Conditional Expressions

In Kotlin, if is an expression and returns a value. Because of this, there is no need for a ternary operator, since if can be used directly:

var value = 1
val otherValue = 5

if (2 < otherValue) value = otherValue

With else:

val firstValue = 1
val secondValue = 2

if (firstValue >= secondValue) {
    println("$firstValue")
} else {
    println("$secondValue")
}

Complete example:

fun main() {
    val firstValue = 1
    val secondValue = 2
    
    if (firstValue >= secondValue) {
        println("$firstValue")
    } else {
        println("$secondValue")
    }
}

An if expression can be used in blocks. In that case, the last expression is the value of the block:

val firstValue = 1
val secondValue = 2

val maxValue = if (firstValue > secondValue) firstValue else secondValue

Complete example:

fun main() {
    val firstValue = 1
    val secondValue = 2
    
    val maxValue = if (firstValue > secondValue) firstValue else secondValue
    
    println(maxValue)  // 2
}

Note

When using if as an expression to return or assign values, else is mandatory to ensure that the flow always has a result.

In Kotlin, when is a conditional expression with multiple branches, similar to the switch statement in other languages such as PHP. The when evaluates its argument and compares it against the conditions of each branch until one is satisfied:

val value = 3

val result = when (value) {
    1 -> "First condition."
    2 -> "Second condition."
    else -> "Default condition (else)."
}

Complete example:

fun main() {
    val value = 3
    
    val result = when (value) {
        1 -> "First condition."
        2 -> "Second condition."
        else -> "Default condition (else)."
    }
    
    println(result)  // Default condition (else).
}

The when also supports ranges in its conditions:

val randomValue = (1..1001).random()

val result = when (randomValue) {
    in 1..100 -> "The value <$randomValue> is between 1 and 100."
    in 101..1000 -> "The value <$randomValue> is between 101 and 1000."
    else -> "The value <$randomValue> is greater than 1000."
}

Complete example:

fun main() {
    val randomValue = (1..1001).random()
    
    val result = when (randomValue) {
        in 1..100 -> "The value <$randomValue> is between 1 and 100."
        in 101..1000 -> "The value <$randomValue> is between 101 and 1000."
        else -> "The value <$randomValue> is greater than 1000."
    }
    
    println(result)
}

The when can be used both as an expression and as a statement. If used as an expression, the value of the first matching condition is the value of the expression. If used as a statement, the values of the individual branches are ignored. As with if, each branch can be a block, and the value of the last expression in the block is the value returned.