Array Initialization in Kotlin

Introduction

Array Initialization is a fundamental concept every Kotlin developer should understand. Arrays store a fixed number of elements in memory with fast index-based access. They are useful when size is known upfront or when interoping with Java APIs.

The Array(size) { index -> value } syntax creates and initializes an array. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• The Array(size) { index -> value } syntax creates and initializes an array.
• Each index can produce a different value using a lambda.
• This is useful for computed initial values.

Syntax

Array(5) { it * 2 }

Array Initialization in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val squares = Array(5) { index -> index * index }
    for (value in squares) {
        print("$value ")
    }
}

Sample Output

0 1 4 9 16

When to use

Use arrays when you need fixed-size storage, primitive arrays without boxing overhead, or compatibility with Java vararg APIs.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val squares = Array(5) { index -> index * index } assigns or updates a value used later in the program.

3. The Array(size) { index -> value } syntax creates and initializes an array.

4. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Prefer listOf or arrayOf factory functions over manual size allocation when possible.
• Use typed arrays (IntArray) for numeric data to avoid boxing overhead.
• Bounds-check mentally: invalid indices throw IndexOutOfBoundsException.

Common Mistakes

• Confusing Array<Int> (boxed) with IntArray (unboxed primitives).
• Assuming arrays are dynamically resizable like ArrayList — they are fixed size.
• Using reference equality (===) when comparing array contents — use contentEquals.

Key Points

• The Array(size) { index -> value } syntax creates and initializes an array.
• Each index can produce a different value using a lambda.
• This is useful for computed initial values.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.