Generating the combinations of a “choose” operation

问题

I want to perform a variation of a "6 choose 2" operation. I have written the following code.

public void choosePatterns(){
    String[] data = {"1", "2", "3", "4", "5", "6"};
    String[] originalPattern = new String[15];
    int index = 0;
    for(int i = 0; i < (6-1); i++)
    {
        for(int j = i+1; j < 6; j++)
        {
            System.out.println(i + "," +j);
        }
    }
}

My code so far generates all the possible combinations of "6 choose 2." However, I would like to vary this and print all the remaining numbers. So far instance, if one of the combinations of "6 choose 2" is "3" and "4," then I would like to print "1," "2," "5," "6."

I am not sure how to most efficiently do this. The long-winded way would be to delete those indices in the "data" array, shift the data so there aren't gaps, then print the array. But is there a faster, more efficient way to do this?

回答1:

Here's a way to do it.

public static void choosePatterns(int total, int toChoose) {
  List<Integer> indices = IntStream.range(1, toChoose + 1).mapToObj(i -> Integer.valueOf(0)).collect(Collectors.toList());
  resetIndex(indices, 0, 1, total);
  while (true) {
    System.out.println("chosen: " + indices);
    System.out.print("not chosen: ");
    for (int i = 1; i <= total; i++) {
      if (! indices.contains(Integer.valueOf(i))) {
        System.out.print(i + " ");
      }
    }
    System.out.println("\n");

    if (! incrementIndices(indices, indices.size() - 1, total)) {
      break;
    }
  }
}

public static boolean resetIndex(List<Integer> indices, int posn, int value, int total) {
  if (value <= total) {
    indices.set(posn, value);
    return posn == indices.size() - 1 ? true : resetIndex(indices, posn + 1, value + 1, total);
  } else {
    return false;
  }
}

public static boolean incrementIndices(List<Integer> indices, int posn, int total) {
  if (indices.get(posn) < total) {
    indices.set(posn, indices.get(posn) + 1);
  } else {
    int resetPosn = posn;
    do {
      if (resetPosn-- == 0) return false;
    } while (! resetIndex(indices, resetPosn, indices.get(resetPosn) + 1, total));
  }
  return true;
}

public static void main(String[] args) throws IOException {
  choosePatterns(6, 2);
}

It prints this:

chosen: [1, 2]
not chosen: 3 4 5 6 

chosen: [1, 3]
not chosen: 2 4 5 6 

chosen: [1, 4]
not chosen: 2 3 5 6 

chosen: [1, 5]
not chosen: 2 3 4 6 

chosen: [1, 6]
not chosen: 2 3 4 5 

chosen: [2, 3]
not chosen: 1 4 5 6 

chosen: [2, 4]
not chosen: 1 3 5 6 

chosen: [2, 5]
not chosen: 1 3 4 6 

chosen: [2, 6]
not chosen: 1 3 4 5 

chosen: [3, 4]
not chosen: 1 2 5 6 

chosen: [3, 5]
not chosen: 1 2 4 6 

chosen: [3, 6]
not chosen: 1 2 4 5 

chosen: [4, 5]
not chosen: 1 2 3 6 

chosen: [4, 6]
not chosen: 1 2 3 5 

chosen: [5, 6]
not chosen: 1 2 3 4 

回答2:

I changed to using an array of ints, and just added 1 to the values for display purposes:

public static void choosePatterns(){
    int[] display = new int[6];
    for(int i = 0; i < (6-1); i++)
    {
        for(int j = i+1; j < 6; j++)
        {
            display[0] = i+1;
            display[1] = j+1;
            int x = 2;
            for(int k = 0; k < 6; k++)
            {
                if((k != i) && (k != j))
                    display[x++] = k+1;
            }
            System.out.println(Arrays.toString(display));
        }
    }
}

回答3:

Well, since you want [6 choose X] to become dynamic (X = 2, 3, 4,..) and don't want to add a specific number of for-loops for every case, I think recursion should do the trick.

Let's see following sample code:

public void choosePatterns(){
    String[] data = {"1", "2", "3", "4", "5", "6"};
    int deep = 3; // << This value is the X above
    deep = deep > data.length ? data.length : deep; // << prevent X over data length
    printCombine(data, new ArrayList<Integer>(), deep); // Main business here
}

printCombine method content:

private void printCombine(final String[] data, final List<Integer> selectedIdxs, final int deep) {
    if(deep == 1) {  // When come to the last combine number, print out
        StringBuilder sb = new StringBuilder();
        for(int i : selectedIdxs) {
            sb.append(data[i]);
            sb.append(", ");
        }
        String prefixCombine = sb.toString();

        for(int i = 0; i < data.length; i++) {
            if(!selectedIdxs.contains(i)) {
                System.out.println(new StringBuilder(prefixCombine).append(data[i]).toString());
            }
        }
    } else {
        for(int i = 0; i < data.length; i++) {
            if(!selectedIdxs.contains(i)) {
                // Mark the selected indices of the combination
                List<Integer> curSelectedIdx = new ArrayList<Integer>();
                curSelectedIdx.addAll(selectedIdxs);
                curSelectedIdx.add(i);
                printCombine(data, curSelectedIdx, deep - 1);
            }
        }
    }
}

来源：https://stackoverflow.com/questions/39400073/generating-the-combinations-of-a-choose-operation