merge two arraylist lists in list1 while it remain sorted

Question

In my assignment the third step is to Call the method merge to merge the two lists in list1 so that the list1 remains sorted.

I write my code but it doesn't work well , the output show wrong because it important to be sorted

 public static void merge (ArrayList<Integer> list1, ArrayList<Integer> list2)
 {
        int i;
        int n=list1.size();
        int pos , j=0;

        for (pos =0 ;pos<n ; pos++)
        {
            for ( i=0 ; i<n ; i++)
                if (list1.get(j)>list2.get(pos))
                    list1.add(pos,list2.get(pos));
                else 
                    j++;
       } 
 }

@Laf I didn't put the tag (homework ) . some one put it 4 me :) — rain
– rain, Commented Oct 25, 2012 at 13:32
What do you mean? Merge Sort is an algorithm for ordered sequences, however they are represented. BTW an ArrayList is a variable length data-structure that wraps an array. — Paul Ruane
– Paul Ruane, Commented Oct 25, 2012 at 13:46
@PaulRuane I am still beginner in java , thanks alot for ur precious time :) — rain
– rain, Commented Oct 25, 2012 at 13:54

Keppil · Accepted Answer · 2012-10-25 14:19:22Z

10

You only need one for loop assuming both lists are sorted:

public static void merge(List<Integer> l1, List<Integer> l2) {
    for (int index1 = 0, index2 = 0; index2 < l2.size(); index1++) {
        if (index1 == l1.size() || l1.get(index1) > l2.get(index2)) {
            l1.add(index1, l2.get(index2++));
        }
    }
}

If l2 isn't sorted, you need two loops:

public static void merge(List<Integer> l1, List<Integer> l2) {
    for (int index2 = 0; index2 < l2.size(); index2++) {
        for (int index1 = 0; ; index1++) {
            if (index1 == l1.size() || l1.get(index1) > l2.get(index2)) {
                l1.add(index1, l2.get(index2));
                break;
            }
        }
    }
}

edited Oct 25, 2012 at 14:19

answered Oct 25, 2012 at 13:48

Keppil

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2 Comments

rain Over a year ago

thanks a lot , but sorry to bother you but the middle element in list2 doesn't sorted with them . for example : list1= 1 7 11 13 , list2 = 4 0 25 list1 after merge = 1 4 0 7 11 13 25

Keppil Over a year ago

@rain: Added a variant for unsorted l2.

atamanroman · Accepted Answer · 2012-10-25 13:32:34Z

2

Easy fix: sort afterwards.

list1.addAll(list2);
Collections.sort(list1);

Use sets to avoid duplicates.

answered Oct 25, 2012 at 13:32

atamanroman

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2 Comments

atamanroman Over a year ago

Works but may (sadly) not be what your assignment expects.

rain Over a year ago

the doctor doesn't teach us this method , maybe he want us to use loops

Harry · Accepted Answer · 2012-10-25 13:35:40Z

1

public static void merge (ArrayList<Integer> list1, ArrayList<Integer> list2)
{
    list1.addAll(list2);
    Collections.sort(list1);
}

answered Oct 25, 2012 at 13:35

Harry

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Comments

Renjith · Accepted Answer · 2012-10-25 13:33:09Z

0

After you merge the lists, call sort method as below.

Collections.sort(list1); // carries out natural ordering.

If you need a customized sorting, use Comparator object

Collections.sort(list1, comparatorObject);

Check Comparator example for more details.

Here is your modified code:

 public static void merge (ArrayList<Integer> list1, ArrayList<Integer> list2)
 {
    list1.add(list2); //merges list 2 to list1
    Collections.sort(list1); //natural ordering
 }

answered Oct 25, 2012 at 13:33

Renjith

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Comments

netmikey · Accepted Answer · 2012-10-25 13:37:19Z

0

If your input Lists aren't too long, I'd suggest just merging the lists and using the Collections.sort() Method to restore the order:

public static void mergeAndSort(List<Integer> list1, List<Integer> list2) {
    List<Integer> combinedList = new ArrayList<Integer>(list1);
    combinedList.addAll(list2);
    Collections.sort(combinedList);
    return combinedList;
}

As a sidenote, you should use the List interface instead of the ArrayList implementation class wherever possible.

answered Oct 25, 2012 at 13:37

netmikey

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Comments

Satish Kumar · Accepted Answer · 2016-04-27 02:30:42Z

0

public list mergeAndSort(List<integer> list1, List<integer> list2){
List<integer> list3;
int list2Size = list2.size();
for(int i=0;i<list2Size;i++){
    list1.add(list2(i));  
}
// Here we got all the elements in 1 list i.e list1

int list1Size = list1.size();
for(i=0;i<list1Size;i++){
    int small = 0;
    for(int j=i;j<list1size;j++){
        if(list1(i)> list2(j)){
            small = list2(j;
        }
    }
    list3.add(small); //Smallest 1 will be added to the new list
}

}

answered Apr 27, 2016 at 2:30

Satish Kumar

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1 Comment

Hank D Over a year ago

Congratulations on your first answer, @Satish. Your answer would be more beneficial to current and future answer seekers if you gave it a little more explanation. See How to Answer

Cemil Dogan · Accepted Answer · 2015-07-03 00:26:27Z

-1

     ArrayList<Integer> a = new ArrayList();
     ArrayList<Integer> b = new ArrayList();
     ArrayList<Integer> c = new ArrayList();

     a.add(1);
     a.add(3);
     a.add(5);
     a.add(7);
     a.add(17);
     a.add(27);
     a.add(37);

     b.add(0); 
     b.add(2);
     b.add(4);

     while( a.size() > 0 || b.size() >0){

        if( a.size() == 0 || b.size() == 0){
            c.addAll(b);
            c.addAll(a);
            break;
        }

        if(a.get(0) < b.get(0)){
            c.add(a.get(0));
            a.remove(0);
        }
        else{
            c.add(b.get(0));
            b.remove(0);
        }

    }

    System.out.println(c.toString());

answered Jul 3, 2015 at 0:26

Cemil Dogan

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1 Comment

user4639281 Over a year ago

Generally, answers without explanations are considered not very useful. Please add an explanation.

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