When expression as powerful switch in Kotlin

Introduction

The when expression helps you choose what to do based on different values, like a smart traffic controller.

You want to run different code depending on a variable's value.

You need to check multiple conditions without writing many <code>if</code> statements.

You want to replace a long <code>if-else</code> chain with cleaner code.

You want to handle different types or ranges of values easily.

Syntax

Kotlin

when (variable) {
    value1 -> action1
    value2 -> action2
    in range -> action3
    else -> defaultAction
}

The when expression checks the variable against each case.

The else branch is like a default catch-all if no other case matches.

Examples

Prints "Two" because x is 2.

Kotlin

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

Prints "Between 5 and 10" because y is 5.

Kotlin

val y = 5
when (y) {
    in 1..4 -> println("Between 1 and 4")
    in 5..10 -> println("Between 5 and 10")
    else -> println("Out of range")
}

Checks the type of obj and prints "It's a string".

Kotlin

val obj: Any = "Hello"
when (obj) {
    is String -> println("It's a string")
    is Int -> println("It's an integer")
    else -> println("Unknown type")
}

Sample Program

This program assigns a letter grade based on the numeric score using when. It prints the score and the grade.

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, Grade: $grade")
}

Important Notes

You can use when without an argument to check complex conditions.

Each branch can have multiple statements if you use curly braces.

Summary

when is a cleaner way to choose actions based on values.

It works like a switch but is more powerful and flexible.

Use else to handle unexpected cases.

Practice

1. What is the main purpose of the when expression in Kotlin?

easy

A. To define functions with multiple parameters

B. To create loops that repeat actions multiple times

C. To select actions based on different values, like a switch statement

D. To declare variables with different types

Solution

1. Step 1: Understand the role of when

   The when expression is used to choose between different actions depending on the value it checks.

2. Step 2: Compare with other options

   The other options describe loops, functions, and variables, which are unrelated to when.

3. Final Answer:

   To select actions based on different values, like a switch statement -> Option C

4. Quick Check:

   when chooses actions by value [OK]

Hint: Think of when as a smarter switch [OK]

Common Mistakes:

• Confusing when with loops
• Thinking when declares variables
• Mixing when with function definitions

2. Which of the following is the correct syntax for a simple when expression in Kotlin?

easy

A. when x { 1: println("One") 2: println("Two") else: println("Other") }

B. switch (x) { case 1: println("One") case 2: println("Two") default: println("Other") }

C. if (x == 1) println("One") else if (x == 2) println("Two") else println("Other")

D. when (x) { 1 -> println("One") 2 -> println("Two") else -> println("Other") }

Solution

1. Step 1: Identify Kotlin when syntax

   The correct syntax uses when (x) { value -> action } with arrows and braces.

2. Step 2: Check other options

   when x { 1: println("One") 2: println("Two") else: println("Other") } uses colons (:) instead of arrows (->) and lacks parentheses. switch (x) { case 1: ... } is Java or JavaScript syntax. if (x == 1) ... is an if-else chain.

3. Final Answer:

   when (x) { 1 -> println("One") 2 -> println("Two") else -> println("Other") } -> Option D

4. Quick Check:

   Kotlin when uses arrows and braces [OK]

Hint: Look for arrows (->) inside braces for when [OK]

Common Mistakes:

• Using colon (:) instead of arrow (->)
• Confusing when with switch syntax
• Omitting parentheses around the variable

3. What will be the output of the following Kotlin code?

val x = 3
when (x) {
  1, 2 -> println("Small")
  in 3..5 -> println("Medium")
  else -> println("Large")
}

medium

A. Medium

B. Small

C. Large

D. No output (runtime error)

Solution

1. Step 1: Check the value of x

   The variable x is 3.

2. Step 2: Match x with when cases

   3 is not in 1 or 2, but it is in the range 3..5, so "Medium" is printed.

3. Final Answer:

   Medium -> Option A

4. Quick Check:

   3 in 3..5 means output is "Medium" [OK]

Hint: Check ranges and multiple values carefully [OK]

Common Mistakes:

• Ignoring ranges and picking wrong case
• Assuming 3 matches 1 or 2
• Thinking else runs when a range matches

4. Identify the error in this Kotlin when expression:

val y = 10
when y {
  5 -> println("Five")
  10 -> println("Ten")
  else -> println("Other")
}

medium

A. Missing parentheses around y in when

B. Missing arrow (->) after else

C. Using when without braces

D. No error, code is correct

Solution

1. Step 1: Check when syntax

   The when expression requires parentheses around the variable: when (y).

2. Step 2: Check other syntax parts

   Braces are present. Arrows (->) are correctly used after all cases including else. The syntax error is missing parentheses around y.

3. Final Answer:

   Missing parentheses around y in when -> Option A

4. Quick Check:

   when needs parentheses around the subject [OK]

Hint: Always put parentheses around the variable in when [OK]

Common Mistakes:

• Omitting parentheses around the variable
• Forgetting arrows after cases
• Using when without braces

5. You want to use a when expression to categorize ages into groups: "Child" for 0-12, "Teen" for 13-19, "Adult" for 20-64, and "Senior" for 65 and above. Which when expression correctly implements this?

hard

A. when (age) {
in 0..12 -> "Child"
in 13..19 -> "Teen"
in 20..64 -> "Adult"
else -> "Senior"
}

B. when {
age >= 0 && age <= 12 -> "Child"
age >= 13 && age <= 19 -> "Teen"
age >= 20 && age <= 64 -> "Adult"
age >= 65 -> "Senior"
else -> "Unknown"
}

C. when (age) {
0..12 -> "Child"
13..19 -> "Teen"
20..64 -> "Adult"
65 -> "Senior"
}

D. when (age) {
in 0..12 -> "Child"
in 13..19 -> "Teen"
in 20..64 -> "Adult"
in 65..100 -> "Senior"
else -> "Unknown"
}

Solution

1. Step 1: Understand when with condition checks

   when { age >= 0 && age <= 12 -> "Child" age >= 13 && age <= 19 -> "Teen" age >= 20 && age <= 64 -> "Adult" age >= 65 -> "Senior" else -> "Unknown" } uses when without argument and checks conditions with boolean expressions, which is valid and flexible.

2. Step 2: Check other options for correctness

   when (age) { in 0..12 -> "Child" in 13..19 -> "Teen" in 20..64 -> "Adult" else -> "Senior" } is valid but misses handling negative ages explicitly; when (age) { 0..12 -> "Child" 13..19 -> "Teen" 20..64 -> "Adult" 65 -> "Senior" } uses invalid syntax for ranges; when (age) { in 0..12 -> "Child" in 13..19 -> "Teen" in 20..64 -> "Adult" in 65..100 -> "Senior" else -> "Unknown" } limits "Senior" to 65-100 and has an else for unknown ages.

3. Step 3: Choose the best option

   when { age >= 0 && age <= 12 -> "Child" age >= 13 && age <= 19 -> "Teen" age >= 20 && age <= 64 -> "Adult" age >= 65 -> "Senior" else -> "Unknown" } covers all age ranges clearly with conditions and includes an else for unknown ages.

4. Final Answer:

   when { age >= 0 && age <= 12 -> "Child" age >= 13 && age <= 19 -> "Teen" age >= 20 && age <= 64 -> "Adult" age >= 65 -> "Senior" else -> "Unknown" } -> Option B

5. Quick Check:

   Use condition checks inside when for flexible ranges [OK]

Hint: Use condition checks inside when for complex ranges [OK]

Common Mistakes:

• Using invalid range syntax without 'in'
• Not handling unexpected values with else
• Limiting ranges incorrectly or missing conditions