Traditional Usage of if…else

The syntax of if…else is:

if (testExpression) {
   // codes to run if testExpression is true
}
else {
  // codes to run if testExpression is false
}

if executes a certain section of code if the testExpression is evaluated to true. It can have optional else clause. Codes inside else clause are executed if the testExpression is false.

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Example: Traditional Usage of if…else

fun main(args: Array<String>) {

    val number = -10

    if (number > 0) {
        print("Positive number")
    } else {
        print("Negative number")
    }
}

When you run the program, the output will be:

Negative number

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Kotlin if expression

Unlike Java (and other many programming languages), if can be used an expression in Kotlin; it returns a value.

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Here is an example:

Example: Kotin if expression

fun main(args: Array<String>) {

    val number = -10

    val result = if (number > 0) {
        "Positive number"
    } else {
        "Negative number"
    }

    println(result)
}

When you run the program, the output will be:

Negative number

The else branch is mandatory when using if as an expression.

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The curly braces are optional if the body of if has only one statement. For example,

fun main(args: Array<String>) {
    val number = -10
    val result = if (number > 0) "Positive number" else "Negative number"
    println(result)
}

 

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Example: if block With Multiple Expressions

If the block of if branch contains more than one expression, the last expression is returned as the value of the block.

fun main(args: Array<String>) {

    val a = -9
    val b = -11

    val max = if (a > b) {
        println("$a is larger than $b.")
        println("max variable holds value of a.")
        a
    } else {
        println("$b is larger than $a.")
        println("max variable holds value of b.")
        b
    }
    println("max = $max")
}

When you run the program, the output will be:

-9 is larger than -11.
max variable holds value of a.
max = -9

Recommended Reading: Kotlin when Statement

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Kotlin if..else..if Ladder

You can return a block of code among many blocks in Kotlin using if..else...if ladder.

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Example: if…else…if Ladder

fun main(args: Array<String>) {

    val number = 0

    val result = if (number > 0)
        "positive number"
    else if (number < 0)
        "negative number"
    else 
        "zero"
    
    println("number is $result")
}

This program checks whether number is positive number, negative number, or zero.

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Kotlin Nested if Expression

An if expression can be inside the block of another if expression known as nested if expression.

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Example: Nested if Expression

This program computes the largest number among three numbers.

fun main(args: Array<String>) {

    val n1 = 3
    val n2 = 5
    val n3 = -2

    val max = if (n1 > n2) {
        if (n1 > n3)
            n1
        else
            n3
    } else {
        if (n2 > n3)
            n2
        else
            n3
    }

    println("max = $max")
}

When you run the program, the output will be:

max = 5