = when (score) {
in 90..100 -> "A"
in 80..89 -> "B"
in 70..79 -> "C"
else -> "F"
}

C. val grade = when {
score >= 90 -> "A"
score >= 80 -> "B"
score >= 70 -> "C"
else -> "F"
}

D. val grade = when {
in 90..100 -> "A"
in 80..89 -> "B"
in 70..79 -> "C"
else -> "F"
}

Solution

1. Step 1: Understand when with condition expressions

   val grade = when { score >= 90 -> "A" score >= 80 -> "B" score >= 70 -> "C" else -> "F" } uses when without argument and conditions like score >= 90, which is valid and clear for ranges.

2. Step 2: Check other options for errors

   val grade = when (score) { in 90..100 -> "A" in 80..89 -> "B" in 70..79 -> "C" else -> "F" } is correct syntax but uses ranges; val grade = when (score) { score >= 90 -> "A" score >= 80 -> "B" score >= 70 -> "C" else -> "F" } incorrectly uses conditions inside when(score) which expects values, not conditions; val grade = when { in 90..100 -> "A" in 80..89 -> "B" in 70..79 -> "C" else -> "F" } uses in without variable, which is invalid.

3. Final Answer:

   val grade = when { score >= 90 -> "A" score >= 80 -> "B" score >= 70 -> "C" else -> "F" } -> Option C

4. Quick Check:

   Use when without argument for conditions [OK]

Hint: Use when without argument for condition checks [OK]

Common Mistakes:

• Using conditions inside when(variable)
• Omitting variable in range checks
• Misplacing in keyword