Why When with multiple conditions in Kotlin? - Purpose & Use Cases

The Big Idea

What if you could replace many confusing checks with one simple, clear expression?

The Scenario

Imagine you have a list of fruits and you want to print a special message for each type. You try to check each fruit one by one using many if-else statements.

The Problem

Using many if-else statements makes your code long, hard to read, and easy to make mistakes. It's like checking every fruit separately instead of grouping similar ones together.

The Solution

The when expression lets you check multiple conditions in a clean and organized way. You can group several cases together, making your code shorter and easier to understand.

Before vs After

✗ Before

if (fruit == "apple" || fruit == "pear") {
    println("This is a sweet fruit")
} else if (fruit == "lemon" || fruit == "lime") {
    println("This is a sour fruit")
}

✓ After

when (fruit) {
    "apple", "pear" -> println("This is a sweet fruit")
    "lemon", "lime" -> println("This is a sour fruit")
}

What It Enables

You can write clear and concise code that handles many conditions together, making your programs easier to read and maintain.

Real Life Example

Think about a traffic light system where you want to perform the same action for both red and yellow lights (stop), and a different action for green (go). Using when with multiple conditions groups these actions neatly.

Key Takeaways

Manual checks with many if-else are long and confusing.

when groups multiple conditions for cleaner code.

This makes your programs easier to read and less error-prone.

Practice

1.

What does the when expression do when multiple conditions are separated by commas in Kotlin?

easy

A. It throws an error if multiple conditions are used.

B. It checks if the value matches any of the listed conditions and executes the corresponding block.

C. It only checks the first condition and ignores the rest.

D. It requires all conditions to be true at the same time to execute the block.

Solution

1. Step 1: Understand how when handles multiple conditions

   In Kotlin, when multiple conditions are separated by commas, it means the when expression checks if the value matches any one of those conditions.

2. Step 2: Interpret the behavior of multiple conditions

   If the value matches any condition in the list, the corresponding block runs. It does not require all conditions to be true simultaneously.

3. Final Answer:

   It checks if the value matches any of the listed conditions and executes the corresponding block. -> Option B

4. Quick Check:

   Multiple conditions = any match triggers block [OK]

Hint: Multiple conditions mean OR logic, not AND [OK]

Common Mistakes:

• Thinking all conditions must be true at once
• Believing only the first condition is checked
• Assuming multiple conditions cause errors

2.

Which of the following is the correct syntax to check if a variable num is either 1, 2, or 3 using when with multiple conditions?

when (num) {
    ? -> println("Number is 1, 2, or 3")
    else -> println("Other number")
}

easy

A. 1, 2, 3

B. 1 || 2 || 3

C. 1 && 2 && 3

D. [1, 2, 3]

Solution

1. Step 1: Recall Kotlin when syntax for multiple conditions

   In Kotlin, multiple conditions in a when case are separated by commas without operators.

2. Step 2: Match the correct syntax

   1, 2, 3 uses commas to separate values: 1, 2, 3, which is correct. Other options use logical operators or list syntax, which are invalid here.

3. Final Answer:

   1, 2, 3 -> Option A

4. Quick Check:

   Multiple conditions use commas [OK]

Hint: Use commas, not operators, to separate conditions [OK]

Common Mistakes:

• Using || or && inside when conditions
• Trying to use list syntax like [1, 2, 3]
• Missing commas between values

3.

What will be the output of the following Kotlin code?

val x = 5
when (x) {
    1, 2, 3 -> println("Low")
    4, 5, 6 -> println("Medium")
    else -> println("High")
}

medium

A. Medium

B. High

C. Low

D. No output

Solution

1. Step 1: Identify the value of x

   The variable x is assigned the value 5.

2. Step 2: Match x with when conditions

   The second case checks if x is 4, 5, or 6. Since x is 5, this condition matches and prints "Medium".

3. Final Answer:

   Medium -> Option A

4. Quick Check:

   5 in 4,5,6 prints Medium [OK]

Hint: Check which group contains the value [OK]

Common Mistakes:

• Choosing the first matching group incorrectly
• Confusing else with matching values
• Assuming no output if multiple conditions exist

4.

Find the error in this Kotlin when expression:

val grade = 'B'
when (grade) {
    'A', 'B' -> println("Excellent")
    'C' || 'D' -> println("Good")
    else -> println("Try again")
}

medium

A. The variable grade should be an Int, not Char.

B. Using commas to separate 'A' and 'B' is wrong.

C. Missing parentheses around conditions.

D. Using '||' inside when conditions is invalid syntax.

Solution

1. Step 1: Review the when condition syntax

   In Kotlin, multiple conditions in a when case must be separated by commas, not logical operators like '||'.

2. Step 2: Identify the incorrect usage

   The line 'C' || 'D' uses '||', which is invalid syntax in when. It should be 'C', 'D'.

3. Final Answer:

   Using '||' inside when conditions is invalid syntax. -> Option D

4. Quick Check:

   Use commas, not logical operators [OK]

Hint: Never use || or && in when conditions [OK]

Common Mistakes:

• Using logical operators instead of commas
• Thinking commas are wrong separators
• Confusing variable types with syntax errors

5.

You want to write a Kotlin when expression that prints "Weekend" if the day is either "Saturday" or "Sunday", "Weekday" if the day is any other day of the week, and "Invalid" for any other input. Which code correctly implements this?

hard

A. when(day) {
in listOf("Saturday", "Sunday") -> println("Weekend")
else -> println("Weekday")
}

B. when(day) {
"Saturday" || "Sunday" -> println("Weekend")
else -> println("Weekday")
}

C. when(day) {
"Saturday", "Sunday" -> println("Weekend")
"Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday")
else -> println("Invalid")
}

D. when(day) {
"Saturday" && "Sunday" -> println("Weekend")
else -> println("Weekday")
}

Solution

1. Step 1: Check correct multiple condition syntax for weekend days

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } correctly uses commas to separate "Saturday" and "Sunday" to match either day.

2. Step 2: Verify handling of other days and else case

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } lists all weekdays explicitly and uses else for invalid inputs, matching the problem requirements.

3. Final Answer:

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } -> Option C

4. Quick Check:

   Use commas for multiple matches and else for fallback [OK]

Hint: List all matches with commas; use else for others [OK]

Common Mistakes:

• Using || or && instead of commas
• Missing else for invalid inputs
• Not listing all weekdays explicitly