Counting Occurences of int an array using List

Question

So I'm trying to make a program that counts the occurences of int in an array. what i tried to do is to make a method that lists the unique integers, then another method to compare the list items to the original array items.

public List listUnique(int[] arr){
    Arrays.sort(arr);
    List <Integer> temp = new ArrayList<>();
    int currentInt = 0;
    for (int i = 0; i < arr.length; i++){
        if(arr[i] != currentInt){
            temp.add(arr[i]);
            currentInt = arr[i];
        }
    }
    return temp;
}
public int[] countDupli(List unique, int[] arr){
    int [] ret = new int[unique.size()];
    Iterator <Integer> iterator = unique.iterator();
    for (int l = 0; l < unique.size(); l++){
        ret[l] = iterator.next().intValue(); 
    }   
    int[] dupli = new int[ret.length];
    for (int j = 0; j < ret.length; j++){
        dupli[j] = 0;
    }
    for (int k = 0; k < ret.length; k++){
        for (int i = 0; i < arr.length; i++){
            if ( ret[k] == arr[i]){
                dupli[k]+= 1;
            }
        }
        k++;
    }
return dupli;
}

It isn't doing what is intended to do tho. For example, an input of {1,2,...,1,2} 10 items of those, prints the correct unique items but only outputs the count of 1 but not 2. dupli = [5,0]. where did the algorithm go wrong? thanks

This is the perfect opportunity to look into it. Debugging gives you the tools to understand your own code, which you're in a much better position to do than anyone else. — shmosel
– shmosel, Commented Mar 8, 2017 at 23:52
Thank you everyone, I found that I incremented my k-index twice in a row. my bad. thanks for the help. I'll definitely look into debugging. — Paul Steven Fantonalgo Nadera
– Paul Steven Fantonalgo Nadera, Commented Mar 8, 2017 at 23:57

Sandesh Nimhan · Accepted Answer · 2017-03-09 01:00:56Z

0

In your code try debugging or just print ret[] values and see if all unique values are present.

Other Suggestions:

List <Integer> temp = new ArrayList<>();
        int currentInt = 0;
        for (int i = 0; i < arr.length; i++){
            if(arr[i] != currentInt){
                temp.add(arr[i]);
                currentInt = arr[i];
            }

You can simply use HashSet, then you don't have to write above code to find unique values. HashSet does not store duplicate values.

 Set<Integer> set = new HashSet<Integer>(Arrays.asList(arr));

Then you can use iterator and compare with your sorted array and increase count simultaneously.

You can skip the following for loop, as java initializes it to 0 by default.

    int[] dupli = new int[ret.length];
    for (int j = 0; j < ret.length; j++){
        dupli[j] = 0;
    }

answered Mar 9, 2017 at 1:00

Sandesh Nimhan

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