Get contents from method with unique combination (no repetition)

Question

I have 5 text files, which contain some elements.

1. A (has 1000 elements)
2. B (has 7 elements)
3. C (has 3 elements)
4. D (has 2 elements)
5. E (has 1 element)

Example of contents inside A:

9864902,9864892,9864891,9864890,9864889,9864888,9864887,9864886,9864885,9864884,9864883,9864882,9864881,9864880,9864879,9864878,9864877,9864876,9864873,9864872,9864870,9864869,9864868,9864867,9864866,9864865,9864864,9864863,9864862,9864861,9864858,9864857,9864856,9864855,9864854,9864853,9864852,9864851,9864850,9864849,9864848,9864847,9864846,9864845,9864844,9864843,9864842,9864841,9864840,9864839,9864838,9864837,9864836,9864835,9864834,9864833,9864832,9864831,9864830,9864829,9864828,9864827,9864826,9864825,9864824,9864823,9864822,9864821,9864820,9864819,9864818,9864817,9864816,9864815,9864814,9864813,9864812,9864811,9864810,9864809,9864808,9864807,9864806,9864805,9864804,9864803,9864802,9864801,9864800,9864799,9864798,9864797,9864796,9864795,9864794,9864793,9864792,9864791,9864790,9864789,9864788,9864787,9864786,9864785,9864784,9864783,9864782,9864781,9864780,9864779,9864778,9864777,9864776,9864775,9864774,9864773,9864772,9864771,9864770,9864769,9864768,9864767,9864766,9864765,9864764,9864763,9864762,9864761

(and many many more, about 18k, but now I wish to get 1st 1000 only.)

here is one of the method to get contents inside text file.

public string getA(int index)
{
    // Contains 1000 elements
    string[] Array_A = File.ReadAllLines("D:\\user\\A.txt");
    string[] A = null;
    for (int i = 0; i < Array_A.Length; i++)
    {
       A = Array_A[i].Split(',');
    } 
    return A[index].ToString();
}

Other methods are same writing as above. Just change the 'A' to other name.

Now I want to write another method to get elements which combining all of elements in text files.

public string[] UniqueCombination(int number)
{
   // Write the code here
}

For example, UniqueCombination(7), I need to get 7 unique combinations.

A(0) B(0) C(0) D(0) E(0)

A(0) B(0) C(0) D(1) E(0)

A(0) B(0) C(1) D(0) E(0)

A(0) B(0) C(1) D(1) E(0)

A(0) B(0) C(2) D(0) E(0)

A(0) B(0) C(2) D(1) E(0)

A(0) B(3) C(0) D(0) E(0)

(Above is the example of combinations, because E just has only 1 element, so always return first position element.)

However, for this case, I would like to get thousands, might up to 5 digits(20,000) combinations. How to do that? I have already wrote another method for random index. But for this case I have no idea.

(p.s the sequence of the elements are doesn't matter, just to promise every combination is unique)

Answer 1

int size = 7;
// read the text and create lists for each file 
var listA = ReadFileToList("D:\\user\\A.txt");
var listB = ReadFileToList("D:\\user\\B.txt");
var listC = ReadFileToList("D:\\user\\C.txt");
var listD = ReadFileToList("D:\\user\\D.txt");

// get the combinations from lists 
var combinations = (from a in listA
                   from b in listB
                   from c in listC
                   from d in listD
                   select new[] { a, b, c, d })
                   .Take(size)
                   .ToList();

if you need randomly select items

var randomCombinations = (from a in listA
               from b in listB
               from c in listC
               from d in listD
               select new[] { a, b, c, d })
               .OrderBy(n => Guid.NewGuid())
               .Take(size)
               .ToList();

You need helper method like below

    public List<string> ReadFileToList(string path)
    {
        List<string> temp = new List<string>();
        using (StreamReader r = new StreamReader(path))
        {
            string line;
            if ((line = r.ReadLine()) != null)
            {
                temp =line.Split(',').ToList();
            }
        }
        return temp;
    }

UPDATE

you can print result as below

Console.WriteLine(string.Join("|",
             randomCombinations.Select(c => string.Join(",", c))));

Result will show something like below

A,D,E,G|A,D,F,G|B,D,F,G|B,C,E,G|A,C,E,G|A,C,F,G|B,C,F,G

OR Extending your code as below

foreach (var item in combinations) { Console.WriteLine(string.Join(",", item)); }