Re: How to remove duplicate values in an array.

Steven Simpson <ss@domain.invalid>
Sun, 13 Oct 2013 10:00:30 +0100
On 13/10/13 06:18, wrote:

I have some code to build an object elementData that stores a list of ints in an array and also keeps track of a boolean unique that determines if duplicates are allowed in a list. I have written a small method to search out duplicates and remove them, but for some reason it is not working. I am not sure if the problem is with the method itself or the placement of the call to the removeDuplicates method.

    // post: places the value in the correct place based on ascending order
    public void add(int value) {
       if (size == 1) {
          elementData[0] = value;

(I'm not sure you need this branch. binarySearch would have to quickly
yield -1, position would take the value 0, the shifting loop would have
zero iterations, and the value would be assigned to position zero.)

       } else {
          int position = Arrays.binarySearch(elementData, 0, size - 1, value);
          if (position < 0 ) {
             position = (-position) - 1;
          for (int i = size - 1; i > position; i--) {
             elementData[i] = elementData[i - 1];
          elementData[position] = value;
          if (unique) {

You're evidently relying on the sequence being sorted, and ensuring it
too - so your removeDuplicates could be limited to looking for them only
where the insertion just took place. Better still, detect a
non-negative from binarySearch, and don't bother inserting.

    //post: removes any duplicate values from the list
    private void removeDuplicates() {
       for(int i = size - 1; i < 0; i--) {

Surely, i > 0?

          if (elementData[i] == elementData[i - 1]){
    // pre : 0 <= index < size() (throws IndexOutOfBoundsException if not)
    // post: removes value at the given index, shifting subsequent values left
    public void remove(int index) {
       for (int i = index; i < size - 1; i++) {
          elementData[i] = elementData[i + 1];

Do you need the result to be sorted, or are you just using that to help
remove duplicates? If it's incidental, you could just add values to a

If sorting is important, but you want to keep duplicates, you could use
a TreeMap:

   SortedMap<Integer, Integer> map = new TreeMap<>();

   void add(int value) {
     Integer count = map.get(value);
     if (count == null) count = 0;
     map.put(value, count);

To sort but avoid duplicates:

   SortedSet<Integer> set = Collections.newSetFromMap(new TreeMap<Integer, Boolean>());

   void add(int value) {

Or use unsorted collections, and sort after.

ss at comp dot lancs dot ac dot uk

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