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Eldar Nezametdinov
Eldar Nezametdinov

Reputation: 1925

Using Java's built-in Set classes to count EACH(keyword) unique element in List values from a list

Given a list that could contain duplicates (like the one below), I need to be able to count Each(keyword) number of unique elements.

List<String> list = new ArrayList<String>();
Set<String> set = new HashSet<String>();
list.add("M1");
list.add("M1");
list.add("M2");
list.add("M3");

set.addAll(list);
System.out.println(set.size());

How do I get the count each unique element from the List? That means i want to know how many "M1" contains in List(list), how many "M2", etc.

The result should be the following:
2 M1
1 M2
1 M3

Upvotes: 2

Views: 2047

Answers (5)

user2652246
user2652246

Reputation:

Much easier way: use Collections.frequency()

System.out.println("M2: "+Collections.frequency(list,"M2");

will output

M2: 1

Upvotes: 0

morgano
morgano

Reputation: 17422

Set won't help you in this case, you need a Map:

List<String> list = new ArrayList<String>();
Set<String> set = new HashSet<String>();
list.add("M1");
list.add("M1");
list.add("M2");
list.add("M3");

// ...

Map<String, Integer> counts = new HashMap<String, Integer>();
for(String element: list) {
    int currentCount;
    if(counts.contains(element)) {
        currentCount = counts.get(element) + 1;
    } else {
        currentCount = 1;
    }
    counts.put(element, currentCount);
}

// ...

for(String element: counts.keySet()) {
    System.out.println("element: " + element + ", times appeared: " + counts.get(element));
}

Upvotes: 2

Jigar Joshi
Jigar Joshi

Reputation: 240928

You are looking for Map<String, Integer> data structure, not Set

Something like

for(iterating over something){ 
    Integer count =map.get(value); 
    if( count == null){
          map.put(value, 1);


    } else{
        count++;
        map.put(value, count);
    }

}

Map is the data structure that maps unique to value

Upvotes: 5

Mohsin Shaikh
Mohsin Shaikh

Reputation: 504

means You want to know how many "M1" contains in List(list), how many "M2", instead of using set interface, you can use the Map interface because Map contain the key, value pair format ,i.e. Map data structure.

 Map<key,Value>

Upvotes: 0

Rob
Rob

Reputation: 6497

I think you are looking for something like this (I didn't compile it, but it should get you going in the right direction):

List<String> list = ArrayList<>();
Map<String, Integer> counts = new HashMap<>();
// Fill list with values....

for (String item:list) {
    Integer count = counts.get(item);
    if (count == null) {
        // This is the first time we have seen item, so the count should be one.
        count = 1;
    } else {
        // Increment the count by one.
        count = count + 1;
    }
    counts.put(item, count);
}

// Print them all out.
for (Entry<String, Integer> entry : counts.entrySet()) {
    System.out.println(entry.getValue() + " " + entry.getKey());
}

Upvotes: 2

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