Kotlin if else Expression

In this tutorial, we will learn how to control the flow of execution in Kotlin using the if-else expressions. Using if-else statements to control the flow of execution is available in almost all the modern programming languages.

In most of the languages, if-else block is used to select a condition based on an expression. If the if expression returns true, if block will be executed. Otherwise, the else block will be executed. For example, if we want to know whether a student passed the examination or not using the score, we can write the following code:

fun main() {
    val marks = 70
    val result:String
    if(marks > 33){
        result = "Hurrah! You passed"
    }
    else{
        result ="Sorry! You failed"
    }
    println(result)
}

Hurrah! You passed

Here, because the marks of the student is greater than 33, he has passed the examination.

But in Kotlin, if-else is not a statement, it is an expression. It means the if-else block can return a value with a slight syntax change.

Let's see the code below:

fun main() {
    val marks = 70
    val result:String
    result = if(marks > 33) {
        "Hurrah! You passed"
    }
    else{
        "Sorry! You failed"
    }
    println(result)
}

Here, based on the condition, the value will be returned from the if-else block and will be assigned to the result variable.

Note: While using the if-else as an expression, it is compulsory to have an else block with if block.

There are various types of if-else expression in Kotlin:

1. Simple if-else expression

2. if-else ladder

3. Nested if expression

Simple if-else expression

Here is a pictorial representation of how an if statement works if used without an else block:

Let's take a code example to see the simple if expression in action:

fun main() {
    val light = "red"
    if(light == "red"){
       print("Stop")
    }
}

Stop

And if we have if and else both specified in the code, then:

We already saw an example of simple if-else expression above. We can simplify this expression even more:

fun main() {
    val marks = 70
    val result:String
    result = if(marks > 33) "Hurrah! You passed" else "Sorry! You failed"
    println(result)
}

Hurrah! You passed

We can remove the curly braces if we have a single line code in if and else block.

The if-else Ladder

If we have more than one condition to check, then we can use if else ladder. Let us understand this with an example.

Suppose we want to assign the grade to a student based on his/her marks; we can use if else ladder:

fun main() {
    val marks = 70
    val grade: String
    grade = if(marks >= 91)
        "A+"
    else if(marks >= 81)
        "A"
    else if(marks >= 71)
        "B+"
    else if(marks >= 61)
        "B"
    else if(marks >= 51)
        "C+"
    else if(marks >= 41)
        "C"
    else if(marks >= 33)
        "D"
    else
        "F"
    println("Your grade is: $grade")
}

Your grade is: B

To make the above code example more interactive, you can take input from the user and then show the grade as output.

Nested if-else expression

When we have an if expression inside another if expression, it is known as nested if expression. Let us understand this with the help of an example.

We will write a program to find the maximum of three numbers.

Algorithm example:

• Input three numbers: num1, num2 and num3.

• If num1 is greater than num2:

  • If num1 is greater than num3, then num1 is maximum

  • Else num3 is maximum

• Else if num2 is greater than num3, num2 is maximum

• Else num3 is maximum

This can be implemented as:

import java.util.*

fun main() {
    val scanner = Scanner(System.`in`)

    // Taking three numbers as input
    println("Enter first number: ")
    val num1 = scanner.nextInt()

    println("Enter second number: ")
    val num2 = scanner.nextInt()

    println("Enter third number: ")
    val num3 = scanner.nextInt()

    val max = if(num1 > num2){
        if(num1 > num3)
            num1
        else
            num3
    }
    else if(num2 > num3)
        num2
    else num3

    println("Maximum number is: $max")
}

Enter first number:
10
Enter second number:
20
Enter third number:
11
Maximum number is: 20

The first section of the program above is taking input of 3 numbers from the user and then we are comparing those numbers to find the maximum number out of the 3 numbers. For this, we use an if else condition inside another if condition, and this is nested if expression.

There is no limit on the nesting. You can have as many if condition inside one if condition or can make a nesting chain.

Summary

In this tutorial, we discussed the if and else expression in Kotlin. In the next tutorial, we will discuss the when expression in Kotlin.