Finding already existing value in Key Value pair

Question

I am storing a string and int value in Key value pair.

var list = new List<KeyValuePair<string, int>>();

While adding i need to check if string(Key) already exists in list, if exists i need to add it to Value instead of adding new key.
How to check and add?

Note that `KeyValuePair` is immutable. You would have to remove the existing entry and create a new one every time you wanted to update the value. Which is why a `Dictionary` is much better as given in the answers. — mellamokb, Jan 23 '13 at 05:44

score 36 · Accepted Answer · answered Jan 23 '13 at 05:38

36

Instead of List you can use Dictionary and check if it contains key then add the new value to the existing key

int newValue = 10;
Dictionary<string, int> dictionary = new Dictionary<string, int>();
if (dictionary.ContainsKey("key"))
    dictionary["key"] = dictionary["key"] + newValue;

answered Jan 23 '13 at 05:38

Habib

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Just a note: Dictionary doesn't allow accessing by index. If that's a requirement (as it is in my case), then this is not a viable solution. – Dan Bechard Jul 22 '15 at 19:54
1

@Dan, `Dictionary` does allow access by index, like `dictionary["myKey"]` I am not sure what you meant. – Habib Jul 22 '15 at 19:58
5

But on the other hand Dictionary doesn't support same keys with different values. – SSpoke Sep 15 '15 at 02:37
@Dan, You can use ElementAt[myInt] if you need to get entries by position. – B H Oct 12 '16 at 19:11
@BH Thanks, I'll try that next time I need it. – Dan Bechard Oct 12 '16 at 19:31
The OP asks for a List why suggest him a Dictionary? Dictionarys cant be sorted and lots of other differences. – nights Jul 24 '18 at 04:44

Ravi Gadag · Answer 2 · 2013-01-23T05:44:20.137

use dictonary. Dictionary in C# and I suggest you to read this post Dictonary in .net

Dictionary<string, int> dictionary =
        new Dictionary<string, int>();
    dictionary.Add("cat", 2);
    dictionary.Add("dog", 1);
    dictionary.Add("llama", 0);
    dictionary.Add("iguana", -1);

to check. use ContainsKey ContainsKey

if (dictionary.ContainsKey("key"))
    dictionary["key"] = dictionary["key"] + yourValue;

score 7 · Answer 3 · answered Aug 09 '19 at 22:23

7

For anyone who has to use a List (which was the case for me, since it does things Dictionary doesn't), you can just use a lambda expression to see if the List contains the Key:

list.Any(l => l.Key == checkForKey);

answered Aug 09 '19 at 22:23

WATYF

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score 5 · Answer 4 · answered Jan 23 '13 at 05:40

5

If you need use the list,you must foreach the list,and look for the keys. Simplely,you can use hashtable.

answered Jan 23 '13 at 05:40

Jhonny

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Karthik T · Answer 5 · 2013-01-23T05:41:19.153

4

Your needs exactly describe the design of Dictionarys?

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

// Add some elements to the dictionary. There are no  
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");

// If a key does not exist, setting the indexer for that key 
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

edited Jan 23 '13 at 05:41

answered Jan 23 '13 at 05:36

Karthik T

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Map is a very Javaish word ;p – leppie Jan 23 '13 at 05:36
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@Olivarsham Check my answer, you want `Dictionary` – Karthik T Jan 23 '13 at 05:40

score 4 · Answer 6 · answered Jan 23 '13 at 06:07

For sure, dictionary is preferable in your case. You can not modify the Value of KeyValue<string,int> class as it is Immutable.

But even if you still want to use List<KeyValuePair<string, int>>();. You can use IEqualityComparer<KeyValuePair<string, int>>. Code will be like.

public class KeyComparer : IEqualityComparer<KeyValuePair<string, int>>
{

    public bool Equals(KeyValuePair<string, int> x, KeyValuePair<string, int> y)
    {
        return x.Key.Equals(y.Key);
    }

    public int GetHashCode(KeyValuePair<string, int> obj)
    {
        return obj.Key.GetHashCode();
    }
}

And use it in Contains like

var list = new List<KeyValuePair<string, int>>();
        string checkKey = "my string";
        if (list.Contains(new KeyValuePair<string, int>(checkKey, int.MinValue), new KeyComparer()))
        {
            KeyValuePair<string, int> item = list.Find((lItem) => lItem.Key.Equals(checkKey));
            list.Remove(item);
            list.Add(new KeyValuePair<string, int>("checkKey", int.MinValue));// add new value
        }

which does not sounds good way.

hope this info helps..

Finding already existing value in Key Value pair

6 Answers6