CODEWITHSUNDEEP
java
Firefly
Firefly

Reputation: 428

How to create an item set for items of a map?

I have a Map<String, Long> that looks something like:

{A=1, B=2, C=3, D=4, E=3, F=2, G=1}

I want to create a new List<Set<String>> that gets a key from the previous map and create all possible pairings with the other keys as Set<String> and put it into the created List<String> so that it would look something like:

[A,B], [A,C], [A,D], [A,E], [A,F], [A,G], [B,C], [B,D], [B,E], [B,F], [B,G],
[C,D], [C,E], [C,F], [C,G], [D,E], [D,F], [D,G], [E,F], [E,G], [F,G]

What is a good way for me to do it?

Upvotes: 1

Views: 195

Answers (2)

Abhijit Sarkar
Abhijit Sarkar

Reputation: 24593

Using Java 11 collection methods and Streams:

var n = map.size();
var keys = new ArrayList<>(map.keySet());

var combinations = IntStream
    .range(0, n)
    .boxed()
    .flatMap(i -> IntStream
    .range(i + 1, n).mapToObj(j -> Set.of(keys.get(i), keys.get(j))))
    .collect(Collectors.toList());
System.out.println(combinations);

[[B, A], [C, A], [D, A], [E, A], [F, A], [G, A], [C, B], [D, B], [E, B], [F, B], [G, B], [D, C], [E, C], [F, C], [G, C], [E, D], [F, D], [G, D], [F, E], [G, E], [G, F]]

Upvotes: 3

kabanus
kabanus

Reputation: 25980

Without worrying too much about efficiency:

List<String> keys = new ArrayList<String>(myMap.keySet());
List<Set<String>> keyPairs = new ArrayList<>();
for (int i = 0; i < keys .size(); i++)
    for (int j = i + 1; j < keys .size(); j++)
        keyPairs.add(Set.of(keys[i], keys[j]))

Upvotes: 5

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