Conditionally remove elements from a list in Kotlin

This article explores different ways to conditionally remove elements from a mutable list in Kotlin.

It is not permissible to modify a list while iterating over it; otherwise, ConcurrentModificationException will be thrown. However, you can use any of the following methods to safely remove elements from the list that doesn’t involve iterating it.

1. Using `removeAll()` function

The idea is to collect a list of elements that match the given predicate and then pass that collection to the removeAll() function. This will remove those elements from the original list.

import java.util.function.Predicate

fun <T> remove(list: MutableList<T>, predicate: Predicate<T>) {

val newList: MutableList<T> = ArrayList()

list.filter { predicate.test(it) }.forEach { newList.add(it) }

list.removeAll(newList)

}

fun main() {

var days: MutableList<String> = mutableListOf("Monday", "Tuesday", "Wednesday",

"Thursday", "Friday", "Saturday", "Sunday")

var weekend = Predicate { day: String -> day == "Sunday" || day == "Saturday" }

remove(days, weekend)

println(days) // [Monday, Tuesday, Wednesday, Thursday, Friday]

}

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2. Using `remove()` function

Since you can’t modify a list while iterating over it, you can create a filtered list of elements to be removed and then remove those elements from the original list. This can be implemented as follows using filter() and forEach() function:

import java.util.function.Predicate

fun <T> remove(list: MutableList<T>, predicate: Predicate<T>) {

list.filter { predicate.test(it) }.forEach { list.remove(it) }

}

fun main() {

var days: MutableList<String> = mutableListOf("Monday", "Tuesday", "Wednesday",

"Thursday", "Friday", "Saturday", "Sunday")

var weekend = Predicate { day: String -> day == "Sunday" || day == "Saturday" }

remove(days, weekend)

println(days) // [Monday, Tuesday, Wednesday, Thursday, Friday]

}

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3. Using `removeIf()` function

The simplest solution is to directly call the removeIf() function, which removes all elements from the list that satisfies the given predicate.

import java.util.function.Predicate

fun <T> remove(list: MutableList<T>, predicate: Predicate<T>) {

list.removeIf { x: T -> predicate.test(x) }

}

fun main() {

var days: MutableList<String> = mutableListOf("Monday", "Tuesday", "Wednesday",

"Thursday", "Friday", "Saturday", "Sunday")

var weekend = Predicate { day: String -> day == "Sunday" || day == "Saturday" }

remove(days, weekend)

println(days) // [Monday, Tuesday, Wednesday, Thursday, Friday]

}

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That’s all about conditionally remove elements from a list in Kotlin.

Conditionally remove elements from a list in Kotlin

1. Using removeAll() function

2. Using remove() function

3. Using removeIf() function

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1. Using `removeAll()` function

2. Using `remove()` function

3. Using `removeIf()` function