Split ArrayList in multiple ArrayLists where values are same

Question

Well im stuck on this point.

im trying to make sort function what does this.

parentarraylist = [1,1,1,2,3,2,3,3,2,2,1];
*magic*
childarraylist1 = [1,1,1,1];
childarraylist2 = [2,2,2,2];
childarraylist3 = [3,3,3];

the magic part is where im stuck.

i have tryied putting it in an for loop(the parent) and checking on the value. like this

int i = 0;
int finder = 0;
ArrayList<int> initArray = new ArrayList();
for(int list : parentarraylist){
    if(i == 0){
        finder = list
    }
    if(finder == list){
           initArray.add(list);
           parentarraylist.remove(list);
    }else{
        new ArrayList *value of list* =  new ArrayList(); 
        finder = list;
        *value of list*.add(list);
    }
}

this results in a view errors like java.util.ConcurrentModificationException and i cant set value of list

what can i do to make this work?

Doing different coding but getting same error but its throwing inconsistent. — AndyBoy, Mar 23 '15 at 12:56
im getting the error always.... im my code im trying to sort on dates — Twizzler, Mar 23 '15 at 12:57
can you explain again what's the input and what's the desired output? — Blackbelt, Mar 23 '15 at 13:02
the input is = `parentarraylist = [1,1,1,2,3,2,3,3,2,2,1];` the desired output is: `childarraylist1 = [1,1,1,1]; childarraylist2 = [2,2,2,2]; childarraylist3 = [3,3,3];` — Twizzler, Mar 23 '15 at 13:05
@Blackbelt my code : http://stackoverflow.com/questions/29208804/android-exception-concurentmodificationexception — AndyBoy, Mar 23 '15 at 13:09
i can afford a hashmap if list.remove is possible (the arrays are max 5000 objects large) — Twizzler, Mar 23 '15 at 13:13

score 2 · Accepted Answer · edited May 23 '17 at 12:06

This little snippet should help you achieve your goal:

//maps will hold ALL unique integer as it's key
HashMap<Integer, ArrayList<Integer>> maps = new HashMap<Integer, ArrayList<Integer>>();

//your initial array, written inline for clarity
ArrayList<Integer> parentarraylist = new ArrayList<Integer>(Arrays.asList( new Integer[] {1,1,1,2,3,2,3,3,2,2,1}));

//get the iterator so that we won't need another temporary int variable for loop
Iterator<Integer> iterator = parentarraylist.iterator();
while(iterator.hasNext()){
    //next = current integer in our array
    Integer next = iterator.next();

    //check if we have already have current integer or not
    if(!maps.containsKey(next)){
        //we don't have it, initialise an arraylist for this specific integer
        ArrayList<Integer> x = new ArrayList<Integer>();
        x.add(next);

        //put it to our map holder
        maps.put(next, x);
    } else {
        //already have it, add directly
        maps.get(next).add(next);
    }
}

This codes will print something like this:

printMap(maps);
//1 = [1, 1, 1, 1]
//2 = [2, 2, 2, 2]
//3 = [3, 3, 3]

printMap() is taken from this answer: Iterate through a HashMap

public static void printMap(Map mp) {
   Iterator it = mp.entrySet().iterator();
   while (it.hasNext()) {
       Map.Entry pair = (Map.Entry)it.next();
       System.out.println(pair.getKey() + " = " + pair.getValue());
       it.remove(); // avoids a ConcurrentModificationException
   }
}

score 1 · Answer 2 · answered Mar 23 '15 at 13:14

You could try something like this:

public static void main(String[] args) {
        ArrayList<Integer> parent = new ArrayList<Integer>();
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        parent.add(3);
        parent.add(1);
        parent.add(1);
        parent.add(2);
        parent.add(3);
        parent.add(3);
        parent.add(1);

        for(int i : parent)
        {
            boolean check = false;
            for(ArrayList<Integer> result : results)
            {
                if(result.size() > 0)
                {
                    if(result.get(0) == i)
                    {
                        check = true;
                        result.add(i);
                        break;
                    }
                }
            }
            if(!check)
            {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                temp.add(i);
                results.add(temp);
            }
        }

        for(ArrayList<Integer> i : results)
        {
            for(int j : i)
            {
                System.out.print("" + j);
            }
            System.out.println();
        }
    }

Output:

333
111 
2

score 0 · Answer 3 · answered Mar 23 '15 at 13:03

0

The problem is that you remove an element from an array over which you are iterating. The

parentArrayList.remove(list);

causes the error here. If you remove this line your program will work. At the moment I do not see the benefit of removing the item from the parentArrayList for your sorting algorithm so just delete it and you are good to go.

answered Mar 23 '15 at 13:03

Deutro

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i dont know if i can afford it(the extra loops) – Twizzler Mar 23 '15 at 13:13

Split ArrayList in multiple ArrayLists where values are same

3 Answers3