How do i get all subsets?

[Charlie Goth]

Is there a way of getting all subsets from a Set? I've tried writing it myself but its doing me head in, and making me think I'm not cut out for this programming

Any help would be greatly appreciated.

[Stefan Krompass]

Hi,

are you sure you need the power set of a set? Be aware that the power set will get huge very quickly. For a set with more than 20 to 30 entires, it will definitely not be representable with any normal Java data structure, and you'll run out of memory long, long before then.
But if you're sure that you need the power set, you might try it with recursion...

Stefan

[Charlie Goth]

Yes I need it, its for calculating the winnings of 'canadian' style bets, that is bets on A, B, C, AB, AC, BC & ABC.

[Michael Dunn]

class CombinationSets
{
  String set = "ABC";
  public CombinationSets()
  {
    for(int x = 1; x <= set.length(); x++)
    {
      int[] array = new int[x];
      for(int y = 0; y < x; y++) array[y] = y;
      printSets(array);
    }
  }
  public void printSets(int[] arr)
  {
    String thisCombo = "";
    while(true)
    {
      thisCombo = "";
      for(int x = 0; x < arr.length ; x++) thisCombo += set.charAt(arr[x]);
      System.out.println(thisCombo);
      if(arr[0] == (set.length()-1)-(arr.length-1)) break;
      if(arr[arr.length-1] == set.length()-1)
      {
        for(int i = 0; i < arr.length;i++)
        {
          if(arr[i] == (set.length()-1)-(arr.length-1-i))
          {
            arr[i-1]++;
            for(int ii = i; ii < arr.length; ii++) arr[ii] = arr[ii-1] + 1;
            break;
          }
        }
      }
      else arr[arr.length-1]++;
    }
  }
  public static void main(String[] args) {new CombinationSets();}
}

[David Weitzman]

If you're willing to use iteration here's a technique that often works nicely:

There is a correspondence between binary numbers and subsets of a given set. Suppose I have a set of things {A, B, C}. I'll say that the subset {A, B, C} corresponds to the binary number 111, {A, C} corresponds to 101, {} corresponds to 000. If I've got an ordering of the items I can just say that a 1 means the item is in a set and 0 means it isn't simple enough.

Now if we want to iterate through all subsets of {A, B, C} we can just count the numbers from 000 to 111. Here's what the code might look like (I haven't tested it, so assume there are bugs):

Thing[] set = ...;
 
int numSubsets = 1 << set.length;
 
for (int subset = 0; subset < numSubsets; subset++) {
  int count = 0;
  int temp = subset;
  Set subsetObj = new HashSet();
  while (temp > 0) {
    if (temp & 1) subsetObj.add(set[count]);
    count++;
    temp >>> 1;
  }
}

[ August 08, 2004: Message edited by: David Weitzman ]