Map operations

Learn map management with put, get and remove.

Map operations: put, get, remove

On a mutable map put(k, v) adds a new pair or updates an existing key. get(k) returns the value as V?; if the key is missing it returns null. remove(k) takes the key out.

val ages = mutableMapOf("Ava" to 25)
ages.put("Liam", 30)
ages["Nora"] = 28
ages.get("Ava")   // 25
ages.get("missing")  // null

The ages[k] = v syntax does the same job as put; it is just shorter to write.

getOrElse and getOrDefault

Sometimes you want a default value instead of null when a key is missing. getOrElse(k) { default } produces the default with a lambda; getOrDefault(k, default) returns a fixed value.

val ages = mapOf("Ava" to 25)
ages.getOrElse("missing") { 0 }    // 0
ages.getOrDefault("missing", 0)    // 0

These helpers fold the later null check into one line and make the code more readable.

Exercise Put a pair and print a value

Start with mutableMapOf("Ava" to 25, "Liam" to 30). Add a new pair with put("Nora", 28), then print Ava's age with println(ages.get("Ava")).

Show example solution

fun main() {
    val ages = mutableMapOf("Ava" to 25, "Liam" to 30)
    ages.put("Nora", 28)
    println(ages.get("Ava"))
}

Test yourself

What does map.get("missing") return?
- -1
- null
- Throws an exception
- Unit
Show answer

null — On a map, get returns null when the key is missing; no exception.
Match method to purpose

put(k, v)

Add/update a key-value pair
get(k)

V? (may be null)
remove(k)

Remove a key
keys

All keys

Fill in the blank: put a pair

ages.____("Nora", 28)

Show answer

ages.put("Nora", 28)

Next lesson: Map iteration

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