How can I replace some element and remove some element in one loop in java?

Question

I wanna do some thing like this :

List<Integer> list = [1,2,1,3];
for (each : list) {
    if (each == 1) {
        remove (each);
    } else {
        each = 4;
    }
}

after the loop the list is supposed to be [4,4].

I have tried the following codes :

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "1", "3"));
for (String i : list) {
    if (i.equals("1")) {
        i = "4";
    }
}

But it doesn't change the value, the list is [2,3].

List<Integer> list = [1,2,1,3];
for (int i = 0; i< list.size(); i++) {
    //do something to replace the element
}

This can replace the element but can not remove the element.

The only way I can come up with is to remove the elements in one loop, and replace the element in another loop, how can I do both in just one loop?

ישו אוהב אותך · Accepted Answer · 2021-03-12 06:39:54Z

1

Your following code doesn't work:

for (String i : list) {
    if (i.equals("1")) {
        i = "4";
    }
}

because you're replacing the value of String i instead of the selected item of list.

You need to use set method:

for (int i = 0; i < list.size(); i++) {
    String item = list.get(i);
    if (item.equals("1")) {
        list.set(i, "4");
    }
}

UPDATE

If you want to remove and change the item in one loop, you need to use listIterator and change for loop with while loop:

ListIterator<String> iterator = list.listIterator();

while(iterator.hasNext()){
   String item = iterator.next();
   
   // Change item value
   if(item.equals("1") iterator.set("4");
  
   // remove item
   if(item.equals("2") iterator.remove();
}

Please be noted, I haven't test the code yet.

edited Mar 12, 2021 at 6:39

answered Mar 12, 2021 at 6:02

ישו אוהב אותך

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2 Comments

lushuming Over a year ago

yeah, I can replace the value throw this but I can't remove some of them without using iterator.

lushuming Over a year ago

Thanks a lot, the updated code works. @ישו אוהב אותך

bliss · Accepted Answer · 2021-03-12 07:17:25Z

0

You need to be careful to avoid a ConcurrentModificationException when attempting to remove an element from a list while iterating over it. You can use this:

list = list.stream()
        .filter(element -> element != 1)
        .map(element -> 4)
        .collect(Collectors.toList());

edited Mar 12, 2021 at 7:17

answered Mar 12, 2021 at 6:26

bliss

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3 Comments

lushuming Over a year ago

Actually, the result of this answer is a list of boolean, it doesn't work.

Alex Sveshnikov Over a year ago

The map should be replaced with map(element -> 4)

bliss Over a year ago

Yeah, typo. Sorry about that.

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