Remove data from list while iterating kotlin

Question

I am new to kotlin programming. What I want is that I want to remove a particular data from a list while iterating through it, but when I am doing that my app is crashing.

for ((pos, i) in listTotal!!.withIndex()) {

            if (pos != 0 && pos != listTotal!!.size - 1) {

                if (paymentsAndTagsModel.tagName == i.header) {
                    //listTotal!!.removeAt(pos)
                    listTotal!!.remove(i)
                }



            }
        }

OR

 for ((pos,i) in listTotal!!.listIterator().withIndex()){
            if (i.header == paymentsAndTagsModel.tagName){
                listTotal!!.listIterator(pos).remove()
            }

        }

The exception which I am getting

java.lang.IllegalStateException

score 39 · Answer 1 · answered Aug 16 '19 at 08:53

39

val numbers = mutableListOf(1,2,3,4,5,6)
val numberIterator = numbers.iterator()
while (numberIterator.hasNext()) {
    val integer = numberIterator.next()
    if (integer < 3) {
        numberIterator.remove()
    }
}

answered Aug 16 '19 at 08:53

murgupluoglu

6,524
4
33
43

1

This should be the accepted answer as the Kotlin docs clearly state the following: "For iterating mutable collections, there is MutableIterator that extends Iterator with the element removal function remove(). So, you can remove elements from a collection while iterating it." – Johann Jun 19 '20 at 16:41

score 37 · Answer 2 · answered Jun 18 '18 at 12:31

37

use removeAll

pushList?.removeAll {  TimeUnit.MILLISECONDS.toMinutes(
      System.currentTimeMillis() - it.date) > THRESHOLD }

answered Jun 18 '18 at 12:31

Mustafa Güven

15,526
11
63
83

1

If you check the code, this is a *very* effective and quick way to filter a MutableList in-place! `.retainAll()` is very similar, but the predicate is negated. – Agoston Horvath Aug 24 '20 at 15:34
What's the difference between `removeAll` and `removeIf` ? It seems both do the same thing, no? – android developer May 06 '21 at 10:54
@androiddeveloper `removeAll` is in `kotlin.collections.MutableCollections` and `removeIf` is in `java.util.Collections` – Archmede Jun 22 '21 at 15:28
@Archmede But do they do the same thing? – android developer Jun 22 '21 at 16:23
@androiddeveloper the javadocs have very similar descriptions – Archmede Jun 22 '21 at 19:26
@Archmede So are they doing the same thing? – android developer Jun 22 '21 at 21:33
This removes the elements, but not while iterating through the list. – Vadim Kotov Oct 20 '21 at 15:44

score 24 · Accepted Answer · answered Feb 02 '18 at 06:56

24

It's forbidden to modify a collection through its interface while iterating over it. The only way to mutate the collection contents is to use Iterator.remove.

However using Iterators can be unwieldy and in vast majority of cases it's better to treat the collections as immutable which Kotlin encourages. You can use a filter to create a new collections like so:

listTotal = listTotal.filterIndexed { ix, element ->
    ix != 0 && ix != listTotal.lastIndex && element.header == paymentsAndTagsModel.tagName
}

answered Feb 02 '18 at 06:56

miensol

39,733
7
116
112

Ok thanks, Sir I used your solution but the thing is I have to notify my adapter for data change but it's not working – Anuj Mody Feb 02 '18 at 07:38
@AnujMody Please provide more details about what adapter you are using. Have you called [notifyDataSetChanged](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html#notifyDataSetChanged()) ? – miensol Feb 02 '18 at 07:42
There is a much better answer from @murgupluoglu now. – Khantahr Dec 30 '21 at 20:18

score 23 · Answer 4 · answered Feb 02 '18 at 09:55

The answer by miensol seems perfect.

However, I don't understand the context for using the withIndex function or filteredIndex. You can use the filter function just by itself.

You don't need access to the index the list is at, if you're using lists.

Also, I'd strongly recommend working with a data class if you already aren't. Your code would look something like this

Data Class

data class Event(
        var eventCode : String,
        var header : String
)

Filtering Logic

fun main(args:Array<String>){

    val eventList : MutableList<Event> = mutableListOf(
            Event(eventCode = "123",header = "One"),
            Event(eventCode = "456",header = "Two"),
            Event(eventCode = "789",header = "Three")
    )


    val filteredList = eventList.filter { !it.header.equals("Two") }

}

Thanks alot! list = list.filter { !(it.ID== ID) }.toMutableList() — Hossein Yousefpour, Sep 04 '19 at 10:31

score 9 · Answer 5 · edited Jul 13 '19 at 09:48

9

The following code works for me:

val iterator = listTotal.iterator()
for(i in iterator){
    if(i.haer== paymentsAndTagsModel.tagName){
        iterator.remove()
    }
}

You can also read this article.

edited Jul 13 '19 at 09:48

double-beep

5,031
17
33
41

answered Jul 13 '19 at 09:22

sadhu

115
1
5

Renetik · Answer 6 · 2022-11-30T08:37:32.777

People didn't break iteration in previous posts dont know why. It can be simple but also with extensions and also for Map:

fun <T> MutableCollection<T>.removeFirst(filter: (T) -> Boolean) =
    iterator().removeIf(filter)

fun <K, V> MutableMap<K, V>.removeFirst(filter: (K, V) -> Boolean) =
    iterator().removeIf { filter(it.key, it.value) }

fun <T> MutableIterator<T>.removeFirst(filter: (T) -> Boolean): Boolean {
    for (item in this) if (filter.invoke(item)) {
        remove()
        return true
    }
    return false
}

score 0 · Answer 7 · answered Jun 04 '19 at 23:32

0

Use a while loop, here is the kotlin extension function:

fun <E> MutableList<E>.removeIfMatch(isMatchConsumer: (existingItem: E) -> Boolean) {
    var index = 0
    var lastIndex = this.size -1

    while(index <= lastIndex && lastIndex >= 0){
        when {
            isMatchConsumer.invoke(this[index]) -> {
                this.removeAt(index)
                lastIndex-- // max is decreased by 1
            }
            else -> index++ // only increment if we do not remove
        }
    }
}

answered Jun 04 '19 at 23:32

Derwrecked

771
1
6
17

I can't find `removeIfMatch`. Instead I see 2 functions that seem like they do the same thing: `removeAll` and `removeIf` . How come? – android developer May 06 '21 at 10:58
@androiddeveloper Make sure you're using a `MutableList` and not a `List` – Archmede Jun 22 '21 at 03:34
@Archmede Hm it's of ArrayList. Important? – android developer Jun 22 '21 at 17:23
@androiddeveloper `ArrayList` implements `MutableList` so it doesn't matter – Archmede Jun 22 '21 at 19:21
@Archmede So it's the same? – android developer Jun 22 '21 at 21:34

score 0 · Answer 8 · answered Mar 16 '22 at 17:26

Typically you can use:

yourMutableCollection.removeIf { someLogic == true }

However, I'm working with an Android app that must support APIs older than 24. In this case removeIf can't be used.

Here's a solution that is nearly identical to that implemented in Kotlin Collections that doesn't rely on Predicate.test - which is why API 24+ is required in the first place

 //This function is in Kotlin Collections but only for Android API 24+
fun <E> MutableCollection<E>.removeIff(filter: (E) -> Boolean): Boolean {
    var removed = false
    val iterator: MutableIterator<E> = this.iterator()
    while (iterator.hasNext()) {
        val value = iterator.next()
        if (filter.invoke(value)) {
            iterator.remove()
            removed = true
        }
    }
    return removed
}

score 0 · Answer 9 · answered Jan 19 '23 at 15:47

Another solution that will suit small collections. For example set of listeners in some controller.

inline fun <T> MutableCollection<T>.forEachSafe(action: (T) -> Unit) {
    val listCopy = ArrayList<T>(this)
    for (element: T in listCopy) {
        if (this.contains(element)) {
            action(element)
        }
    }
}

It makes sure that elements of collection can be removed safely even from outside code.

Remove data from list while iterating kotlin

9 Answers9