Remove duplicate items in ArrayList using for loop only

Question

You are given the following sequence of numbers, 1, 652 ,5, 15, 385, 4 , 55, 666, 13, 2, 4658, 9, 55, -588, 10, 1083, 17 ,4. You can remove the duplicate numbers by only using For loop and ArrayList.

public class Ex {
    public static void main(String[] args) {

        ArrayList list = new ArrayList();
        list.add(1);
        list.add(652);
        list.add(5);
        list.add(15);
        list.add(385);
        list.add(4);
        list.add(55);
        list.add(666);
        list.add(13);
        list.add(2);
        list.add(4658);
        list.add(9);
        list.add(55);
        list.add(-588);
        list.add(10);
        list.add(1083);
        list.add(17);
        list.add(4);


        System.out.println("Before remove : " + list);

        for (int i = 0; i < list.size(); i++) {
            for (int j = 1; j < list.size(); j++) {
                if (list.get(i) == list.get(j)) {
                    list.remove(j);
                }
            }
        }
        System.out.println("After remove duplicate items : "+list);
    }
}

Output

Before remove : [1, 652, 5, 15, 385, 4, 55, 666, 13, 2, 4658, 9, 55, -588, 10, 1083, 17, 4]
After remove duplicate items : [1, 5, 385, 55, 13, 4658, -588, 1083, 4]

Some items that not duplicated is missing. Ex 10 and 652.

use list.sort and then iterate (once) again.If you have dublicates (and not triplecates) , you'll be fine. — Tsakiroglou Fotis
– Tsakiroglou Fotis, Commented Oct 23, 2018 at 8:32

Eran · Accepted Answer · 2018-10-23 08:31:39Z

4

You have many issues:

You will remove non-duplicate records when i == j. To avoid that the inner loop should start with int j = i + 1.
When you remove records, you should decrement j, since all the elements following the removed element are shifted one index down.
You should compare Integers with equals, not with ==

edited Oct 23, 2018 at 8:31

answered Oct 23, 2018 at 8:30

Eran

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2 Comments

Andrew Over a year ago

4. the raw type

Eran Over a year ago

@AndrewTobilko good point, but that doesn't affect the correctness of the code.

Bastmark · Accepted Answer · 2018-10-23 08:59:44Z

The inner loop always starts with j = 1 which means you will remove every single element of the list except the one at index 0. You should instead let j = i + 1. The inner loop will then only check the following elements of the list and not the one you are comparing. After removing you should use j-- so the index of the next element corresponds to j. And you can use == when comparing primitive types (int) When comparing reference types (Integer) you should use equals.

This is what i suggest:

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

Oleg Cherednik · Accepted Answer · 2018-10-23 09:12:56Z

What about other values: 15 or 666? Are you sure you expect correct output? It is much better to use Iterator to remove elements from the list:

public static ArrayList<Integer> removeDuplicates(ArrayList<Integer> numbers) {
    ListIterator<Integer> it = numbers.listIterator();
    numbers.removeIf(num -> isDuplicated(numbers, it.previousIndex(), num));
    return numbers;
}

private static boolean isDuplicated(ArrayList<Integer> numbers, int toPos, int val) {
    for (int pos = 0; pos < toPos; pos++)
        if (numbers.get(pos) == val)
            return true;

    return false;
}

Output:

Before remove : [1, 652, 5, 15, 385, 4, 55, 666, 13, 2, 4658, 9, 55, -588, 10, 1083, 17, 4]
After remove duplicate items : [1, 652, 5, 15, 385, 4, 55, 666, 13, 2, 4658, 9, 55, -588, 10, 1083, 17, 4]

Meysam Keshvari · Accepted Answer · 2020-09-09 06:04:35Z

0

public class ArrayListExample {
    public static void main(String[] args) {
        // ArrayList with duplicate elements
        ArrayList<Integer> numbersList = new ArrayList<>(Arrays.asList(1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8));
        System.out.println(numbersList);
        Log.d("DBG",numbersList+"");
        // ArrayList without duplicate elements
        LinkedHashSet<Integer> hashSet = new LinkedHashSet<>(numbersList); 
        ArrayList<Integer> listWithoutDuplicates = new ArrayList<>(hashSet);
        System.out.println(listWithoutDuplicates);
        Log.d("DBG",listWithoutDuplicates+"");
    }
}

input

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

output

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

edited Sep 9, 2020 at 6:04

answered Sep 23, 2019 at 9:06

Meysam Keshvari

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Comments

Ankit Tayal · Accepted Answer · 2021-02-01 20:05:09Z

0

// Code for Duplicates not allowed ... for upper example
for(int i = 0; i < list.size(); i++) {
            for(int j = i + 1; j < list.size(); j++) {
                if(list.get(i).equals(list.get(j))){
                    list.remove(j);
                    j--;
                }
            }

answered Feb 1, 2021 at 20:05

Ankit Tayal

1

1 Comment

sonali Over a year ago

Please explain your answer

rushikesh sarolkar · Accepted Answer · 2022-12-15 18:59:54Z

ublic class RemoveDublicate {

public static void main(String[] args) {
    List<Integer> list=new ArrayList<Integer>();
    list.add(1);
    list.add(652);
    list.add(5);
    list.add(15);
    list.add(385);
    list.add(4);
    list.add(55);
    list.add(666);
    list.add(13);
    list.add(2);
    list.add(4658);
    list.add(9);
    list.add(55);
    list.add(-588);
    list.add(10);
    list.add(1083);
    list.add(17);
    list.add(4);
        System.out.println(list);
        
        for(int i=0;i<list.size();i++)
        {
            for(int j=i+1;j<list.size();j++)
            {
                
                if(list.get(i)==list.get(j))
                {
                    list.remove(j);
                }
            }
        }
        System.out.println(list);


}

}

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