Lesson 4 - Conditions (branching) in Kotlin

In the previous exercise, Solved tasks for Kotlin lesson 3, we've practiced our knowledge from previous lessons.

{KOTLIN_CONSOLE}
println("Enter your age:")
val age: Int = (readLine()!!).toInt()
if (age == 25) {
    println("Hey, I'm 25 too!"); // printed only when 25 is entered
}
println("Thanks for your age!") // printed always
{/KOTLIN_CONSOLE}

{KOTLIN_CONSOLE}
println("Enter your age:")
val age: Int = (readLine()!!).toInt()
if (age == 25) {
    println("Hey, I'm 25 too!")  // printed only when 25 is entered
    println("Nice to meet you!")  // printed only when 25 is entered
}
else {
    println("Nice to meet you, I'm 25!") // printed for any other age
}
println("Thanks for your age!") // printed always
{/KOTLIN_CONSOLE}

{KOTLIN_CONSOLE}
println("Enter your age:");
var age: Int = (readLine()!!).toInt()
if (age == 25) {
    println("Hey, I'm 25 too!") // printed only when 25 is entered
    println("Nice to meet you!") // printed only when 25 is entered
}
else {
    if (age == 20 || age == 14) {
        println("One my brother is of that age too!") // printed only when 20 is entered
    }
    println("Nice to meet you, I'm 25!") // printed for any other age than 25
}
println("Thanks for your age!") // printed always
{/KOTLIN_CONSOLE}

{KOTLIN_CONSOLE}
println("Welcome to our calculator")
println("Enter the first number:")
val a = readLine()!!.toDouble()
println("Enter the second number:")
val b = readLine()!!.toDouble()
println("Choose one of the following operations:")
println("1 - addition")
println("2 - subtraction")
println("3 - multiplication")
println("4 - division")
val option = readLine()!!.toInt()
var result = 0.0
when (option) {
    1 -> result = a + b
    2 -> result = a - b
    3 -> result = a * b
    4 -> result = a / b
}
if ((option > 0) && (option < 5)) {
    println("Result: $result")
} else {
    println("Invalid option");
}
println("Thank you for using our calculator. Press any key to end the program")
{/KOTLIN_CONSOLE}

In the previous lesson, Solved tasks for Kotlin lesson 3, we discussed Kotlin data types in details. We need to react somehow to different situations if we want to program something. It may be, for example, a value entered by the user, according to which we would like to change the running of the program. We metaphorically say that the program branches, and for branching we use conditions. We will pay attention to those in today's article. We're going to create a program which calculates square roots, and which we're going to use to improve our calculator.

Conditions

In Kotlin, conditions are exactly the same as in all C-like languages, either way, I will explain everything for beginners. Advanced programmers will probably be bored for a moment

We write conditions using the if keyword, which is followed by a logical expression. If the expression is true, the following statement will be executed. If it's not true, the following statement will be skipped, and the program will continue with the next statement. Let's try it out:

if (15 > 5)
    println("True")
println("The program continues here...")

The output:

True
The program continues here...

If the condition is true, the command which writes text to the console will be executed. In both cases the program continues. Of course, a variable can also be part of the expression:

println("Enter a number")
val a = readLine()!!.toInt()
if (a > 5)
    println("The number you entered is greater than 5!")
println("Thanks for the input!")

Operators

Let's look at the relational operators which can be used in expressions:

We use the == operator for equality to avoid confusing it with a normal assignment to a variable (the = operator). If we want to negate an expression, we enclose it in parentheses and write an exclamation mark before the expression. If we want to execute more than one command, we have to insert commands into a block of curly brackets:

import kotlin.math.*
println("Enter some number and I'll calculate a square root:")
val a = readLine()!!.toInt()
if (a > 0) {
    println("The number you entered is greater than 0, so I can calculate it!")
    val root = sqrt(a.toDouble());
    println("The square root of $a is $root")
}
println("Thanks for the input")

The output:

Enter some number and I'll calculate a square root:
144
You've entered a number greater than 0, I can calculate it!
Square root of 144 is 12.0
Thanks for the input

The program retrieves a number from the user, and if it's greater than 0, it calculates the square root. The sqrt() function returns the square root as Double but we have to convert our input to Double first. To be able to use these math function in Kotlin, it's neccessary to import kotlin.math, see the first line of the source code above. It'd be nice if our program warned us if we entered a negative number. With what we know up until now, we could write something like this:

import kotlin.math.*
println("Enter some number and I'll calculate a square root:")
val a = readLine()!!.toInt()
if (a > 0) {
    println("The number you entered is greater than 0, so I can calculate it!")
    val root = sqrt(a.toDouble());
    println("The square root of $a is $root")
}

if (a <= 0) {
    println("I can't calculate the square root of a negative number!")
println("Thanks for the input")

We must keep in mind the case where a == 0, but also when it's less than 0. The code can be greatly simplified by using the `else keyword which executes the following statement or block of statements if the condition was not true:

import kotlin.math.*
println("Enter some number and I'll calculate a square root:")
val a = readLine()!!.toInt()
if (a > 0) {
    println("The number you entered is greater than 0, so I can calculate it!")
    val root = sqrt(a.toDouble());
    println("The square root of $a is $root")
} else
    println("I can't calculate the square root of a negative number!")
println("Thanks for the input")

The code is much clearer, and we don't have to make up the negate condition which could be very difficult with complex conditions sometimes. In the case of multiple commands, there would be a { } block again after the else keyword.

We also use the else keyword when we need to modify the variable used in a condition so we can't evaluate it later again. The program remembers that the condition didn't apply and it'll move to the else branch. Let's look at an example: Consider a number which value will be either 0 or 1 and we'll be asked to swap those values (if there is 0, we'll put a 1 there, and the other way around). Naively, we could write a code like this:

int a = 0 // the variable is initialized with a value of 0

if (a == 0) { // if the value is 0, we change its value to 1
    a = 1
}
if (a == 1) { // if the value is 1, we change its value to 0
    a = 0
}

println(a);

It doesn't work, does it? Let's take a closer look at the program. At the very beginning, a contains the value 0, the first condition is undoubtedly fulfilled and it assigns 1 into a. Well, suddenly, the second condition becomes true as well. What should we do? When we swap the conditions, we'll have the same problem with 1. Now, how do we solve this? You guessed it, using else!

int a = 0 // the variable is initialized with a value of 0

if (a == 0) { // if the value is 0, we change its value to 1
    a = 1
} else { // if the value is 1, we change its value to 0
    a = 0
}

println(a)

Conditions can be composed by using two basic logical operators:

Let's take a look at the example:

println("Enter a number between 10-20:")
val a = readLine()!!.toInt()
if (a >= 10 && a <= 20) {
    println("The condition has been met.")
} else {
    println("You did it wrong.")
}

Of course operators can be combined with parentheses:

println("Enter a number between 10-20 or 30-40:")
val a = readLine()!!.toInt()
if (((a >= 10) && (a <= 20)) || ((a >=30) && (a <= 40))) {
    println("The condition has been met.")
} else {
    println("You did it wrong.")
}

When

When is a construct taken from the C language, like most of Kotlin's syntax. It allows us to relatively simplify the usage of if-else command sequences. Let's remember our calculator from the first lesson, which had read two numbers and calculated all 4 operations. Now, we want to choose the operation. Without the when, we would write the code like this:

println("Welcome to our calculator")
println("Enter the first number:")
val a = readLine()!!.toDouble()
println("Enter the second number:");
val b = readLine()!!.toDouble()
println("Choose one of the following operations:")
println("1 - addition")
println("2 - subtraction")
println("3 - multiplication")
println("4 - division")
val choice = readLine()!!.toInt()
var result = 0.0
if (choice == 1) {
    result = a + b
} else if (choice == 2) {
    result = a - b
} else if (choice == 3) {
    result = a * b
} else if (choice == 4) {
    result = a / b
}
if ((choice > 0) && (choice < 5)) {
    println("result: $result")
} else {
    println("Invalid choice")
}
println("Thank you for using our calculator.")

The output:

Welcome to our calculator
Enter the first number:
3.14
Enter the second number:
2.72
Choose one of the following operations:
1 - addition
2 - subtraction
3 - multiplication
4 - division
2
result: 0.42
Thank you for using our calculator. Press any key to end the program.

Notice that we've declared the variable result at the beginning, so we could later assign something to it. If we declared it at every assignment, Kotlin would not compile the code and report an error since the variable would be already declared. A variable can be declared (initialized in memory) only once. Unfortunately, Kotlin is not able to tell whether a value has been already assigned to the result variable. It would report an error on the line where we're printing to the console because Kotlin doesn't like the fact that the variable being printed is not guaranteed to contain a value. For this reason, we have to assign zero to the result variable at the beginning. Another trick is validating the user's choice. The program should still work the same even without all the elses (but why keep on asking if we already have a result).

Now here's the same program using when:

println("Welcome to our calculator")
println("Enter the first number:");
val a = readLine()!!.toDouble()
println("Enter the second number:")
val b = readLine()!!.toDouble()
println("Choose one of the following operations:")
println("1 - addition")
println("2 - subtraction")
println("3 - multiplication")
println("4 - division")
val choice = readLine()!!.toInt()
var result = 0.0
when (choice) {
    1 -> result = a + b
    2 -> result = a - b
    3 -> result = a * b
    4 -> result = a / b
}
if ((choice > 0) && (choice < 5)) {
    println("Result: $result")
} else {
    println("Invalid choice")
}
println("Thank you for using our calculator.")

As you can see, the code is a bit clearer now. If we needed to execute multiple commands in any branch of the when, we'd write them into the { } block. In the original switch of the C language, individual branches had to be closed using the break command. In Kotlin, this is not the case. Next to the x -> option, the when structure can also contain else -> which will be executed if neither of the cases applied. It's up to you whether you use when or not. Generally, it's useful only for a larger amount of branches and you always could replace it with an if-else sequence. Of course, when can be used for String variables as well.

That is all for today. In the next lesson, Solved tasks for Kotlin lesson 4, we'll take a look at arrays and loops, i.e. finish up with the absolute basics of the Kotlin language. Look forward to it

In the following exercise, Solved tasks for Kotlin lesson 4, we're gonna practice our knowledge from previous lessons.

Did you have a problem with anything? Download the sample application below and compare it with your project, you will find the error easily.