How to remove duplicate combinations from a List<string> using LINQ

Question

I'm having a List of String like

List<string> MyList = new List<string>
{ 
    "A-B", 
    "B-A", 
    "C-D", 
    "C-E", 
    "D-C",
    "D-E",
    "E-C",
    "E-D",
    "F-G",
    "G-F"
};

I need to remove duplicate from the List i.e, if "A-B" and "B-A" exist then i need to keep only "A-B" (First entry)

So the result will be like

"A-B"   
"C-D"
"C-E"   
"D-E"
"F-G"

Is there any way to do this using LINQ?

Answer 1

This returns the sequence you look for:

var result = MyList
    .Select(s => s.Split('-').OrderBy(s1 => s1))
    .Select(a => string.Join("-", a.ToArray()))
    .Distinct();

foreach (var str in result)
{
    Console.WriteLine(str);
}

In short: split each string on the - character into two-element arrays. Sort each array, and join them back together. Then you can simply use Distinct to get the unique values.

Update: when thinking a bit more, I realized that you can easily remove one of the Select calls:

var result = MyList
    .Select(s => string.Join("-", s.Split('-').OrderBy(s1 => s1).ToArray()))
    .Distinct();

Disclaimer: this solution will always keep the value "A-B" over "B-A", regardless of the order in which the appear in the original sequence.

Answer 2

Implement IEqualityComparer witch returns true on Equals("A-B", "B-A"). And use Enumerable.Distinct method

Answer 3

You can use the Enumerable.Distinct(IEnumerable<TSource>, IEqualityComparer<TSource>) overload.

Now you just need to implement IEqualityComparer. Here's something for you to get started:

class Comparer : IEqualityComparer<String>
{

    public bool Equals(String s1, String s2)
    {
        // will need to test for nullity
        return Reverse(s1).Equals(s2);
    }

    public int GetHashCode(String s)
    {
        // will have to implement this
    }

}

For a Reverse() implementation, see this question

Answer 4

You need to implement the IEqualityComparer like this:

public class CharComparer : IEqualityComparer<string>
{
    #region IEqualityComparer<string> Members

    public bool Equals(string x, string y)
    {
        if (x == y)
            return true;

        if (x.Length == 3 && y.Length == 3)
        {
            if (x[2] == y[0] && x[0] == y[2])
                return true;

            if (x[0] == y[2] && x[2] == y[0])
                return true;
        }

        return false;
    }

    public int GetHashCode(string obj)
    {
        // return 0 to force the Equals to fire (otherwise it won't...!)
        return 0;
    }

    #endregion
}

The sample program:

class Program
{
    static void Main(string[] args)
    {
        List<string> MyList = new List<string>
        { 
            "A-B", 
            "B-A", 
            "C-D", 
            "C-E", 
            "D-C",
            "D-E",
            "E-C",
            "E-D",
            "F-G",
            "G-F"
        };

        var distinct = MyList.Distinct(new CharComparer());
        foreach (string s in distinct)
            Console.WriteLine(s);

        Console.ReadLine();
    }
}

The result:

"A-B"   
"C-D"
"C-E"   
"D-E"
"F-G"

Answer 5

Very basic, but could be written better (but it's just working):

class Comparer : IEqualityComparer<string>
  {
      public bool Equals(string x, string y)
      {
          return (x[0] == y[0] && x[2] == y[2]) || (x[0] == y[2] && x[2] == y[0]);
      }

      public int GetHashCode(string obj)
      {
          return 0;
      }
  }

var MyList = new List<String>
{ 
    "A-B", 
    "B-A", 
    "C-D", 
    "C-E", 
    "D-C",
    "D-E",
    "E-C",
    "E-D",
    "F-G",
    "G-F"
}
.Distinct(new Comparer());

foreach (var s in MyList)
{
    Console.WriteLine(s);
}

Answer 6

int checkID = 0;
while (checkID < MyList.Count)
{
 string szCheckItem = MyList[checkID];
 string []Pairs = szCheckItem.Split("-".ToCharArray());
 string szInvertItem = Pairs[1] + "-" + Pairs[0];
 int i=checkID+1;
 while (i < MyList.Count)
 {
  if((MyList[i] == szCheckItem) || (MyList[i] == szInvertItem))
  {
   MyList.RemoveAt(i);
   continue;
  }
  i++;
 }

 checkID++;
}