Kotlin Collection Write operations

In Kotlin, when we work with collections like lists or sets, sometimes we need to change or update their contents. If a collection allows us to modify its elements, we call it a MutableCollection. This means we can add, remove, or update its elements anytime. In this article, we will explore the different ways we can perform these write operations in a Kotlin MutableCollection.

1. Adding Elements

add() Function

We use the add() function to insert a new element into the collection. The element is added to the end of the collection.

Example:

fun main() {
    val numbers = mutableListOf(11, 12, 13, 14)
    numbers.add(15)
    println(numbers)
}

Output:

[11, 12, 13, 14, 15]

addAll() Function

The addAll() function helps us add multiple elements at once. We can pass an Iterable, Sequence, or Array to this function.

Example:

fun main() {
    val list1 = mutableListOf(1, 2, 2, 13, 14)
    val list2 = listOf(5, 6)
    list1.addAll(list2)
    println(list1)

    list1.addAll(listOf(7, 8))
    println(list1)
}

Output:

[1, 2, 2, 13, 14, 5, 6]
[1, 2, 2, 13, 14, 5, 6, 7, 8]

plusAssign (+=) Operator

We can also use the += operator to add an element or a group of elements to the collection.

Example:

fun main() {
    val items = mutableListOf("one", "two", "seven", "four", "five")
    items += "three"
    println(items)
}

Output:

[one, two, seven, four, five, three]

2. Removing Elements

remove() Function

The remove() function deletes the first occurrence of the given element from the collection. If the element is not found, nothing happens.

Example:

fun main() {
    val numbers = mutableListOf(11, 22, 33, 44, 33)
    numbers.remove(33)
    println(numbers)
}

Output:

[11, 22, 44, 33]

removeAll() Function

We can use removeAll() to remove all elements that match the given collection or condition.

Example:

fun main() {
    val items = mutableListOf("one", "two", "three", "four")
    items.removeAll(listOf("one", "two", "four"))
    println(items)
}

Output:

[three]

minusAssign (-=) Operator

The -= operator removes the specified element or collection of elements.

Example:

fun main() {
    val items = mutableListOf("one", "two", "three", "four")
    items -= "two"
    println(items)
}

Output:

[one, three, four]

retainAll() Function

This function keeps only those elements that match the given condition and removes the rest.

Example:

fun main() {
    val numbers = mutableListOf(11, 22, 33, 44)
    numbers.retainAll { it > 30 }
    println(numbers)
}

Output:

[33, 44]

clear() Function

If we want to remove all elements from the collection, we use the clear() function.

Example:

fun main() {
    val numbers = mutableListOf(12, 23, 34, 45)
    numbers.clear()
    println(numbers)
}

Output:

[]

3. Updating Elements

Using the [] Operator

We can use the square brackets [] operator to change the value of an element at a specific index.

Example:

fun main() {
    val items = mutableListOf("one", "two", "three")
    items[2] = "two"
    println(items)
}

Output:

[one, two, two]

fill() Function

If we want to replace all the elements in the collection with a single value, we can use the fill() function.

Example:

fun main() {
    val numbers = MutableList(4) { 0 }
    numbers.fill(3)
    println(numbers)

    numbers.fill(0)
    println(numbers)
}

Output:

[3, 3, 3, 3]
[0, 0, 0, 0]