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I did see this and this post on it, but it is not in Java or Kotlin, so it doesn't help me. Now, coming to the problem, I want to be able to create all the possible combinations for an array. For example, I have this array:

[1,2,3,4]

And then all the possible combinations give us this:

[1,2,3,4]
[1,2,4,3]
[1,3,2,4]
[1,3,4,2]
[1,4,2,3]
[1,4,3,2]

[2,1,4,3]
[2,1,3,4]
[2,3,1,4]
[2,3,4,1]
[2,4,3,1]
[2,4,1,3]

[3,1,2,4]
[3,1,4,2]
[3,2,4,1]
[3,2,1,4]
[3,4,1,2]
[3,4,2,1]

[4,1,2,3]
[4,1,3,2]
[4,2,3,1]
[4,2,1,3]
[4,3,2,1]
[4,3,1,2]

I don't want to have a space between these two different starts or numbers. I just added it for readability.

It should not only work for Integers but also for objects like strings, our classes, etc...

So, how can I achieve this in Java or Kotlin?

I also just came across this* post, but it wanted for two arrays and needed for one. Also, it was closed, so I felt it better to ask another one.

  • The post is now deleted
Sambhav Khandelwal
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  • Quote: "Just let me know what can be better." What does that mean? There is no code in your post. – lukas.j May 31 '22 at 09:27
  • stackoverflow is not a code writing service. Post the code you have developed so far. – lukas.j May 31 '22 at 09:30
  • @lukas.j I don't actually need the entire code. Just a good logic would be good. And, I don't have any idea of what might be used. Why? Becuase I never know the size of the array. Just an example – Sambhav Khandelwal May 31 '22 at 09:32
  • The technical term for this is ‘[permutations](https://en.wikipedia.org/wiki/Permutation)’. ‘[Combinations](https://en.wikipedia.org/wiki/Combination)’ are unordered subsets: for example, there are 6 combinations of 2 from {1, 2, 3, 4}: {1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4}, {3, 4}. – gidds May 31 '22 at 09:33
  • [here](https://stackoverflow.com/questions/2920315/permutation-of-array) you can find the answer of your question. – Black Bear May 31 '22 at 09:50

3 Answers3

2

It seems that you expect to generate all possible permutations (N!), not combinations of given set of objects. Please check Guava's library Collections2.permutations.

It takes collection of elements, and return Collection of Lists of those elements.

public class Permutations {

public static void main(String[] args) {
    List<Integer> myList = List.of(1,2,3,4);
    // generate permutations of myList
    var permutations = Collections2.permutations(myList);
}

}

Krzysztof Strzała
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1

If the task is not to implement the algorithm which generates the permutations you need, I would recomend to use a library like combinatoricslib3. If you happen to use Guava or Apache Commons in your projects, those have also some methods to generate combinations / permutations from a given collection. With combinatoricslib3 your code could look something like:

import org.paukov.combinatorics3.Generator;

public class Example {

    public static void main(String[] args) {
        Integer[] array = {1,2,3,4};
        Generator.permutation(array)
                .simple()
                .stream()
                .forEach(System.out::println);

        //Example with strings

        Generator.permutation("apple", "orange", "cherry")
                .simple()
                .stream()
                .forEach(System.out::println);
    }
}

output:

[1, 2, 3, 4]
[1, 2, 4, 3]
[1, 4, 2, 3]
[4, 1, 2, 3]
[4, 1, 3, 2]
....

and

[apple, orange, cherry]
[apple, cherry, orange]
[cherry, apple, orange]
[cherry, orange, apple]
[orange, cherry, apple]
[orange, apple, cherry]
Eritrean
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0

Without using a library:

fun List<Any>.permutations(): List<List<Any>> {
  if (isEmpty()) return listOf(emptyList())
  return indices.fold(emptyList()) { result, i ->
    (result + (this - this[i])
      .permutations()
      .fold(mutableListOf()) { acc, item ->
        acc.add(item + this[i])
        acc
      }
    ).toMutableList()
  }
}

listOf(1, 2, 3, 4).permutations().forEach(::println)

listOf("A", "B", "C", "D").permutations().forEach(::println)

data class Test (val name: String)
listOf(Test("A"), Test("B"), Test("C"), Test("D")).permutations().forEach(::println)
lukas.j
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