Deleting Duplicate Array Entries

Question

I'm trying to use a Scanner to read in a text file with fraction Strings of the form "a/b", store them in an array, then print to the console each unique one along with its count. I can print each separate entry in the text file with an accurate count, but I can't figure out how to print the contents of the array without any duplicates.

I'm also trying to accomplish this entirely within main. Here's what I've got so far:

public static void main(String[] args) throws FileNotFoundException
{

        ArrayList<String> fracs = new ArrayList<String>();
        Scanner input = new Scanner(new File("input.txt"));
        ArrayList<String> fracsDisplay = new ArrayList<String>();

        while(input.hasNext())
            fracs.add(input.next());

        for(int i = 0; i < fracs.size(); i++)
        {
            int count = 0;
            for(int j = 0; j < fracs.size(); j++)
            {
                if(fracs.get(i).equals(fracs.get(j)))
                    count++;
            }
            System.out.println(fracs.get(i) + ": " + count);
        }

        input.close();
}

hmm, you could before adding the elements check for fracs.contains(...) if the array contains already an element and if so just increment the counter or avoid adding the element again. A Set is preventing duplicate entries by default. And why don't you use a Map if you want to print the entry-name as well as its count? Lookup the count (=value) via the unique entry (=key) — Roman Vottner
– Roman Vottner, Commented Jan 12, 2014 at 2:21

jamie-owen · Accepted Answer · 2014-01-12 02:34:10Z

1

The easiest way to get rid of duplicates would be to use a Set, as you are storing Strings duplicates won't be added.

Instead of ArrayList<String> fracsDisplay = new ArrayList<String>(); you could use:

Set<String> fracsDisplay = new HashSet<>();
fracsDisplay.addAll(fracs);
for (String s : fracsDisplay) {
    System.out.println(s);
}

if some kind of order needs to be maintained then I would suggest a TreeSet, but either will handle the removal of duplicates... which seems to be what you want in this case.

edited Jan 12, 2014 at 2:34

answered Jan 12, 2014 at 2:28

jamie-owen

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2 Comments

Roman Vottner Over a year ago

disadvantage if the entries should be available in an ordered structure

jamie-owen Over a year ago

True, in this case a TreeSet could be used, OP didn't state whether order was important or not... answer updated to reflect your suggestion!

Zhou Tai · Accepted Answer · 2014-01-12 02:42:35Z

If a grasp it, this task is a "Word Count"-like task. To do such a task, HashMap is the best choice. I suggest you use a HashMap to do this task. Your algorithm has a O(n^2) time complexity. You can use a HashMap to reduce it down to O(n). Using a HashMap also helps you deduplicate. Hava a try.

        HashMap<String, Integer> fracs = new HashMap<String, Integer>();
        Scanner input = new Scanner(new File("input.txt"));

        while(input.hasNext()) {
            String frac = input.next();
            if (! fracs.containsKey(frac)) {
                fracs.put(frac, 1);
            } else {
                fracs.put(frac, fracs.get(frac) + 1);
            }
        }

        input.close();

        for (Map.Entry<String, Integer> fracCount : fracs.entrySet()) {
            System.out.println(fracCount.getKey() + " " + fracCount.getValue());
        }

If fraction strings should keep the order of occurance, use LinkedHashMap; If fraction String should keep the lexicographical order, or some spercific order, use SortedHashMap. Here I suggest you read "Java Generics and Collections" which focus on Java Collection framework.

nmargaritis · Accepted Answer · 2014-01-12 03:34:44Z

0

Should you store everything into an ArrayList? You could use either a HashSet or a TreeSet since dublications there are not allowed.

edited Jan 12, 2014 at 3:34

answered Jan 12, 2014 at 2:27

nmargaritis

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Comments

rohan-patel · Accepted Answer · 2014-01-12 03:49:13Z

0

You can use HashSet as Set collection will not allow duplicate values. If your order of array is important then you can use LinkedHashSet.

Set<T> set = new HashSet<T>();
Collections.addAll(set, array);

answered Jan 12, 2014 at 3:49

rohan-patel

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