Combination of an string array in c# without repetition

Question

I am new to c# and I need to perform a permutation without repetition for the arrays. For example, Below are the 2 string arrays.

string[] str1 = String[]{"E1","E2","E3"};
string[] str2 = String[]{"E1","E2","E3","E4"};

And I need to generate output for str1 as

{"E1","E2","E3","E1 E2","E1 E3","E2 E3","E1 E2 E3"}

and for str2 as

{"E1","E2","E3","E4","E1 E2","E1 E3","E1 E4","E2 E3","E2 E4",
 "E3 E4","E1 E2 E3","E1 E2 E4","E1 E3 E4","E2 E3 E4","E1 E2 E3 E4"}`

Also, I would like to compare the output of str1 & str2. If any of the results are matching(ex.:E2 E3& E2 E3) then I need to return "Output is matching" but I am stuck on how to proceed with the below coding in c# and the output is repeating and also array length is static. Please guide me.

String[] str1 = { "E1", "E2", "E3" };

List<string> resultedValue = new List<string>();

for (int i = 0; i < str1.Length; i++)
 {
  for (int j = 0; j < str1.Length; j++)
   {
     if (i == j) resultedValue.Add(string.Format("[ {0} ]", str1[i]));
     else resultedValue.Add(string.Format("[ {0} {1} ]", str1[i], str1[j]));
   }
 }
 Console.WriteLine(resultedValue.ToArray());

In other words: please show us your attempt first, then we can help you fix it — Xerillio
– Xerillio, Commented Jun 9, 2020 at 16:48
What you want is all the combinations of all lengths. Check out this blog by Eric Lippert that goes over how to create combinations of one length ericlippert.com/2014/10/13/producing-combinations-part-one — juharr
– juharr, Commented Jun 9, 2020 at 17:09
FYI this is not a duplicate of stackoverflow.com/questions/5128615/c-sharp-string-permutation as the OP has mistakenly used the term permutations, but they actually want combinations. — juharr
– juharr, Commented Jun 9, 2020 at 17:11

NetMage · Accepted Answer · 2020-06-11 00:32:02Z

I couldn't remember where I got this code (possibly translated from another language), and it isn't very efficient, but it also isn't length constrained, but using LINQ, you can get the powerset of an IEnumerable, minus the empty set:

public static class IEnumerableExt {
    public static string Join(this IEnumerable<string> src, string sep) => String.Join(sep, src);

    public static bool IsEmpty<T>(this IEnumerable<T> src) => !src.Any();

    public static IEnumerable<IEnumerable<T>> AllCombinations<T>(this IEnumerable<T> start) {
        IEnumerable<IEnumerable<T>> HelperCombinations(IEnumerable<T> items) {
            if (items.IsEmpty())
                yield return items;
            else {
                var head = items.First();
                var tail = items.Skip(1);
                foreach (var sequence in HelperCombinations(tail)) {
                    yield return sequence; // Without first
                    yield return sequence.Prepend(head);
                }
            }
        }

        return HelperCombinations(start).Skip(1); // don't return the empty set
    }
}

With these extension methods, you can do:

var comboStr1 = str1.AllCombinations().Select(ss => ss.Join(" "));
var comboStr2 = str2.AllCombinations().Select(ss => ss.Join(" "));

var hsComboStr1 = comboStr1.ToHashSet();
var ans = comboStr2.Any(s2combo => hsComboStr1.Contains(s2combo));

to determine if the powersets of str1 and str2 share any members.

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