How do I split Arraylist into several Arraylists?

Question

I am trying to split the Arraylist into numbers of Arraylists with the rule:

Split the arraylist into several sub-arraylists when the value of index i in arraylist is bigger than the value of index i+1.

For example, if there is an arraylist {6,7,8,3,9,10,1,4}, it should be stored as { {6,7,8}, {3,9,10}, {1,4} }.

How can I do this one?

any index `i` is by definition less than `i+1` (it is, in fact, one less), so you're going to have be more precise in your description. Did you mean the _element_ at index `i` and `i+1`? Also, "it should be stored in another arraylist" suggests that in "6,7,8,3,9", only 8 should be put in its own list, so again: you need edit your question to be a lot more precise. — Mike 'Pomax' Kamermans, Mar 25 '21 at 05:11
One of the easiest and fastest way is : https://stackoverflow.com/questions/12099721/how-to-use-sublist — Stanislav Zaiac, Mar 25 '21 at 05:50
As I understand your requirements, each sub-list must contain numbers in descending order only. From your sample input, you should only have two sub-lists as follows: `{8,3}` and `{10, 1}`. — Abra, Mar 25 '21 at 07:41

score 2 · Accepted Answer · answered Mar 25 '21 at 05:36

By looping through the original arraylist and using the sublist method the below code should solve your problem.

    ArrayList<ArrayList<Integer>> lists = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(6,7,8,3,9,10,1,4));

    int indexToSplitFrom = 0;
    for(int i = 0; i < numbers.size() - 1; i++){
        System.out.println(numbers.get(i));
        if(numbers.get(i) > numbers.get(i+1)){
            lists.add(new ArrayList<>(numbers.subList(indexToSplitFrom, i + 1)));
            indexToSplitFrom = i + 1;
        }
    }
    lists.add(new ArrayList<>(numbers.subList(indexToSplitFrom,  + numbers.size())));

Amir Reza Mohammadi · Answer 2 · 2021-03-25T07:29:27.863

1

This code should work for you

        List<Integer> mainList = new ArrayList<>();
        List<List<Integer>> listOfLists = new ArrayList<>();
        int listIndex = 0;
        for (int i = 0; i < mainList.size(); i++) {
            if (i == 0) {
                listOfLists.add(new ArrayList<>());
            } else if (mainList.get(i) < mainList.get(i - 1)) {
                listOfLists.add(new ArrayList<>());
                listIndex++;
            }
            listOfLists.get(listIndex).add(mainList.get(i));
        }

edited Mar 25 '21 at 07:29

answered Mar 25 '21 at 05:38

Amir Reza Mohammadi

76
7

At the first time in the loop, `listOfLists.get (listIndex)` throws an exception. – Mar 25 '21 at 06:12
its because of not instantiated sub-lists. I edited the code – Amir Reza Mohammadi Mar 25 '21 at 07:30

score 1 · Answer 3 · answered Mar 25 '21 at 08:50

public static void splitList(ArrayList<Integer> input) {
        ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
        System.out.println("Input : "+input);
        for (int count = 0; count < input.size(); count++) {

            ArrayList<Integer> subList = new ArrayList<Integer>();
            for (int index = count; index < input.size(); index++) {
                if((index == input.size() - 1) || (input.get(index) > input.get(index + 1))){
                    subList.add(input.get(index));
                    count = index;
                    break;
                }else {
                    subList.add(input.get(index));
                }
            }
            output.add(subList);
        }
        System.out.println("Output : "+output);
    }

How do I split Arraylist into several Arraylists?

3 Answers3