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Thread: Problem ordering nested collections

  1. #1
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    Default Problem ordering nested collections

    Hi all,

    I know how to use Java collections but at beginner's level.

    I have the list:

    [10, 20, 30, 12, 13, 8, 1, 2, 3]

    With the following methods:

    1) List<List<Integer>> getSubSequences( Iterator<Integer> it, Comparator<Integer> cmp )
    2) List<List<Integer>> getSubSequencesAscendentOrder( Iterator<Integer> it )

    The outputs must be:

    1) [[10, 20, 30], [12, 13], [8], [1, 2, 3]]

    and

    2) [[1, 2, 3], [8], [10, 20, 30], [12, 13]]


    I've implemented the first method and allready works as follows:


     
    class MyComparator implements Comparator<Integer> 
    {
        public int compare(Integer i1, Integer i2)
        {
            return i1 - i2;
        }
    }
     
     
    public static List<List<Integer>> getSubSequences(Iterator<Integer> it, Comparator<Integer> cmp)
    {
     
        List<List<Integer>> list = new ArrayList<List<Integer>>();
        ArrayList<Integer> subList = new ArrayList<Integer>();
     
        int aux1 = 0;
     
        if(it.hasNext())
        {
            aux1 = it.next();
            subList.add(aux1);
        }
     
        while(it.hasNext())
        {
            int aux2 = it.next();
     
            if(cmp.compare(aux2, aux1) < 0 )
            {
                list.add(subList);
                subList = new ArrayList<Integer>();
            }
            subList.add(aux2);
            aux1 = aux2;
        }
        list.add(subLista);
        return list;
     
    }


    In public static main() I have:


     
     
        int array[] = {10, 20, 30, 12, 13, 8, 1, 2, 3};
     
        ArrayList<Integer> li = new ArrayList<Integer>(array.length);
     
        for(int i = 0; i < array.length; i++) li.add(array[i]);
     
        Iterator<Integer> it = li.iterator();
     
        Comparator cmp = new MyComparator();
        List<List<Integer>> res = getSubSequences(it, cmp) ;
     
        it = ((List)res).iterator();
     
        while(it.hasNext())
                System.out.print(it.next() + " ");
        System.out.println();


    This works fine and the output is [10, 20, 30] [12, 13] [8] [1, 2, 3]

    Not exactly [[10, 20, 30], [12, 13], [8], [1, 2, 3]] but close.


    I think the method 2) must be very similar to method 1), however I can't implement that.

    Can someone give some clues?


    Thank you in advance

    Paulo


  2. #2
    Super Moderator jps's Avatar
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    Default Re: Problem ordering nested collections

    Welcome PauloSotto

    If the desired outputs are:
    1) [[10, 20, 30], [12, 13], [8], [1, 2, 3]]
    and:
    2) [[1, 2, 3], [8], [10, 20, 30], [12, 13]]
    ...output 2 looks like output 1 sorted by the first element in ascending order.

  3. #3
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    Default Re: Problem ordering nested collections

    Quote Originally Posted by jps View Post
    Welcome PauloSotto

    If the desired outputs are:
    1) [[10, 20, 30], [12, 13], [8], [1, 2, 3]]
    and:
    2) [[1, 2, 3], [8], [10, 20, 30], [12, 13]]
    ...output 2 looks like output 1 sorted by the first element in ascending order.

    You are absolutely right.

    The problem is I don't know how to sort a List<List<Integer>> or say in other terms
    I don't know how to sort the "inner" list inside the "outer" list.

    Can you help me please ?


    Thank you
    Paulo

  4. #4
    Super Moderator jps's Avatar
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    Default Re: Problem ordering nested collections

    The method signature of method2 shows an Iterator<Integer> as the parameter
    Where will you get an Iterator<Integer>?
    What happens if you sort the array before the first method runs? What is the output of the first method?

  5. #5
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    Default Re: Problem ordering nested collections

    Quote Originally Posted by jps View Post
    The method signature of method2 shows an Iterator<Integer> as the parameter
    Where will you get an Iterator<Integer>?
    What happens if you sort the array before the first method runs? What is the output of the first method?

    Thank you very much for your patience.

    I think is more informative paste the code than say what it does.

    Lets see the entire code (it's not to big):

    MyComparator.java
    public class MyComparator implements Comparator<Integer> 
    {
        public int compare(Integer i1, Integer i2)
        {
            return i1 - i2;
        }
    }


    Main.java
    public class Main
    {
     
        public static List<List<Integer>> getSubSequences(Iterator<Integer> it, Comparator<Integer> cmp)
        {
     
            List<List<Integer>> lista = new ArrayList<List<Integer>>();
            ArrayList <Integer> subLista = new ArrayList<Integer>();
     
            int aux1 = 0;
     
            if(it.hasNext())          // if has another element
            {
                aux1 = it.next();   // return element
                subLista.add(aux1);
            }
     
            while(it.hasNext())
            {
                int aux2 = it.next();
     
                if(cmp.compare(aux2, aux1) < 0 )
                {
                    lista.add(subLista);
                    subLista = new ArrayList<Integer>();
                }
                subLista.add(aux2);
                aux1 = aux2;
            }
            lista.add(subLista);
     
            return lista;
     
        }
     
     
     
        public static List<List<Integer>> getSubSequencesAscendentOrder(Iterator<Integer> it)
        {
            // THIS IS THE METHOD TO IMPLEMENT        
        }
     
     
     
    //  M A I N
     
        public static void main(String args[])
        {
     
            int array[] = {10, 20, 30, 12, 13, 8, 1, 2, 3};
     
            // Create ArrayList with the same length of the array
            ArrayList<Integer> li = new ArrayList<Integer>(array.length);
     
            // Copy array to ArrayList
            for(int i = 0; i < array.length; i++)
            {
                li.add(array[i]);
            }
     
            Iterator<Integer> it = li.iterator();
     
            Comparator cmp = new MyComparator();
            List<List<Integer>> res = getSubSequences(it, cmp) ;
     
            it = ((List)res).iterator();
     
            while(it.hasNext()) 
                    System.out.print(it.next() + " ");
            System.out.println();
     
        }
    }


    Method 1 => List<List<Integer>> getSubSequences( Iterator<Integer> it, Comparator<Integer> cmp )

    Output:

    [10, 20, 30] [12, 13] [8] [1, 2, 3]


    This output however, is not accurate.

    What was expected is:

    [[10, 20, 30], [12, 13], [8], [1, 2, 3]]

    (The leading '[' and trailing ']' and the commas between sub-lists are missing).



    Method 2 => List<List<Integer>> getSubSequencesAscendentOrder( Iterator<Integer> it )


    Expected output:

    [[1, 2, 3], [8], [10, 20, 30], [12, 13]]


    If you can help me I'll appreciate so much.


    Thank you in advance.

    Paulo

  6. #6
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    Default Re: Problem ordering nested collections

    public static List<List<Integer>> getSubSequencesAscendentOrder(Iterator<Integer> it)
        {
            // THIS IS THE METHOD TO IMPLEMENT        
        }
    What have you tried here so far?
    What happened when you tried my suggestion in post #4?

  7. #7
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    Default Re: Problem ordering nested collections

    Quote Originally Posted by jps View Post
    What have you tried here so far?
    What happened when you tried my suggestion in post #4?
    In post #4 you suggested:

    What happens if you sort the array before the first method runs? What is the output of the first method?
    According to your suggestion, I get:

    [1, 2, 3, 8, 10, 12, 13, 20, 30]

    Since the array is sorted before calling first method, I'll get only one sub-collection because
    the array can't be split in sub-arrays.

    In second method I haven't tried nothing, because I don't know why can I sort the inner Lists inside the outer list.


    Thank you

    Paulo

  8. #8
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    Default Ordering nested collections (question reformulated)

    Hi all,

    I don't know how to sort data structures of type List<List< ...>> or any collection of collections in general

    I'll put here my question, but now, reformulated with a more concise (and simple) example.

    Consider this code:

        public static void main(String args[])
        {
            ArrayList<String> list1 = new ArrayList<String>();
            ArrayList<String> list2 = new ArrayList<String>();
            ArrayList<String> list3 = new ArrayList<String>();
            ArrayList<ArrayList<String>> listOfList = new ArrayList<ArrayList<String>>();
     
            list1.add("9");
            list1.add("6");
            list1.add("3");
            listOfList.add(list1);
     
            list2.add("8");
            list2.add("5");
            list2.add("2");
            listOfList.add(list2);
     
            list3.add("7");
            list3.add("4");
            list3.add("1");
            listOfList.add(list3);
     
            Iterator<ArrayList<String>> it = listOfList.iterator();        
            while (it.hasNext())
            {
                System.out.println(it.next());
            }
     
        }


    Output:

    [9, 6, 3]
    [8, 5, 2]
    [7, 4, 1]


    Now, I add the following code to the end of my program:

            Collections.sort(listOfList.get(0)); // sort the 1st sublist => 0 -> 1st sublist; 1 -> 2nd sublist; 2 -> 3rd sublist
     
            it = listOfList.iterator();        
            while (it.hasNext())
            {
                System.out.println(it.next());
            }

    Output:

    [9, 6, 3]
    [8, 5, 2]
    [7, 4, 1]

    [3, 6, 9] <= Now I have the 1st sublist by ascending order
    [8, 5, 2]
    [7, 4, 1]


    But this is NOT what I want.

    I want to sort not inside one sublist, but the sublists each other like this:

    Before sorting:
    [9, 6, 3] <= 1st sublist
    [8, 5, 2] <= 2nd sublist
    [7, 4, 1] <= 3rd sublist

    After sorting:
    [7, 4, 1] <= 1st sublist
    [8, 5, 2] <= 2nd sublist
    [9, 6, 3] <= 3rd sublist

    I want to sort the sublists by the first element of each list.


    Can anyone help me?


    Thank you in advance.

    Paulo Sotto

  9. #9
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    Default Re: Ordering nested collections (question reformulated)

    Sounds like a job for a custom Comparator.
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  10. #10
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    Default Re: Problem ordering nested collections

    Please do not double post your question.
    Threads merged.

    ublic static List<List<Integer>> getSubSequencesAscendentOrder(Iterator<Integer> it)
        {
            // THIS IS THE METHOD TO IMPLEMENT        
        }
    This does not show any effort to solve this problem. What have you tried?

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