Control Flow in Kotlin

Control flow statements are essential for creating dynamic and responsive applications. Kotlin provides several ways to control the flow of your program, including if expressions, when statements, and various types of loops.

If Expressions

In Kotlin, if is an expression that returns a value:

// Basic if expression
val max = if (a > b) a else b

// Multi-line if expression
val max = if (a > b) {
    println("Choose a")
    a
} else {
    println("Choose b")
    b
}

Traditional if-else

if (score >= 90) {
    grade = "A"
} else if (score >= 80) {
    grade = "B"
} else if (score >= 70) {
    grade = "C"
} else {
    grade = "F"
}

When Expression

Kotlin’s when expression is a powerful replacement for switch statements:

// Basic when
when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> print("x is neither 1 nor 2")
}

// When with multiple conditions
when (x) {
    0, 1 -> print("x == 0 or x == 1")
    else -> print("otherwise")
}

// When with ranges
when (x) {
    in 1..10 -> print("x is in the range")
    in validNumbers -> print("x is valid")
    !in 10..20 -> print("x is outside the range")
    else -> print("none of the above")
}

For Loops

Range-based loops

// Basic range
for (i in 1..5) {
    print(i)
}

// Down to
for (i in 5 downTo 1) {
    print(i)
}

// Step
for (i in 0..10 step 2) {
    print(i)
}

Collection iteration

val items = listOf("apple", "banana", "orange")

// Basic iteration
for (item in items) {
    println(item)
}

// With index
for ((index, value) in items.withIndex()) {
    println("$index: $value")
}

While Loops

While

var x = 0
while (x < 5) {
    println(x)
    x++
}

Do-while

var x = 0
do {
    println(x)
    x++
} while (x < 5)

Break and Continue

Break

for (i in 1..10) {
    if (i == 5) break
    println(i)
}

Continue

for (i in 1..10) {
    if (i % 2 == 0) continue
    println(i)
}

Return at Labels

fun findUser(users: List<User>): User? {
    users.forEach { user ->
        if (user.isActive) {
            return@forEach  // Returns from the lambda
        }
    }
    return null
}

Best Practices

Prefer when over if-else chains

// Good
when {
    x > 0 -> "positive"
    x < 0 -> "negative"
    else -> "zero"
}

// Avoid
if (x > 0) "positive"
else if (x < 0) "negative"
else "zero"

Use ranges effectively

// Good
for (i in 0 until size)

// Avoid
for (i in 0..size-1)

Leverage forEach for collections

// Good
items.forEach { println(it) }

// Avoid
for (item in items) {
    println(item)
}

Conclusion

Kotlin’s control flow features are designed to be:

Expressive and readable
Safe and null-aware
Flexible and powerful
Concise and efficient

Understanding these control flow mechanisms is crucial for writing clean and maintainable Kotlin code.

Stay tuned for our next post where we’ll explore functions in Kotlin!

Control Flow in Kotlin