Kotlin Maps

A map stores data as key-value pairs. Each key is unique, and each key maps to exactly one value. Think of a map like a dictionary: each word (key) has its own definition (value). Maps are ideal when you need to look up data by a name or identifier.

Creating a Map

val ages = mapOf("Alice" to 30, "Bob" to 25, "Carol" to 28)

val scores = mapOf(
    "Math"    to 92,
    "English" to 88,
    "Science" to 95
)

Map Structure

mapOf("Alice" to 30, "Bob" to 25, "Carol" to 28)

  Key     │ Value
 ─────────┼───────
  "Alice" │  30
  "Bob"   │  25
  "Carol" │  28

Access by key: map["Alice"] → 30

Accessing Values

val capitals = mapOf("India" to "New Delhi", "Japan" to "Tokyo", "France" to "Paris")

println(capitals["India"])         // New Delhi
println(capitals["Japan"])         // Tokyo
println(capitals["Germany"])       // null (key not found)

println(capitals.getValue("France"))        // Paris
// capitals.getValue("Germany")             // throws NoSuchElementException

println(capitals.getOrDefault("Germany", "Unknown"))  // Unknown
println(capitals.getOrElse("Germany") { "Not in map" }) // Not in map

Map Properties

val map = mapOf("a" to 1, "b" to 2, "c" to 3)

println(map.size)            // 3
println(map.isEmpty())       // false
println(map.containsKey("b"))   // true
println(map.containsValue(2))   // true
println("a" in map)          // true (checks key)

Iterating a Map

val prices = mapOf("Apple" to 80, "Mango" to 120, "Banana" to 40)

// Iterate key-value pairs
for ((item, price) in prices) {
    println("$item costs ₹$price")
}

// Keys only
for (key in prices.keys) { println(key) }

// Values only
for (value in prices.values) { println(value) }

Mutable Map

val inventory = mutableMapOf("Pens" to 50, "Notebooks" to 30)

// Add new entry
inventory["Stapler"] = 10

// Update existing
inventory["Pens"] = 45

// Remove
inventory.remove("Notebooks")

println(inventory)   // {Pens=45, Stapler=10}

Useful Map Functions

val marks = mapOf("Alice" to 88, "Bob" to 72, "Carol" to 95, "Dan" to 60)

// Filter entries where value meets condition
val toppers = marks.filter { it.value >= 85 }
println(toppers)   // {Alice=88, Carol=95}

// Transform values
val scaled = marks.mapValues { it.value * 1.1 }
println(scaled)    // {Alice=96.8, Bob=79.2, Carol=104.5, Dan=66.0}

// Transform keys
val keysUpper = marks.mapKeys { it.key.uppercase() }
println(keysUpper) // {ALICE=88, BOB=72, CAROL=95, DAN=60}

Practical Example: Word Frequency Counter

fun main() {
    val sentence = "kotlin is fun kotlin is easy kotlin is great"
    val words = sentence.split(" ")

    val frequency = mutableMapOf()
    for (word in words) {
        frequency[word] = (frequency[word] ?: 0) + 1
    }

    println("Word Frequencies:")
    frequency.entries.sortedByDescending { it.value }.forEach {
        println("  ${it.key.padEnd(8)} → ${it.value} times")
    }
}

Output:

Word Frequencies:
  kotlin   → 3 times
  is       → 3 times
  fun      → 1 times
  easy     → 1 times
  great    → 1 times