CODEWITHSUNDEEP
c#idictionaryoledbdatareader
Arianule
Arianule

Reputation: 9043

adding List values to a dictionary

I am trying to populate a dictionary of which the subject value which is unique have various Code values that should be matched with it.

CODE    SUBJECT

7DIM-062  Recruitment and Selection

7DIM-063    Recruitment and Selection

7DIM-064    Recruitment and Selection

7DIM-065    Recruitment and Selection

7DIM-066    Recruitment and Selection

7DIM-067    Recruitment and Selection

7DIM-068    Recruitment and Selection

So what I want is only for Reqruitment and Selection to be added to the Dictionary once as the unique key and then all the corresponding codes to be added to a List. How would I go about in doing this?

Dictionary<string, List<string>> dict = new Dictionary<string,List<string>>();

this is my query

OleDbDataReader dbReader = cmd.ExecuteReader();
while (dbReader.Read())
{
    string code = (string)dbReader["CODE"];
    string subject = (string)dbReader["SUBJECT"];
    //???? this is the point where I would want to add the values
    dict.Add(subject, new List<string>().Add(code);

Upvotes: 0

Views: 127

Answers (3)

Igor Korkhov
Igor Korkhov

Reputation: 8558

You can use Lookup<string, string>:

var subjects = new List<KeyValuePair<string, string>>();
while (dbReader.Read())
{
    string code = (string)dbReader["CODE"];
    string subject = (string)dbReader["SUBJECT"];

    subjects.Add(new KeyValuePair<string, string>(subject, code));
}
// ...
var lookup = subjects.ToLookup(x => x.Key, x => x.Value);
var recruitmentAndSelectionCodes = lookup["Recruitment and Selection"].ToList();
// returns
//     7DIM-062 
//     7DIM-063 
//     etc.

Upvotes: 2

Tim Schmelter
Tim Schmelter

Reputation: 460138

You could use Dictionary.TryGetValue to look if your dictionary already contains that subject. Then you can add the new code to it, otherwise add subject+code:

Dictionary<string, List<string>> dict = new Dictionary<string,List<string>>();
while (dbReader.Read())
{
    string code = (string)dbReader["CODE"];
    string subject = (string)dbReader["SUBJECT"];

    List<string> codes;
    if (dict.TryGetValue(subject, out codes))
    {
        codes.Add(code);
    }
    else
    {
        codes = new List<string>() { code };
        dict.Add(subject, codes);
    }
}

It's just more efficient than to lookup twice.

This method combines the functionality of the ContainsKey method and the Item property. If the key is not found, then the value parameter gets the appropriate default value for the type TValue; for example, 0 (zero) for integer types, false for Boolean types, and null for reference types. Use the TryGetValue method if your code frequently attempts to access keys that are not in the dictionary. Using this method is more efficient than catching the KeyNotFoundException thrown by the Item property. This method approaches an O(1) operation.

Upvotes: 2

gzaxx
gzaxx

Reputation: 17600

First check if your dictionary already has the key, if not add new key with List initialization. 

if (!dict.ContainsKey(subject))
{
    dict[subject] = new List<string>();    
}

dict[subject].Add(code);

Upvotes: 5

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