How to use LINQ to select some fields with total combination from a set items.

Question

Hello:
I have this requirements, the following is the data:
var A1 = ("A", "EUR/USD", "EURO");
var A2 = ("A", "CHF/USD", "CHF");
var A3 = ("A", "EUR/GBP", "EURO");
var A4 = ("B", "EUR/USD", "EURO");
var A5 = ("B", "CHF/USD", "CHF");
var A6 = ("B", "EUR/GBP", "EURO");
var A7 = ("C", "EUR/USD", "EURO");
var A8 = ("C", "CHF/USD", "CHF");
List<(string, string, string)> forex_pairs = new()
{
A1, A2, A3, A4, A5, A6, A7, A8
};
I want to select the first item from the list of tuples, where the last 2 items are in the same group, also I want to list all the combinations of the first item.
In the above example, I want to get the following results:
Group#1: ("EUR/USD", "EURO") have the following List<(string, string)> members: List<("A", "B"), ("A", "C"),("A", "C"));
Group#2: ("CHF/USD", "CHF") have the following List<("A", "B"), ("A", "C"),("A", "C"));
Group#3: ("EUR/GBP", "EURO") have the following List<("A", "B"));

Answer 1

@john zyd, Welcome to Microsoft Q&A,

First, we need to use Groupby to group by the last 2 items.

var result = forex_pairs.GroupBy(x => new { x.Item2, x.Item3 })

Second, please use tolookup method to select the correspond item1.

Select(m => m.ToLookup(p => p.Item1.ToString())

Third, we need to select the final key.

Select(m => m.Select(n => n.Key)).ToList()

Finally, we could use the following code to get Permutations.

static IEnumerable<IEnumerable<T>> GetPermutations<T>(IEnumerable<T> items, int count)
        {
            int i = 0;
            foreach (var item in items)
            {
                if (count == 1)
                    yield return new T[] { item };
                else
                {
                    foreach (var result in GetPermutations(items.Skip(i + 1), count - 1))
                        yield return new T[] { item }.Concat(result);
                }

                ++i;
            }
        }

Here is the completed code example:

static void Main(string[] args)
        {
            var A1 = ("A", "EUR/USD", "EURO");
            var A2 = ("A", "CHF/USD", "CHF");
            var A3 = ("A", "EUR/GBP", "EURO");
            var A4 = ("B", "EUR/USD", "EURO");
            var A5 = ("B", "CHF/USD", "CHF");
            var A6 = ("B", "EUR/GBP", "EURO");
            var A7 = ("C", "EUR/USD", "EURO");
            var A8 = ("C", "CHF/USD", "CHF");
            var forex_pairs = new List<(string, string, string)>(){
              A1, A2, A3, A4, A5, A6, A7, A8};
            var result = forex_pairs.GroupBy(x => new { x.Item2, x.Item3 }).Select(m => m.ToLookup(p => p.Item1.ToString())).Select(m => m.Select(n => n.Key)).ToList(); ;
            foreach (var item in result)
            {
                var result1 = GetPermutations(item, 2);
                foreach (var liststr in result1)
                {
                    foreach (var str in liststr)
                    {
                        Console.WriteLine(str); 
                    }
                    Console.WriteLine("-----");
                }
                Console.WriteLine("***");


            }

        }
        static IEnumerable<IEnumerable<T>> GetPermutations<T>(IEnumerable<T> items, int count)
        {
            int i = 0;
            foreach (var item in items)
            {
                if (count == 1)
                    yield return new T[] { item };
                else
                {
                    foreach (var result in GetPermutations(items.Skip(i + 1), count - 1))
                        yield return new T[] { item }.Concat(result);
                }

                ++i;
            }
        }

Result:

Best Regards,

Jack

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