Kotlin Lists

A list is an ordered collection of items. Items sit in a specific sequence, and each item has a numbered position called an index. Kotlin provides two kinds of lists: read-only (List) and editable (MutableList). This topic covers read-only lists.

Creating a List

val fruits = listOf("Apple", "Mango", "Banana", "Grape")
val numbers = listOf(10, 20, 30, 40, 50)
val mixed = listOf(1, "hello", true, 3.14)   // mixed types (not recommended)

List Structure

listOf("Apple", "Mango", "Banana", "Grape")

Index:    0        1        2        3
        ┌──────┬───────┬────────┬───────┐
        │Apple │ Mango │Banana  │ Grape │
        └──────┴───────┴────────┴───────┘

Accessing Elements

val fruits = listOf("Apple", "Mango", "Banana", "Grape")

println(fruits[0])          // Apple
println(fruits[2])          // Banana
println(fruits.first())     // Apple
println(fruits.last())      // Grape
println(fruits.get(1))      // Mango  (same as fruits[1])

Safe Access

Accessing an index that does not exist crashes the program. Use getOrNull to avoid this:

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

println(items.getOrNull(1))    // B
println(items.getOrNull(10))   // null (no crash)
println(items.getOrNull(10) ?: "Not found")  // Not found

List Properties and Functions

val scores = listOf(85, 92, 78, 95, 60, 88)

println(scores.size)         // 6
println(scores.count())      // 6
println(scores.isEmpty())    // false
println(scores.isNotEmpty()) // true
println(scores.contains(92)) // true
println(92 in scores)        // true (same as contains)
println(scores.indexOf(78))  // 2 (position of 78)

Aggregation

val numbers = listOf(4, 8, 15, 16, 23, 42)

println(numbers.sum())       // 108
println(numbers.average())   // 18.0
println(numbers.min())       // 4
println(numbers.max())       // 42
println(numbers.count { it > 10 })  // 4 (items greater than 10)

Transforming a List

val names = listOf("alice", "bob", "charlie")

val upper = names.map { it.uppercase() }
println(upper)    // [ALICE, BOB, CHARLIE]

val long = names.filter { it.length > 3 }
println(long)     // [alice, charlie]

val sorted = names.sorted()
println(sorted)   // [alice, bob, charlie]

val reversed = names.reversed()
println(reversed) // [charlie, bob, alice]

Iterating a List

val cities = listOf("Delhi", "Mumbai", "Pune", "Jaipur")

for (city in cities) {
    println(city)
}

cities.forEach { println(it) }  // same result, using lambda

cities.forEachIndexed { index, city ->
    println("$index: $city")
}

Sublist

val letters = listOf('A', 'B', 'C', 'D', 'E', 'F')

println(letters.subList(1, 4))   // [B, C, D]  (from index 1 to 3)
println(letters.take(3))         // [A, B, C]
println(letters.drop(3))         // [D, E, F]
println(letters.takeLast(2))     // [E, F]

Practical Example: Exam Scores Report

fun main() {
    val scores = listOf(72, 88, 45, 95, 61, 83, 52, 79)

    val passing  = scores.filter { it >= 60 }
    val failing  = scores.filter { it < 60 }
    val topScore = scores.max()
    val average  = scores.average()

    println("All Scores  : $scores")
    println("Passing     : $passing")
    println("Failing     : $failing")
    println("Top Score   : $topScore")
    println("Average     : ${"%.1f".format(average)}")
    println("Pass Rate   : ${passing.size * 100 / scores.size}%")
}

Output:

All Scores  : [72, 88, 45, 95, 61, 83, 52, 79]
Passing     : [72, 88, 95, 61, 83, 79]
Failing     : [45, 52]
Top Score   : 95
Average     : 71.9
Pass Rate   : 75%