Question 88080 in ASP.NET Question posted in
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How to search for the keys from dictionary of <int, List<string>> – Asp.net

I have dictionary Dictionary<int, List<string>> taskList = new Dictionary<int, List<string>>();

which gives me the out like:

Task ID: 1664003         Values:
                        "2"
                        "5"
                        "1"
                        "4"
                        "3"

Task ID: 1664004         Values:
                        "1"
                        "2"
                        "3"
                        "5"
                        "4"

Task ID: 1664005         Values:
                        "1"
                        "2"
                        "5"
                        "4"
                        "3"

Now I want to search for keys of zero index of pair value like below:

Values: "2"     Task Id: 1664003
Value: "1"      Task Id: 1664004, 1664005

I want to achieve it using lambda expression

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3

Answers


  1. 0

    You’ll need to iterate over every entry of the dictionary, get the first item of the list and compare it to what you’re looking for, like so:

    var search = 1;
var dictionary = // your dictionary
var output = new List<int>();

foreach (var kvp in dictionary)
{
    if (kvp.Value.First() == search)
        output.Add(kvp.Key);
}

Console.WriteLine($"The following tasks have the value {search} in the dictionary:");
foreach (var result in output)
    Console.WriteLine(result);

    P.S You might want to rethink your data structure, as this seems like a misuse of a dictionary

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  2. 0 
                Dictionary<int, List<string>> taskList = new Dictionary<int, List<string>>();
            List<string> list1 = new List<string>() { "2", "5", "1", "4", "3" };
            List<string> list2 = new List<string>() { "1", "2", "3", "5", "4" };
            List<string> list3 = new List<string>() { "1", "2", "5", "4", "3" };
            taskList.Add(1664003, list1);
            taskList.Add(1664004, list2);
            taskList.Add(1664005, list3);

            string search = "1";

            var result = new List<int>();

            foreach (var kvp in taskList)
            {
                if (kvp.Value[0] == search)
                    result.Add(kvp.Key);
            }

            Console.WriteLine($"Output: {string.Join(", ", result)}");

    Output:

    Output: 1664004, 1664005
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  3. 0

    Your dictionary is the wrong way round for this kind of search. Consider creating a dictionary that is the right way round, then search it many times

    var d = new Dictionary<int, List<string>>()
{
   { 1664003, new List<string>() {"2","5","1","4","3"}},
   { 1664004, new List<string>() {"1","2","3","5","4"}},
   { 1664005, new List<string>() {"1","2","5","4","3"}}
};

//invert the dictionary (only take first element of each list)
var dRev = d.GroupBy(kvp => kvp.Value.First(), kvp => kvp.Key)
  .ToDictionary(g => g.Key, g => g.ToList());


//now you can search it many times
var x = dRev["1"];     //x is a list containing {1664004,1664005}
var y = dRev["2"];     //y is a list containing {1664003}

    Ideally you can keep your dRev for as long as is reasonably practical; if you have a lot of searches to do in a batch, then rebuilding it makes sense. There’s no point rebuilding it every time you search for a single item, but perhaps consider taking the thing that builds the dictionary you have right now and adding to it so it builds a reverse dictionary too, if you’re going to be searching in this direction often, but for a single item at a time.

    If your searches are only ever in this direction, your dictionary may be the wrong way round entirely and the thing that maintains it should be reworked to reverse it permanently

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