All Possible Combinations of a list of Values

Question

I have a list of integers in my C# program. However, I know the number of items I have in my list only at runtime.

Let us say, for the sake of simplicity, my list is {1, 2, 3} Now I need to generate all possible combinations as follows. {1, 2, 3} {1, 2} {1, 3} {2, 3} {1} {2} {3}

Can somebody please help with this?

Answer 1

Here's a generic solution using recursion

public static ICollection<ICollection<T>> Permutations<T>(ICollection<T> list) {
    var result = new List<ICollection<T>>();
    if (list.Count == 1) { // If only one possible permutation
        result.Add(list); // Add it and return it
        return result;
    }
    foreach (var element in list) { // For each element in that list
        var remainingList = new List<T>(list);
        remainingList.Remove(element); // Get a list containing everything except of chosen element
        foreach (var permutation in Permutations<T>(remainingList)) { // Get all possible sub-permutations
            permutation.Add(element); // Add that element
            result.Add(permutation);
        }
    }
    return result;
}

I know this is an old post, but someone might find this helpful.

Answer 2

A slightly more generalised version for Linq using C# 7. Here filtering by items that have two elements.

static void Main(string[] args)
{
    foreach (var vals in Combos(new[] { "0", "1", "2", "3" }).Where(v => v.Skip(1).Any() && !v.Skip(2).Any()))
        Console.WriteLine(string.Join(", ", vals));
}

static IEnumerable<IEnumerable<T>> Combos<T>(T[] arr)
{
    IEnumerable<T> DoQuery(long j, long idx)
    {
        do
        {
            if ((j & 1) == 1) yield return arr[idx];
        } while ((j >>= 1) > 0 && ++idx < arr.Length);
    }
    for (var i = 0; i < Math.Pow(2, arr.Length); i++)
        yield return DoQuery(i, 0);
}

Answer 3

What about

static void Main(string[] args)
{
     Combos(new [] { 1, 2, 3 });
}

static void Combos(int[] arr)
{
    for (var i = 0; i <= Math.Pow(2, arr.Length); i++)
    {
        Console.WriteLine();
        var j = i;
        var idx = 0;
        do 
        {
            if ((j & 1) == 1) Console.Write($"{arr[idx]} ");
        } while ((j >>= 1) > 0 && ++idx < arr.Length);
    }
}

Answer 4

Here are two generic solutions for strongly typed lists that will return all unique combinations of list members (if you can solve this with simpler code, I salute you):

// Recursive
public static List<List<T>> GetAllCombos<T>(List<T> list)
{
  List<List<T>> result = new List<List<T>>();
  // head
  result.Add(new List<T>());
  result.Last().Add(list[0]);
  if (list.Count == 1)
    return result;
  // tail
  List<List<T>> tailCombos = GetAllCombos(list.Skip(1).ToList());
  tailCombos.ForEach(combo =>
  {
    result.Add(new List<T>(combo));
    combo.Add(list[0]);
    result.Add(new List<T>(combo));
  });
  return result;
}

// Iterative, using 'i' as bitmask to choose each combo members
public static List<List<T>> GetAllCombos<T>(List<T> list)
{
  int comboCount = (int) Math.Pow(2, list.Count) - 1;
  List<List<T>> result = new List<List<T>>();
  for (int i = 1; i < comboCount + 1; i++)
  {
    // make each combo here
    result.Add(new List<T>());
    for (int j = 0; j < list.Count; j++)
    {
      if ((i >> j) % 2 != 0)
        result.Last().Add(list[j]);
    }
  }
  return result;
}

// Example usage
List<List<int>> combos = GetAllCombos(new int[] { 1, 2, 3 }.ToList());

Answer 5

Firstly, given a set of n elements, you compute all combinations of k elements out of it (nCk). You have to change the value of k from 1 to n to meet your requirement.

See this codeproject article for C# code for generating combinations.

In case, you are interested in developing the combination algorithm by yourself, check this SO question where there are a lot of links to the relevant material.

Answer 6

try this:

static void Main(string[] args)
{

    GetCombination(new List<int> { 1, 2, 3 });
}

static void GetCombination(List<int> list)
{
    double count = Math.Pow(2, list.Count);
    for (int i = 1; i <= count - 1; i++)
    {
        string str = Convert.ToString(i, 2).PadLeft(list.Count, '0');
        for (int j = 0; j < str.Length; j++)
        {
            if (str[j] == '1')
            {
                Console.Write(list[j]);
            }
        }
        Console.WriteLine();
    }
}