1. The if else statement
Programs would be of little use if they did the same thing regardless of how external circumstances change. A program needs to be able to adapt to different situations and do one thing in some cases and something else in others.
In Kotlin, this is implemented using a conditional statement—a special keyword that allows you to execute different blocks of commands depending on whether the condition is true.
A conditional statement consists of three parts: a condition, command 1, and command 2. If the condition is true, command 1 is executed; otherwise command 2 is executed. The commands are never executed at the same time. The general form of this statement is:
if (condition)
command 1
else
command 2if-else
if translates as “if”, and else as “otherwise”. In plain language, it can be written like this:
If condition is true, then
execute command1
else
execute command2if-else in plain language
Examples:
// The following text will be printed: You are still a child
val age = 17
if (age < 18)
println("You are still a child")
else
println("You are already an adult")
// The following text will be printed: Warm
val temperature = 5
if (temperature < 0)
println("It's freezing outside")
else
println("Warm")
// The following text will be printed: You can apply for a driver's license
val age = 18
if (age == 18)
println("You can apply for a driver's license")
else
println("You can save up for a car")
2. A block of commands
If, when a condition is met (or not met), you want your program to execute several commands, you can combine them into a block of commands.
To combine commands into a block, you need to “wrap” them in curly braces. Here is what it looks like in the general case:
{
command1
command2
command3
}There can be as many commands in a block as you want. Even zero.
Examples combining the if-else statement and a command block:
// The following text will be printed: You are still a child. Don't argue with adults
val age = 17
if (age < 18)
{
println("You are still a child")
println("Don't argue with adults")
}
else
{
println("You are already an adult")
println("Kids these days")
}
// The following text will be printed: Warm
val temperature = 5
if (temperature < 0)
{
println("It's freezing outside")
println("Put on a hat")
}
else
println("Warm")
// An empty block of commands will be executed. Nothing will be printed.
val age = 21
if (age == 18)
println("You can get a driver's license")
else
{
}
3. Shortened syntax of the if statement
If the code in the else block is missing, it can be omitted. Then the if statement will look like this:
if (condition)
{
command1
}if
Example:
val age = 19
if (age >= 18)
{
println("You are an adult!")
}
How does it work?
- If the condition is true, the code block is executed. If it is false, nothing happens—the program continues.
- The else block exists in the program, but it is empty (there are no commands between the curly braces). You can simply remove it: nothing will change in the program.
4. Combining conditions: else if
Sometimes you need to check not just “yes/no”, but choose from several options.
This is where the else if chain helps:
if (condition1)
{
// Executed if condition1 is true
}
else if (condition2)
{
// Executed if condition2 is true and condition1 is false
}
else
{
// Executed if none of the conditions above matched
}
Example:
val hour = 13
if (hour < 12)
{
println("Good morning!")
}
else if (hour < 18)
{
println("Good afternoon!")
}
else
{
println("Good evening!")
}
What is happening here?
- If the time is before 12, it's morning.
- If it's from 12 to 18, it's afternoon.
- If it's later, it's evening.
- The program chooses only ONE option! As soon as a condition matches, the others are not even checked.
5. Schematic illustration: branching flowchart
Here is what it looks like as a flowchart:
And if there are multiple conditions:
This really resembles forks in tabletop RPGs: go left—one thing, go right—another, go straight—something else!
6. What conditions can you check?
Inside the parentheses after if there must be a LOGICAL expression—the result of which is “true” true or “false” false.
Main comparison operators:
| Operator | Meaning | Example |
|---|---|---|
|
Equal | |
|
Not equal | |
|
Greater than | |
|
Less than | |
|
Greater than or equal | |
|
Less than or equal | |
Examples of conditions:
if (number == 10)
if (age != 18)
if (temperature > 36)
if (balance <= 0)
Note: = (one sign) is assignment. == (two signs) is comparison. This is one of the most popular mistakes among beginners! If you accidentally write:
if (x = 5) // ERROR!
the compiler will scold you—and it will be absolutely right.
7. Working with strings in conditions
Comparison using == in Kotlin compares values (contents). Therefore, strings can be compared with == (unlike Java). At the same time, you need to be more careful with floating-point numbers (Double/Float): due to rounding errors, comparison via == can produce unexpected results.
We will talk more about the subtleties of string comparison later. And how to compare strings correctly, you can see from the example below:
val password = readln()
if (password == "qwerty")
{
println("You are logged in!")
}
8. Features and nuances:
Curly braces: needed or not?
If after if or else there is only one command, curly braces can be omitted:
if (age > 18)
println("You are older than 18!")
// But it is safer to ALWAYS write them:
if (age > 18)
{
println("You are older than 18!")
}
Tip: always write braces. This protects you from a lot of mistakes, especially when extending code.
Logic errors: be careful with conditions!
If you write this:
if (age > 18)
println("You are older than 18!")
println("Welcome!")
then the compiler will read this code like this:
if (age > 18)
{
println("You are older than 18!")
}
println("Welcome!")
The second line will always be executed—braces are needed so that both lines are inside the “branch”.
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