C# Combinations in a list

Question

Ok, here is the problem that I'm having at the moment: I have a list of elements with which I want to make pairs of all possible combinations.

For example: I have a list of elements "A", "B", "C", "E".

Out of those elements I want to make pairs of all possible combinations (without duplicate elements that already exist while pairing elements), so it would become:

AB

AC

AE

BC

BE

CE

ABC

ABE

ACE

BCE

ABCE

So far, my code doesn't make all combinations like in the example above and it seems to be having a problem with duplicates and I've ran out of ideas how to approach this any further.

    List<char> charList = new List<char> { 'A', 'B', 'C', 'E' };
    List<string> copy = new List<string>();
    List<string> stringList = new List<string>();

    for (int i = 0; i < charList.Count() - 1; i++)
    {
        for (int j = i + 1; j < charList.Count(); j++)
        {
            stringList.Add(charList[i].ToString() + charList[j].ToString());
            copy = stringList.ToList();

        }
    }


    for (int i = 0; i < charList.Count() - 1; i++)
    {
        for (int j = i + 1; j < charList.Count(); j++)
        {
            for (int g = 0; g < stringList.Count(); g++)
            {

                if (!stringList[g].Contains(charList[i]))
                {
                    stringList[g] += charList[i];
                    copy.Add(stringList[g]);
                }
                else if (!stringList[g].Contains(charList[j]))
                {
                    stringList[g] += charList[j];
                    copy.Add(stringList[g]);
                }
            }

        }
    }


    foreach (string value in copy)
    {
        Console.WriteLine(value);
    }

Thanks.

Eric Lippert has a series about permutations, maybe this will help: ericlippert.com/2013/04/15/producing-permutations-part-one — germi
– germi, Commented Nov 21, 2013 at 14:22
Use this stackoverflow.com/questions/5128615/c-sharp-string-permutation — Oleh
– Oleh, Commented Nov 21, 2013 at 14:30

avivr · Accepted Answer · 2013-11-21 14:31:53Z

1

you can use library like this one on CodeProject: http://www.codeproject.com/Articles/26050/Permutations-Combinations-and-Variations-using-C-G

The code will look something like this:

for (int i = 1; i <= charList.Count(); i++)
{
  Combinations<char> combinations = new Combinations<char>(charList , i);
  foreach(IList<char> c in combinations)
  {
     Console.WriteLine("{{{0}}}", String.Join(" ", chars));
  }
}

answered Nov 21, 2013 at 14:31

avivr

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Comments

Vladimir · Accepted Answer · 2013-11-21 14:59:31Z

1

var charList = new List<char> { 'A', 'B', 'C', 'E' };
List<string> stringList = new List<string>();

for (var i = 1; i < Math.Pow(2, charList.Count); i++)
{
    var sb = new StringBuilder();
    for (var j = 0; j < charList.Count; j++)
    {
        int power = (int)Math.Pow(2, j);
        if ((i & power) == power) sb.Append(charList[j]);
    }
    var s = sb.ToString();
    if (s.Length > 1) stringList.Add(sb.ToString());
}

// Sort results.
stringList.Sort((s1, s2) => s1.Length != s2.Length
    ? s1.Length.CompareTo(s2.Length) : s1.CompareTo(s2));

edited Nov 21, 2013 at 14:59

answered Nov 21, 2013 at 14:26

Vladimir

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6 Comments

Paul Sinnema Over a year ago

It doesn't it look like an error to me. Maybe he ment ((i & power) == 1)

Paul Sinnema Over a year ago

Result does not look good AB ABC ABCE ABE AC ACE AE BC BCE BE CE

Vladimir Over a year ago

I tried and got: AB, AC, BC, ABC, AE, BE, ABE, CE, ACE, BCE, ABCE.

Sani Huttunen Over a year ago

@PaulSinnema: Just add sorting and the result will be as expected. stringList = stringList.OrderBy(s => s).OrderBy(s => s.Length).ToList();

Paul Sinnema Over a year ago

Yeah. I added stringlist.Sort() but there are way too little results are there not? I get pages of them

|

Paul Sinnema · Accepted Answer · 2013-11-21 14:32:45Z

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        string[] list = new string[] { "A", "B", "C", "D" };
        List<string> combined = new List<string>();

        for (int i = 0; i < list.Length; i++)
        {
            combined.Add(list[i]);
            for (int j = 0; j < list.Length; j++)
            {
                combined.Add(list[i] + list[j]);
                for (int k = 0; k < list.Length; k++)
                {
                    combined.Add(list[i] + list[j] + list[k]);
                    for (int l = 0; l < list.Length; l++)
                    {
                        combined.Add(list[i]+list[j] + list[k] + list[l]);
                    }
                }
            }
        }

        combined.ForEach(item => Console.WriteLine(item));

        Console.ReadKey();
    }
}

}

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