# Basics

## **Literals**

* **Integer numbers**
* **Characters**
* **Strings**

## **Variables**

* `val` (for *value*) declares an **immutable variable** (just a **named value** or a **constant**), which cannot be changed after it has been initialized (this is actually not entirely true).

It is always possible to change the internal state of a `val` variable: while it is prohibited to reassign the variable, its content can be modified in some other ways.

```kotlin
// list creation
val myMutableList = mutableListOf(1, 2, 3, 4, 5)
// trying to update the list
myMutableList = mutableListOf(1, 2, 3, 4, 5, 6) // error line
// adding a new element
myMutableList.add(6)   // it works
// printing the list
println(myMutableList) // [1, 2, 3, 4, 5, 6]
```

* `var` (for *variable*) declares a **mutable variable**, which can be changed (as many times as needed).

{% hint style="info" %}
The more mutable variables (those declared with the keyword `var`) in your code, the harder it is to read. 

Remember, immutable variables (those declared with the keyword `val`) help write more readable code.
{% endhint %}

* `const` modifier is used before the `val` keyword to declare a compile-time constant.

```kotlin
const val CONST_INT = 127
const val CONST_DOUBLE = 3.14
const val CONST_CHAR = 'c'
const val CONST_STRING = "I am constant"
const val CONST_ARRAY = arrayOf(1, 2, 3) 
                                // error: only primitives and strings are allowed

fun main() {
    const val MY_INT_2 = 2048 
            // error: Modifier 'const' is not applicable to 'local variable'
}
```

## Data types

Kotlin determines the data types of variables automatically. This mechanism is called **type inference**.

The type of a variable can also be specified when declaring it:

```kotlin
val/var identifier: Type = initialization
```

## Functions

All functions **return** a result, even the `println` function.

```kotlin
val result = println("text")
println(result) // kotlin.Unit
```

The result is a special value called `Unit`, which practically means `no result`. When your function returns nothing, it means it returns `Unit`, you can think of it as `Void`.