arraylist.remove not working properly

Question

I have a sorted array of integers and want to remove the duplicates, I wrote the following code

package practice;
import java.util.*;

public class pb1 {

public static void removeDup(int[] theArray){       

    ArrayList<Integer> hold=new ArrayList<Integer>();


    for(int i=0;i<theArray.length-1;i++){


            hold.add(theArray[i]);

            //System.out.println(hold);

}
    for(int i=0;i<hold.size()-1;i++){
        if(hold.get(i)==hold.get(i+1)){
            hold.remove(i);
        }

    }
    System.out.println(hold);
}


public static void main(String[]args){
    int[] now={1,1,2,2,3,4,4,4,5,5,5,5};
    removeDup(now);



}

}

trying to delete the duplicates using the arraylist.remove method, but I can still see duplicates in my printed arraylist. I dont know why, pls can someone help me? thanks

Answer 1

An alternative solution. This takes advantage of the fact that a Set does not allow duplicate entries, and greatly simplifies your code.

public class pb1 
{

public static void removeDup(int[] theArray)
{       
    Set<Integer> hold=new TreeSet<Integer>();
    for(int i=0;i<theArray.length-1;i++)
    {
        hold.add(theArray[i]);
    }
    System.out.println(hold);
}


public static void main(String[]args)
    {
     int[] now={1,1,2,2,3,4,4,4,5,5,5,5};
     removeDup(now);
    }
}

Output:

[1, 2, 3, 4, 5]

Answer 2

Iterate backwards. Deleting items forward will delete the current element THEN increment the counter/index thus skipping possible duplicates. Replace your loop with:

for(int i = hold.size() - 1; i >= 1;i--){
    if(hold.get(i)==hold.get(i - 1)){
        hold.remove(i);
    }
}

EDIT: Your code fails at the three consecutive 4's When your index is at the first 4:

{1,1,2,2,3,>4,4,4,5,5,5,5}

it checks the next element. It deletes the current element because it sees a 4 ahead of it BUT, this is where the bug comes in.

{1,1,2,2,3,4,>4,5,5,5,5}

Upon deletion of the first 4, the succeeding array elements MOVE BACK and then you INCREMENT your counter thus skipping the other 4. Ergo, duplicates.

Answer 3

Why do you want to manually remove duplicates? You can copy list to set to remove duplicates and then copy from set to list again. Something like this

ArrayList aList = new ArrayList();
LinkedHashSet link = new LinkedHashSet();
link.addAll(aList);
aList.clear();
aList.addAll(link);

Answer 4

Try this:

for (int i = 0; i < theArray.length - 1; i++) {

           if (!hold.contains(theArray[i]))

                hold.add(theArray[i]);

}

System.out.println(hold);