When as expression returning value in Kotlin - Mini Project: Build & Apply

Using When as an Expression Returning a Value in Kotlin

📖 Scenario: You are creating a simple program that assigns a message based on a day number. This is like checking the day of the week and giving a friendly note.

🎯 Goal: Build a Kotlin program that uses when as an expression to return a message string based on a day number.

📋 What You'll Learn

Create an integer variable day with the value 3

Create a variable message that uses when as an expression to assign a string based on day

Use when with cases for days 1 to 7, returning the correct day name

Print the message variable

💡 Why This Matters

🌍 Real World

Using <code>when</code> expressions helps in apps to decide actions based on user input or data values, like showing messages or choosing options.

💼 Career

Understanding <code>when</code> expressions is important for Kotlin developers to write clean and readable decision-making code.

1

Create the day variable

Create an integer variable called day and set it to 3.

Kotlin

# Create an integer variable called day and set it to 3
# Your code here

Hint

Use val day = 3 to create the variable.

2

Set up the when expression

Create a variable called message that uses when as an expression on day. Start the when expression with cases for 1 and 2 returning "Monday" and "Tuesday" respectively.

Kotlin

val day = 3
# Create a variable message using when expression on day with cases 1 and 2
# Your code here

Hint

Use val message = when(day) { 1 -> "Monday"; 2 -> "Tuesday"; else -> "Unknown" }.

3

Add all days to the when expression

Extend the when expression for message to include all days from 1 to 7 returning "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", and "Sunday" respectively.

Kotlin

val day = 3
val message = when(day) {
    1 -> "Monday"
    2 -> "Tuesday"
    # Add cases for 3 to 7 here
    else -> "Unknown"
}

Hint

Add all days as cases in the when expression.

4

Print the message

Write a println statement to print the message variable.

Kotlin

val day = 3
val message = when(day) {
    1 -> "Monday"
    2 -> "Tuesday"
    3 -> "Wednesday"
    4 -> "Thursday"
    5 -> "Friday"
    6 -> "Saturday"
    7 -> "Sunday"
    else -> "Unknown"
}
# Print the message variable
# Your code here

Hint

Use println(message) to show the result.

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