In Kotlin, you can loop through collections in several common ways depending on whether you need the element, the index, or both.

1. Using for

Use for when you want a simple, readable loop over elements.

val names = listOf("Alice", "Bob", "Charlie")

for (name in names) {
    println(name)
}

Output:

Alice
Bob
Charlie

This works with many Kotlin types, including:

val numbers = arrayOf(1, 2, 3)

for (number in numbers) {
    println(number)
}

---

2. Using forEach

Use forEach when you prefer a functional style.

val names = listOf("Alice", "Bob", "Charlie")

names.forEach { name ->
    println(name)
}

If the lambda has only one parameter, you can use it:

names.forEach {
    println(it)
}

forEach is useful for concise operations, but a regular for loop is often clearer if you need break, continue, or more complex control flow.

---

3. Looping with indices

Use indices when you need the index of each element.

val names = listOf("Alice", "Bob", "Charlie")

for (i in names.indices) {
    println("Index $i: ${names[i]}")
}

Output:

Index 0: Alice
Index 1: Bob
Index 2: Charlie

indices gives the valid index range for the collection, such as 0..lastIndex.

---

4. Using withIndex

If you need both the index and the value, withIndex() is often cleaner than indexing manually.

val names = listOf("Alice", "Bob", "Charlie")

for ((index, name) in names.withIndex()) {
    println("Index $index: $name")
}

---

5. Using forEachIndexed

The forEach equivalent for index + value is forEachIndexed.

val names = listOf("Alice", "Bob", "Charlie")

names.forEachIndexed { index, name ->
    println("Index $index: $name")
}

---

Summary

val items = listOf("A", "B", "C")

// Element only
for (item in items) {
    println(item)
}

// Element only, functional style
items.forEach { item ->
    println(item)
}

// Index only / index-based access
for (i in items.indices) {
    println("items[$i] = ${items[i]}")
}

// Index and value
for ((index, item) in items.withIndex()) {
    println("$index -> $item")
}

// Index and value, functional style
items.forEachIndexed { index, item ->
    println("$index -> $item")
}

Use:

• for (item in items) for simple iteration
• items.forEach { ... } for concise functional-style iteration
• items.indices when you need index-based access
• withIndex() or forEachIndexed when you need both index and value