C#:Conversion of Dictionary<String,String> to Dictionary<String, Dictionary<String,String>>

Question

The First Dictionary is like

        Dictionary<String, String> ParentDict = new Dictionary<String, String>();
        ParentDict.Add("A_1", "1");
        ParentDict.Add("A_2", "2");
        ParentDict.Add("B_1", "3");
        ParentDict.Add("B_2", "4"); 
        ParentDict.Add("C_1", "5");

i need to convert this into a new Dictionary<String, Dictionary<String,String>>

The result will contain

Key                    Value

              Key                   Value             
_________________________________________________

"A"             "A_1"                    "1"
                "A_2"                    "2"

"B"             "B_1"                    "1"
                "B_2"                    "2"

"C"             "C_1"                    "1"

Now i'm using nested for loop to do this.

How can i do this using LNQor LAMBDA Expression?

Answer 1

Try:

var result = from p in ParentDict
             group p by p.Key[0] into g
             select new { Key = g.Key, Value = g };

This should give you a list of { Key, Value }, where Key will be "A", "B", "C", etc, and Value will be an original instance of KeyValuePair from ParentDict.

You can find more LINQ sample queries on this MSDN page: 101 Linq Samples

Answer 2

The reason to do this I suspect is because you need to be able to lookup all entries for a particular key-letter. In that case, a Lookup is a better match, generally:

var letterLookup = ParentDict.ToLookup(kv=>kv.Key[0]);

Usable like this:

//letterLookup['A'] is an IEnumerable<KeyValuePair<string,string>>...

Console.WriteLine(string.Join(", ",
        letterLookup['A'].Select(kv=>kv.ToString()).ToArray()
    )); // [A_1, 1], [A_2, 2]

Console.WriteLine(new XElement("root",
        letterLookup['B'].Select(kv=>new XElement(kv.Key,kv.Value))
    ));// <root><B_1>3</B_1><B_2>4</B_2></root>


Console.WriteLine(letterLookup['B'].Any()); //true
Console.WriteLine(letterLookup['Z'].Any()); //false

The advantage of a lookup over a dictionary is that it may contain multiple values for any given key (unlike a dictionary), and that it has a consistent API if a certain key is absent: it then returns the empty enumerable, whereas a dictionary containing enumerables might either throw KeyNotFoundException, or return null, or return the empty enumerable, all depending on how you created it.

Answer 3

 var result = ParentDict.GroupBy(p => p.Key[0].ToString())
                        .ToDictionary(g => g.Key, g => g.ToDictionary(x => x.Key, x => x.Value));