Map

A map is a collection of key/value pairs, where each key is unique and is used to retrieve the corresponding value. For creating maps, there are functions mapOf() and mutableMapOf(). Using the to infix function makes initialization less noisy. A read-only view of a mutable map can be obtained by casting it to Map.

const val POINTS_X_PASS: Int = 15
val EZPassAccounts: MutableMap<Int, Int> = mutableMapOf(1 to 100, 2 to 100, 3 to 100)   // 1
val EZPassReport: Map<Int, Int> = EZPassAccounts                                        // 2

fun updatePointsCredit(accountId: Int) {
    if (EZPassAccounts.containsKey(accountId)) {                                        // 3
        println("Updating $accountId...")                                               
        EZPassAccounts[accountId] = EZPassAccounts.getValue(accountId) + POINTS_X_PASS  // 4
    } else {
        println("Error: Trying to update a non-existing account (id: $accountId)")
    } 
}

fun accountsReport() {
    println("EZ-Pass report:")
    EZPassReport.forEach {                                                              // 5
        k, v -> println("ID $k: credit $v")
    }
}

fun main() {
    accountsReport()                                                                    // 6
    updatePointsCredit(1)                                                               // 7
    updatePointsCredit(1)                                                               
    updatePointsCredit(5)                                                               // 8 
    accountsReport()                                                                    // 9
}

1. Creates a mutable Map.
2. Creates a read-only view of the Map.
3. Checks if the Map's key exists.
4. Reads the corresponding value and increments it with a constant value.
5. Iterates immutable Map and prints key/value pairs.
6. Reads the account points balance, before updates.
7. Updates an existing account two times.
8. Tries to update a non-existing account: prints an error message.
9. Reads the account points balance, after updates.