When as expression returning value in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign the correct string based on the number using when expression.

Kotlin

val result = when (number) {
    1 -> "One"
    2 -> "Two"
    else -> [1]
}

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

A"Other"

B"Unknown"

C"Zero"

D"None"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to return a string in else branch.

Using a number instead of a string in else.

The else branch must return a string value. "Other" is the correct default string here.

2fill in blank

medium

Complete the when expression to return "Even" for even numbers and "Odd" for odd numbers.

Kotlin

val parity = when {
    number % 2 == 0 -> [1]
    else -> "Odd"
}

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

A"Odd"

B"Zero"

C"Even"

D"None"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning "Odd" for even numbers.

Using numbers instead of strings as return values.

The first condition checks if number is even, so it should return "Even".

3fill in blank

hard

Fix the error in the when expression to correctly return a string for the input.

Kotlin

val description = when (input) {
    is String -> "Text"
    is Int -> [1]
    else -> "Unknown"
}

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

Ainput.toString()

Binput.length

Cinput

D"Number"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning input directly which is an Int, causing type mismatch.

Returning input.length which is invalid for Int.

The when expression must return a string. "Number" is the correct string for Int input.

4fill in blank

hard

Fill both blanks to create a when expression that returns the length of a string or zero otherwise.

Kotlin

val length = when (val value = input) {
    is String -> [1]
    else -> [2]
}

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

Avalue.length

B0

Cinput.length

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using input.length instead of value.length inside when.

Returning null instead of 0 for else case.

For a String input, return its length. Otherwise, return 0.

5fill in blank

hard

Fill all three blanks to create a when expression that returns a message based on the score value.

Kotlin

val message = when {
    score >= [1] -> "Excellent"
    score >= [2] -> "Good"
    else -> [3]
}

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

A90

B60

C"Try again"

D"Average"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the threshold values.

Returning a number instead of a string in else.

Scores 90 or above get "Excellent", 60 or above get "Good", else "Try again".

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