When as expression returning value in Kotlin

Introduction

The when expression in Kotlin lets you choose a value based on conditions. It returns a result, so you can use it directly where a value is needed.

Choosing a message to show based on a user's age

Setting a color based on a traffic light signal

Returning a discount percentage based on customer type

Picking an action based on a button clicked

Syntax

Kotlin

val result = when (variable) {
    condition1 -> value1
    condition2 -> value2
    else -> defaultValue
}

The when expression returns the value of the matched branch.

You must cover all cases or use else as a fallback.

Examples

Returns "Weekend" if day is Saturday or Sunday, otherwise "Weekday".

Kotlin

val dayType = when (day) {
    "Saturday", "Sunday" -> "Weekend"
    else -> "Weekday"
}

Uses conditions without a variable to check if x is positive, negative, or zero.

Kotlin

val numberType = when {
    x > 0 -> "Positive"
    x < 0 -> "Negative"
    else -> "Zero"
}

Sample Program

This program assigns a letter grade based on the numeric score using when as an expression.

Kotlin

fun main() {
    val score = 85
    val grade = when (score) {
        in 90..100 -> "A"
        in 80..89 -> "B"
        in 70..79 -> "C"
        in 60..69 -> "D"
        else -> "F"
    }
    println("Score: $score")
    println("Grade: $grade")
}

Important Notes

You can use multiple conditions separated by commas in one branch.

The when expression is like a smarter switch that returns a value.

Summary

when can return a value directly.

Use it to replace multiple if-else statements for clearer code.

Always handle all cases or provide an else branch.

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