Creating a Dictionary Composite Key out of Every P

问题:

I have a Dictionary<string,string> that is acting as the composite key for another Dictionary.

I also have some string[] arrays that contains the necessary strings to be used as the composite key.

For example:

//Dictionary that will use composite key
Dictionary<Dictionary<string, string>, decimal> lookUpDictionary = new Dictionary<Dictionary<string, string>, decimal>();

//The composite key dictionary
Dictionary<string, string> compositeKey = new Dictionary<string, string>();

//Array containing strings
string[] array1 = { "aa", "bb", "cc" };
string[] array2 = { "xx", "yy", "zz" };

//A container for the array of individual units
string[][] arrayOfArrays = {array1,array2};

I would like to make the composite key out of every possible combination pair of strings within each array. For example, I'd like to fill the compositeKey thats a Dictionary<string, string> with every permutation of the array1 string[]...Such as:

aa aa
aa bb
aa cc
bb aa
bb bb
bb cc
cc aa
cc bb
cc cc

and for the second array:

xx xx
xx yy
xx zz
yy xx
yy yy
yy zz
zz xx
zz yy
zz zz

What I have tried is this nested loop algorithm that seems to work for getting the permutations I want in the right order.

Dictionary<string, string> compositeKey = new Dictionary<string, string>();

string[] array1 = { "aa", "bb", "cc" };
string[] array2 = { "xx", "yy", "zz" };

string[][] arrayOfArrays = {array1,array2};


//for every string[] in the arrayofarrays[][]
foreach(string[] array in arrayOfArrays)
{
    //for every string in each of those string[] arrays in the arrayOfArrays[][]
    foreach(string word in array)
    {
        //get every combination including itself
        foreach(string wordsPair in array)
        {
            string[] permutation = { word, wordsPair };
            Console.WriteLine(permutation[0] + permutation[1]);
            try
            {
                //compositeKey.Add(permutation[0], permutation[1]);
                //string value = compositeKey[permutation[0]];
                //Console.WriteLine(value);
            }
            catch
            {

            }


        }

    }
}

The section within the try catch block is where I am having trouble. I can't seem to create the compositeKey correctly. Is this because of the nested loop, or am I using dictionarys completely wrong.

Edit: It would seem that I am trying to use dictionary in a wrong way. Is there any suggestion on the type of datatype that should be used here?

Any advice is appreciated.

回答1:

When creating all possible combinations, you are looking for a Cartesian Join, in your case an array with itself. It can be easily implemented with a help of Linq

  string[] array1 = { "aa", "bb", "cc" };
  //string[] array2 = { "xx", "yy", "zz" };

  string[][] arrayOfArrays = array1
    .SelectMany(left => array1, (left, right) => new string[] { left, right })
    .ToArray();

Test:

  string test = string.Join(Environment.NewLine, arrayOfArrays
    .Select(line => $"[{string.Join(", ", line)}]"));

  Console.WriteLine(test);

Outcome:

[aa, aa]
[aa, bb]
[aa, cc]
[bb, aa]
[bb, bb]
[bb, cc]
[cc, aa]
[cc, bb]
[cc, cc]

Please, notice, that you shouldn't use string[] as a key in a dictionary since array's implementation doesn't override GetHashCode and Equals:

   // Don't do this! Do not use string[] as a key...
   Dictionary<string[], string> demo = new Dictionary<string[], string>() {
     {new string[] {"a", "b"}, "c"},
   };

   string[] key = new string[] {"a", "b"};     

   // ... And that's the reason why:
   if (!demo.ContainsKey(key))
     Console.WriteLine("Oops!");

You may want to choose Tuple<string, string> instead:

  string[] array1 = { "aa", "bb", "cc" };

  Dictionary<Tuple<string, string>, string> dictionary = array1
    .SelectMany(left => array1, (left, right) => new Tuple<string, string>(left, right))
    .ToDictionary(item => item, 
                  item => "value for " + string.Join(" & ", item.Item1, item.Item2));

  Console.WriteLine(string.Join(Environment.NewLine, dictionary));

Outcome:

[(aa, aa), value for aa & aa]
[(aa, bb), value for aa & bb]
[(aa, cc), value for aa & cc]
[(bb, aa), value for bb & aa]
[(bb, bb), value for bb & bb]
[(bb, cc), value for bb & cc]
[(cc, aa), value for cc & aa]
[(cc, bb), value for cc & bb]
[(cc, cc), value for cc & cc]