1. Basic syntax of a nested if
In real life, you rarely encounter a situation where a decision is made based on just one criterion. Usually, you need to check several conditions at once, and they often depend on each other. For example, to get into a concert, it’s not enough to simply be an adult — sometimes you also need to be on the guest list. This is where so-called multi-level logic comes in, and nested if operators help us do it.
Nested conditions are when one if lives inside another. Imagine a box inside a box: if the first box opens (the condition is met), then you look at what’s inside and, possibly, open the second one. From the perspective of code and logic, this is a very powerful tool.
Let’s start with something simple:
if (conditionA)
{
// actions if conditionA is true
if (conditionB)
{
// actions if both conditions are true
}
}if inside another
if
If this reminds you of nesting dolls, you’ve got it exactly right! The nested if inside the outer one will run only if the outer one triggers.
Example: getting into a club
Let’s extend our training app where the user previously entered their name and age. Now we’ll add a second level of checking: suppose that to get into a private club, you must be older than 21 and have an invitation code.
fun main() {
print("Enter your name: ")
val name = readln()
print("Enter your age: ")
val age = readln().toInt()
if (age >= 21) {
print("Enter invitation code: ")
val code = readln()
if (code == "VIP2026") {
println("Welcome, $name, to the VIP club!")
} else {
println("Sorry, invalid code. Access denied.")
}
} else {
println("Sorry, entry is only for ages 21+!")
}
}
Note: the inner if checks the code only if the age has already passed the check. This saves resources (and doesn’t annoy the bouncer).
2. How many nesting levels are acceptable?
Technically — as many as you want. But in practice — the more nested levels there are, the harder it is to read and maintain such code. In real projects, nesting deeper than 2–3 levels is rarely used. If you end up with “staircases” of 4 or more nested if statements, it’s worth thinking: maybe there’s a way to simplify the logic or move part of the checks into separate functions (but that’s a topic for a separate lecture!).
3. Nested if and else: traps and nuances
Beginner mistake: sloppy formatting of curly braces.
When you write nested conditions, especially when using else, Kotlin strictly follows the syntax rules. If you don’t add curly braces {}, then only the first line after if or else belongs to it. This is a common source of frustrating bugs.
Example with a potential mistake:
if (age >= 21)
if (code == "VIP2026")
println("Welcome!");
else
println("Invalid code.");Many people expect else to belong to the age check, but in fact it belongs to the nearest preceding if (the invitation-code check). Because of this, messages may be shown differently than expected. It’s better to always use braces — even if there’s only one line inside!
To avoid confusion, I’ll show it again. The compiler interprets the code above like this:
if (age >= 21)
{
if (code == "VIP2026")
println("Welcome!");
else
println("Invalid code.");
}Important! Once again: If curly braces are not placed in an if-else construct, the else belongs to the previous (closest) if.
4. Alternatives: else if in nested conditions
Sometimes inside an outer if, you want not only to check one additional condition, but also to provide an alternative. For example, if the invitation code doesn’t work, you can offer to buy a ticket. This is done with a nested else if/else construct:
fun main() {
print("Enter your age: ")
val age = readln().toInt()
if (age >= 21) {
print("Enter invitation code: ")
val code = readln()
if (code == "VIP2026") {
println("Welcome to the club!")
} else if (code == "GUEST") {
println("Guest mode: access only until 23:00.")
} else {
println("Sorry, access is not possible.")
}
} else {
println("Entry is only for ages 21+.")
}
}
In this example, we made a nested choice among three scenarios: VIP code, guest code, and rejection.
5. Practice: discount calculator with multi-level logic
Let’s write an app like this: the user gets a store discount based on age and whether they have a club card.
- If the user’s age is greater than or equal to 60 — the discount is 15%.
- If the age is lower, but the user has a club card — the discount is 7%.
- In all other cases, there is no discount.
How do we structure this?
fun main() {
print("Enter your age: ")
val age = readln().toInt()
var discount = 0.0
if (age >= 60) {
discount = 0.15
} else {
print("Do you have a club card (yes/no)? ")
val hasCard = readln()
if (hasCard == "yes") {
discount = 0.07
}
}
println("Your discount: ${discount * 100}%")
}
Here we used a nested if inside the else branch. If the age doesn’t qualify, only then do we ask about the card.
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