How do I check if a key in a dictionary contain a value?

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So, I'm making a dictionary for which each key can take multiple values(i.e Dictionary <string, List<string>>) My problem is that for each key I want it to have distinct values aka no redundancy in values for a certain key so I need to check if a key contains a value in the list of string

Posted 6-Dec-15 12:33pm

Member 12160712

Updated 6-Dec-15 12:36pm

v3

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PIEBALDconsult 6-Dec-15 18:38pm

You can use a HashSet rather than a List. And if you want to be sure that no two sets contain the same value, you can have another HashSet outside the Dictionary.
https://msdn.microsoft.com/en-us/library/bb359438(v=vs.110).aspx

Member 12160712 6-Dec-15 20:00pm

Thank you so much!

BillWoodruff 7-Dec-15 2:53am

Indeed, using a HashSet may be useful here ... when we know what the OP really wants :)

PIEBALDconsult 7-Dec-15 18:23pm

I'll tell you want he wants, what he really really wants...

Sergey Alexandrovich Kryukov 6-Dec-15 23:24pm

What's wrong with just reading MSDN help? Everything is explained quite clearly.
—SA

BillWoodruff 7-Dec-15 2:50am

Are you saying that you want a Dictionary where the Key Type is 'string and the Value 'Type is List<string> where each Value is unique ?

The question then becomes: what does "unique" mean ? Does it mean that anytime you add a new string to any of the Values, that string must never occur in any of the Values ?

Or, does it mean that if the Value of the Key "hello" is {"a,"b,"c"} and the value of the Key "goodbye" is {"a","b"} that you would not allow the list associated with "goodbye" to add the value "c" to its list ?

Please clarify. Those two scenarios lead to different implementations.

Member 12160712 7-Dec-15 10:40am

No, I meant what you said second is that for anytime I add a new string to a certain key for eg "A" its values mustn't have duplicates i.e(B,C,C,D) I need it to be (B,C,D) And I think that the hashset was the answer to it as suggested by PIEBaldconsult. Thank you!

2 solutions

Solution 1

All you need is reading MSDN help. To check up if an element is in an instance of a HashSet advised by PIEBALDconsult, you can use System.Collections.Generic.HashSet<T>.Contains(T):
https://msdn.microsoft.com/en-us/library/bb356440(v=vs.110).aspx[^].

With Dictionary, you can use System.Collections.Generic.Dictionary<Key, Value>.ContainsKey(Key):
https://msdn.microsoft.com/en-us/library/kw5aaea4(v=vs.110).aspx[^],
see also https://msdn.microsoft.com/en-us/library/a63811ah(v=vs.110).aspx[^].

However, in most cases, it is not the most efficient way, because more often you need combine two steps: get the value by key and determine if the key already exists. This is done by the method TryGetValue:
https://msdn.microsoft.com/en-us/library/bb347013(v=vs.110).aspx[^].

That's all. Do read MSDN before asking questions like that.

—SA

Posted 6-Dec-15 17:32pm

Sergey Alexandrovich Kryukov

Comments

BillWoodruff 7-Dec-15 2:52am

Suggest you read the post again: they are not asking about Keys, but about a Value object which is a List of String.

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BillWoodruff · Accepted Answer · 2015-12-06T22:12:00

Assuming your goal is to ensure the each 'Value field of the Key-Value Pairs contained in your Dictionary has no duplicate Values:

C#

// required
// using System.Collections.Generic;

public class DistinctList : List<string>
{
    public new void Add(string value)
    {
        if (this.Contains(value)) return;

        base.Add(value);
    }
}

public class ValueDistinctDictionary : Dictionary<string, DistinctList>
{
    public void Add(string key, string value)
    {
        if (!this.ContainsKey(key))
        {
            base.Add(key, new DistinctList());
        }

        base[key].Add(value);
    }
}

// test in some method
// set break-points, examine 'vDict
ValueDistinctDictionary vDict = new ValueDistinctDictionary();

vDict.Add("hello", "goodbye");
vDict.Add("hello", "hello again");
vDict.Add("hello", "goodbye");

vDict.Add("hello1", "hello");
vDict.Add("hello1", "hello");
vDict.Add("hello1", "hello");