Understanding Maps in Kotlin: A Comprehensive Guide to Key-Value Pairs

Maps are an essential part of Kotlin. The map allows us to store data in a structured way. A map data is stored in Key-Value pair format, and each key should be unique and connected to a specific value. In today’s tutorial, we will explore Map in depth. Map’s main purpose is to retrieve values based on their Key.

Basic types of Maps:

Immutable Map: Immutable maps cannot be modified after declaration. Once you define an immutable map you cannot perform a CURD operation on it.

val fruits = mapOf("apple" to 1, "banana" to 2, "orange" to 3)

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val fruits = mapOf("apple" to 1, "banana" to 2, "orange" to 3)
Mutable Map: Mutable maps can be modified.

val mutableFruits = mutableMapOf("apple" to 1, "banana" to 2)

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val mutableFruits = mutableMapOf("apple" to 1, "banana" to 2)

Adding duplicate items in Map:

If we intentionally add duplicate items to the Map, it will not add the same item with the same key. But we can assign the same value to a unique key. See the mentioned example for more clarity.

fun main() {
    
   val mutableFruits = mutableMapOf("apple" to 1, "banana" to 2, "apple" to 1, "mango" to 1)
   
 // "apple" is the KEY and "1" is its value.
 // "banana" is the KEY and "2" is its value.
 // "mango" is the KEY and "1" is its value.

   println(mutableFruits)

}

// Output
{apple=1, banana=2, mango=1}

fun main() {

val mutableFruits = mutableMapOf("apple" to 1, "banana" to 2, "apple" to 1, "mango" to 1)

// "apple" is the KEY and "1" is its value.

// "banana" is the KEY and "2" is its value.

// "mango" is the KEY and "1" is its value.

println(mutableFruits)

}

// Output

{apple=1, banana=2, mango=1}

Understanding Maps in Kotlin: A Comprehensive Guide to Key-Value Pairs

String Key Int Value Map.

val stringToIntMap = mutableMapOf("apple" to 1, "banana" to 2, "cherry" to 3)

// Add a new entry
stringToIntMap["date"] = 4

// Update an existing entry
stringToIntMap["apple"] = 10

// Remove an entry
stringToIntMap.remove("banana")

// Print the map
println(stringToIntMap) 

// Output: {apple=10, cherry=3, date=4}

val stringToIntMap = mutableMapOf("apple" to 1, "banana" to 2, "cherry" to 3)

// Add a new entry

stringToIntMap["date"] = 4

// Update an existing entry

stringToIntMap["apple"] = 10

// Remove an entry

stringToIntMap.remove("banana")

// Print the map

println(stringToIntMap)

// Output: {apple=10, cherry=3, date=4}

Int Key String Value Map.

val intToStringMap = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

// Add a new entry
intToStringMap[4] = "four"

// Update an existing entry
intToStringMap[2] = "TWO"

// Remove an entry
intToStringMap.remove(3)

// Print the map
println(intToStringMap) 


// Output: {1=one, 2=TWO, 4=four}

val intToStringMap = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

// Add a new entry

intToStringMap[4] = "four"

// Update an existing entry

intToStringMap[2] = "TWO"

// Remove an entry

intToStringMap.remove(3)

// Print the map

println(intToStringMap)

// Output: {1=one, 2=TWO, 4=four}

String Key Boolean Value Map.

val stringToBooleanMap = mutableMapOf("isActive" to true, "isVerified" to false)

// Add a new entry
stringToBooleanMap["isSubscribed"] = true

// Update an existing entry
stringToBooleanMap["isActive"] = false

// Remove an entry
stringToBooleanMap.remove("isVerified")

// Print the map
println(stringToBooleanMap) 


// Output: {isActive=false, isSubscribed=true}

val stringToBooleanMap = mutableMapOf("isActive" to true, "isVerified" to false)

// Add a new entry

stringToBooleanMap["isSubscribed"] = true

// Update an existing entry

stringToBooleanMap["isActive"] = false

// Remove an entry

stringToBooleanMap.remove("isVerified")

// Print the map

println(stringToBooleanMap)

// Output: {isActive=false, isSubscribed=true}

Double Key String Value Map.

val doubleToStringMap = mutableMapOf(1.1 to "Low", 2.5 to "Medium")

// Add a new entry
doubleToStringMap[3.9] = "High"

// Update an existing entry
doubleToStringMap[1.1] = "Very Low"

// Remove an entry
doubleToStringMap.remove(2.5)

// Print the map
println(doubleToStringMap) 


// Output: {1.1=Very Low, 3.9=High}

val doubleToStringMap = mutableMapOf(1.1 to "Low", 2.5 to "Medium")

// Add a new entry

doubleToStringMap[3.9] = "High"

// Update an existing entry

doubleToStringMap[1.1] = "Very Low"

// Remove an entry

doubleToStringMap.remove(2.5)

// Print the map

println(doubleToStringMap)

// Output: {1.1=Very Low, 3.9=High}

String Key Double Value Map.

val intToPairMap = mutableMapOf(1 to ("ProductA" to 29.99), 2 to ("ProductB" to 39.99))

// Add a new entry
intToPairMap[3] = "ProductC" to 49.99

// Update an existing entry
intToPairMap[1] = "ProductA" to 35.99

// Remove an entry
intToPairMap.remove(2)

// Print the map
println(intToPairMap) 


// Output: {1=(ProductA, 35.99), 3=(ProductC, 49.99)}

val intToPairMap = mutableMapOf(1 to ("ProductA" to 29.99), 2 to ("ProductB" to 39.99))

// Add a new entry

intToPairMap[3] = "ProductC" to 49.99

// Update an existing entry

intToPairMap[1] = "ProductA" to 35.99

// Remove an entry

intToPairMap.remove(2)

// Print the map

println(intToPairMap)

// Output: {1=(ProductA, 35.99), 3=(ProductC, 49.99)}

A real-life example of Map – Student grading system in Kotlin:

I have created a map-based student grading system where the Student’s name is the Key and their grades are the corresponding values.

fun main() {
    // Create a mutable map to store student names and their grades
    val studentGrades = mutableMapOf<String, Int>()

    // Adding student grades
    studentGrades["Adam"] = 85
    studentGrades["Cris"] = 92
    studentGrades["Charlie"] = 78
    studentGrades["Peter"] = 90
    studentGrades["Mopit"] = 88

    // Display all student grades
    println("Student Grades:")
    for ((name, grade) in studentGrades) {
        println("$name: $grade")
    }

    // Updating a student's grade
    studentGrades["Adam"] = 80
    println("\nUpdated Grades:")
    println("Adam: ${studentGrades["Adam"]}")

    // Removing a student from the map
    studentGrades.remove("Charlie")
    println("\nGrades after removing Charlie:")
    for ((name, grade) in studentGrades) {
        println("$name: $grade")
    }

    // Check if a student exists and retrieve their grade
    val studentToCheck = "Peter"
    if (studentToCheck in studentGrades) {
        println("\n$studentToCheck's grade: ${studentGrades[studentToCheck]}")
    } else {
        println("\n$studentToCheck is not in the grade list.")
    }

    // Getting the average grade
    val averageGrade = studentGrades.values.average()
    println("\nAverage Grade: $averageGrade")
}


// Output

Student Grades:
Adam: 85
Cris: 92
Charlie: 78
Peter: 90
Mopit: 88

Updated Grades:
Adam: 80

Grades after removing Charlie:
Adam: 80
Cris: 92
Peter: 90
Mopit: 88

Peter's grade: 90

Average Grade: 87.5

fun main() {

// Create a mutable map to store student names and their grades

val studentGrades = mutableMapOf<String, Int>()

// Adding student grades

studentGrades["Adam"] = 85

studentGrades["Cris"] = 92

studentGrades["Charlie"] = 78

studentGrades["Peter"] = 90

studentGrades["Mopit"] = 88

// Display all student grades

println("Student Grades:")

for ((name, grade) in studentGrades) {

println("$name: $grade")

}

// Updating a student's grade

studentGrades["Adam"] = 80

println("\nUpdated Grades:")

println("Adam: ${studentGrades["Adam"]}")

// Removing a student from the map

studentGrades.remove("Charlie")

println("\nGrades after removing Charlie:")

for ((name, grade) in studentGrades) {

println("$name: $grade")

}

// Check if a student exists and retrieve their grade

val studentToCheck = "Peter"

if (studentToCheck in studentGrades) {

println("\n$studentToCheck's grade: ${studentGrades[studentToCheck]}")

} else {

println("\n$studentToCheck is not in the grade list.")

}

// Getting the average grade

val averageGrade = studentGrades.values.average()

println("\nAverage Grade: $averageGrade")

}

// Output

Student Grades:

Adam: 85

Cris: 92

Charlie: 78

Peter: 90

Mopit: 88

Updated Grades:

Adam: 80

Grades after removing Charlie:

Adam: 80

Cris: 92

Peter: 90

Mopit: 88

Peter's grade: 90

Average Grade: 87.5

If there is anything in your mind please feel free to ask in the comment section, There are also more functions available of Map in Kotlin’s official documentation. You can read them from HERE. Happy coding.