Thread: 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

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.

Originally Posted by jps

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

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?

Originally Posted by jps

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

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?

Originally Posted by jps

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

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

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?