Maps and key-based lookup

A teacher wants to retrieve a mark by student name rather than by position. A Map associates unique keys with values of a common type. Looking up an absent key returns a nullable value, which allows an explicit missing-student path.

Kotlin Map reads key-value associations, and MutableMap permits updates. A missing key lookup returns null. Keys should represent the intended identity; use sortedKeys or an explicitly sorted key list when report order is part of the contract. Assigning null does not universally mean removal as it does in some languages.

Connect this to nullable types and safe calls and data classes, equality and copying.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun main() {
  val scores = mutableMapOf("Diya" to 80, "Saira" to 90)
  scores["Diya"] = 85
  println(scores["Diya"])
  println(scores["Noor"] == null)
  scores.remove("Saira")
  println(scores.keys.sorted())
}

Expected output

85
true
[Diya]

Follow the execution

Diya’s value becomes 85 and Noor has no entry. Remove explicitly deletes Saira. The sorted remaining key list prints Diya, independently of any general map implementation’s iteration policy.

Try this in the Kotlin compiler by pasting the complete program.

Your task: How should this program delete Saira’s entry?

Kotlin/JVM · complete program

fun main() {
  val scores = mutableMapOf("Diya" to 80, "Saira" to 90)
  scores["Diya"] = 85
  println(scores["Diya"])
  println(scores["Noor"] == null)
  scores.remove("Saira")
  println(scores.keys.sorted())
}

Worked answer

Call scores.remove("Saira"). The removal operation makes the intended action explicit.

When values are themselves nullable, a null lookup can mean either a missing key or a stored null; containsKey distinguishes them. Equal names are not always equal student identities.

Call scores.remove("Saira"). The removal operation makes the intended action explicit. When values are themselves nullable, a null lookup can mean either a missing key or a stored null; containsKey distinguishes them.

Further reading: official Kotlin documentation for this topic.