[FIXED] How to generate combinations from char array?

Issue

How to generate combinations from char array? here is my code https://anotepad.com/notes/icjsc5ct I have an array with 5 characters: a, b, c, d, e And I want to generate combinations of 3 characters

1. [a, b, c];
2. [a, b, d];
3. [b, c, d];
4. [b, c, e];
5. [c, d, e]

My code just can generate 2 combinations of 3 character:

1. [a, b, c];
2. [a, b, d];

I don't know how to increase index of Array[0] and Array[1];

public ArrayList< char[] > generate02( int r ) {

    ArrayList< char[] > combinationsList = new ArrayList<>();
    char[] data = new char[ r ];
    // initialize with lowest lexicographic combination
    for ( int i = 0; i < r; i++ ) {
        data[ i ] = CharArray01[ i ];
    }
    PrintData( CharArray01 );
    int n = CharArray01.length;
    while ( IndexInt( data[ r - 1 ] ) < n - 1 ) {
        int t01 = r - 1;
        System.out.println( " IndexInt( data[ r - 1 ] ) < n " );
        System.out.println( " data[ r - 1 ] ) = " + data[ t01 ]
                + ";  IndexInt( data[ r - 1 ] ) = "
                + IndexInt( data[ r - 1 ] )
                + ";  n = " + n );
        combinationsList.add( data.clone() );
        // generate next combination in lexicographic order
        int t02 = n - r + t01;
        while ( t01 != 0 && IndexInt( data[ t01 ] ) == t02 ) {
            t01--;
        }
        int k1 = IndexInt( data[ r - 1 ] );
        int k2 = k1 + 1;
        data[ r - 1 ] = IndexChar( k2 );
        System.out.println( " data[ r - 1 ] ) = " + data[ t01 ]
                + ";  IndexInt( data[ r - 1 ] ) = "
                + IndexInt( data[ r - 1 ] ) );
        System.out.println( "t01 = " + t01 + ";  n = " + n );
        int i = 0;
        for ( i = t01 + 1; i < r; i++ ) {
            int index02 = IndexInt( data[ i - 1 ] );
            int index03 = index02 + 1;
            data[ i ] = data[ index03 ];
        }
    }
    return combinationsList;
}
```

• Function PrintData convert a char array into String and Display it for my debug

  public void PrintData( char[] CharArray02 ) {
      int length = CharArray02.length;
      String string01 = "";
      for ( int i = 0; i < length; i++ ) {
          string01 = string01 + CharArray02[ i ] + "  ";
      }
      System.out.println( "PrintData:  String01 = " + string01 );
  }
  
  
  

• Function IndexInt return position of char (start from 0) in sample string "abcde"

  public int IndexInt( char char01 ) {
      int result = 0;
      String string01 = "abcde";
      int length = string01.length();
      for ( int i = 0; i < length; i++ ) {
          if ( char01 == (char) string01.charAt( i ) ) { return i; }
      }
      return result;
  }
  

• Function IndexChar return char at int position in sample string "abcde"

  public char IndexChar( int index01 ) {
      char result = 'a';
      String string01 = "abcde";
      result = string01.charAt( index01 );
      return result;
  }
  

• Function public ArrayList<char[]> generate02(int r) is the copy and modified code from algorithm integer combination. I really don't understand how the algorithm work

Solution

If it is the solution/example you are are looking for I can refer you to an API I have created to generate combinations of objects in a memory efficient way: https://github.com/3venthorizon/meta/blob/master/meta-mathics/src/main/java/com/devlambda/meta/mathics/CombinationGenerator.java

@Test
public void charComboTest() {
   List<Character> domain = Arrays.asList('a', 'b', 'c', 'd', 'e');
   CombinationGenerator<Character> combinations = new CombinationGenerator<>(domain, 3);
   while (combinations.hasNext()) {
      List<Character> combination = combinations.next();
      System.out.println(combination.stream().map(Object::toString)
            .collect(Collectors.joining(", ", "[", "]")));
   }
}

This prints out the the following result:

[a, b, c]
[a, b, d]
[a, b, e]
[a, c, d]
[a, c, e]
[a, d, e]
[b, c, d]
[b, c, e]
[b, d, e]
[c, d, e]

Answered By - Dewald Pretorius