Java: Simple Combination of a set of element of k order

Question

Hi, I have a set of numbers {'1','13','25','32','49',...}, i want to calculate all possible combinations of this numbers of order k.

Esample1:

set = {'1','5','23','41,'54','63'};
k = 4;

Output1:

1 5 23 41
1 5 23 54
1 5 23 63
1 5 41 54
1 5 41 63
1 5 54 63
1 23 41 54
1 23 41 63
1 23 54 63
1 41 54 63
5 23 41 54
5 23 41 63
5 23 54 63
5 41 54 63
23 41 54 63

---

Example2:

set = {'a','v','f','z'};
k=3;

Output2:

a v f
a v z
a f z
v f z

in Java plaese.

Thank you!

Answer 1

You should be able to find an appropriate algorithm in D.Knuth's The Art of Computer Programming, Volume 4, fascicle 3 - Generating All Combinations, which can be downloaded from his website.

Answer 2

public static void main(String[] args)
{
    java.util.List<Character> chars = Arrays.asList('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h');
    int k = 4;

    java.util.List<java.util.List<String>> result = new ArrayList<List<String>>();

    for (int i = 0; i <= chars.size() - k; i++)
    {
        List<String> column = new ArrayList<String>();
        for (int j = i + k - 1; j < chars.size(); j++)
        {
            StringBuilder sb = new StringBuilder();
            for (int m = i; m < i + k - 1; m++)
            {
                sb.append(chars.get(m)).append(' ');
            }
            sb.append(chars.get(j));
            System.out.println(sb.toString());
            column.add(sb.toString());
        }
        result.add(column);
    }
    String[][] arrayResult = to2dMatrix(result);
    for (String [] c : arrayResult)
    {
        System.out.println(Arrays.toString(c));
    }
}

private static String[][] to2dMatrix(List<List<String>> input)
{
    String[][] result = new String[input.size()][];
    for (int i = 0; i < input.size(); i++)
    {
        String[] s = new String[0];
        result[i] = input.get(i).toArray(s);
    }
    return result;
}

Answer 3

Thank you Boris!

And if i would an String[total_comb][k] as output?

like this:

out = {{'1','2','3'}, {'1','2','4'}, {'1','3','4'},...}

Answer 4

@boris

this is the output:

a b c e
a b c f
a b c g
a b c h
b c d e
b c d f
b c d g
b c d h
c d e f
c d e g
c d e h
d e f g
d e f h
e f g h