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Search For keys Matching values in a dictionary of list in C#

I have a dictionary of lists.
var dicAclWithCommonDsEffectivity = new Dictionary<string, List<int>>();
var list1 = new List<int>(){1,2,3};
var list2 = new List<int>(){2,4,6};
var list3 = new List<int>(){3,7,6};
var list4 = new List<int>(){8,7,6};
dicAclWithCommonDsEffectivity.Add("ab",list1);
dicAclWithCommonDsEffectivity.Add("bc",list2);
dicAclWithCommonDsEffectivity.Add("cd",list3);
dicAclWithCommonDsEffectivity.Add("de",list4);
I want to get the keys in dictionary for which atleast one matching value with the current key list.
for key "ab"(first list). I should get: "ab","bc" and "cd".Since these lists contain one of the matching element in {1,2,3}
Is there a way without looping through each item in the list of dictionary value.

Is there a way without looping through each item in the list of dictionary value.
Something has to loop - dictionaries are only designed to look up by key, and you're not doing that other than for the first check.
You can do this fairly easily though:
private IEnumerable<string> GetMatchingKeys(
Dictionary<string, List<int>> dictionary, string key)
{
// TODO: Use TryGetValue if key might not be in dictionary
HashSet<int> elements = new HashSet<int>(dictionary[key]);
return dictionary.Where(pair => pair.Value.Any(x => elements.Contains(x)))
.Select(pair => pair.Key);
}
This uses the fact that Dictionary implements IEnumerable<KeyValuePair<TKey, TValue>> - so the Where clause checks a particular entry by spotting if any of the elements of its value matches any of the elements of the original value. The Select clause then projects the pair to just the key.
If you need to do this a lot and you're concerned about efficiency, another alternative would be to build a second dictionary from int to List<string> - basically a reverse mapping. You'd need to maintain that, but then you could easily fetch all the "original keys" mapping to each of the values corresponding to the given key, and just use Distinct to avoid duplicates.

It's always better to check for existence of the searchKey in the Dictionary before accessing them. Then you can get the associated list in the dictionary for that particular key.
You can also try like this:
string searchKey = "ab";
if (dicAclWithCommonDsEffectivity.ContainsKey(searchKey))
{
var ListToSearch = dicAclWithCommonDsEffectivity[searchKey];
var resultKeys = dicAclWithCommonDsEffectivity.Where(x =>
x.Value.Any(y => ListToSearch.Contains(y)))
.Select(x => x.Key)
.ToList();
}
else
{
// Specified key was not found
}

If what you mean is to visibly (not logically) remove the looping, you could use LINQ with proper Where filter to do that and Select the keys from the Dictionary which have any value element(s) intersect(s) with the selected List (List in the Dictionary with key == "ab") like this:
string key = "ab";
List<int> selectedList = dicAclWithCommonDsEffectivity[key];
var results = dicAclWithCommonDsEffectivity
.Where(x => x.Value.Any(y => selectedList.Contains(y)))
.Select(x => x.Key);
If you want to logically remove the looping too, please consider Mr. Skeet's answer.

c# Finding pairs in dictionary

For my project i made a dictionary that has a random double and a string that belongs to that double:
Dictionary<double, string> myDict = new Dictionary<double, string>();
For this project i know that the double is a random value, and within the dictionaries all strings are unique, with the exception that about 80% one of them is twice in the dictionary.
So what i want to do, is find the 2 strings that are a pair (the same string) and find the 2 double values that belong to these 2 string.
Basically my idea of doing this is by using IEnumerator counter = myDict.GetEnumerator(); and use the while (counter.MoveNext() == true) to start another IEnumerator that loops again through all the entries of the dictionary and compares by string, so if will find the pairs this way.
So for each entry in the dictionary, it will loop through the whole dictionary again to find pairs.
Now i get the feeling this might not be the best solution to handle this. Are there alternatives to find the pairs in the dictionary, or is this looping through the only real way of doing this?

I believe, you are looking to get Keys for those items where there is a pair of string available in Values.
var result = myDict.GroupBy(r => r.Value)
.Where(grp => grp.Count() == 2)
.SelectMany(grp => grp.Select(subItem => subItem.Key))
.ToList();
If you want to get keys for those items which have multiple string values, (more than two) then modify the condition to:
.Where(grp => grp.Count() >= 2)
Another thing to add, you are adding keys as Random values in the dictionary. Remember, Random doesn't mean Unique. You could end up with an exception since Dictionary keys are unique.
If your dictionary is defined as:
Dictionary<double, string> myDict = new Dictionary<double, string>
{
{1, "ABC"},
{2, "ABC"},
{3,"DEF"},
{4,"DEF"},
{5,"DEF2"},
{6,"XYZ"}
};
For output after the LINQ expression:
foreach (var d in result)
{
Console.WriteLine(d);
}
Output:
1
2
3
4

Understanding lambda expressions in C#

I'm new to lambdas and they seemed fairly straight-forward until I tried to do something more complex.
I have this dictionary.
Dictionary<int, int> dict = new Dictionary<int,int>();
of which I want to obtain the key of the key-val pair with the largest value. What I tried is:
dict.Keys.Max(g => dict[g])
The reasoning being that out of the list of Keys, pick that one for which dict[key] is largest. However, this picks the largest value itself, rather than its corresponding key.

dict.Keys.OrderByDescending(g => dict[g]).First() will accomplish what you want, but may be inefficient for large dictionaries. MaxBy in John Skeet's MoreLinq will do exactly what you want efficiently.

var maxValue = dict.Max((maxPair) => maxPair.Value);
var maxPairs = dict.Where((pair) => pair.Value == maxValue);
This will give you a list of all of the pairs that have the maximum value.
If you just want the keys, you can do this afterwards:
var maxKeys = maxPairs.Select((pair) => pair.Key);

I decided to add an answer based on my thoughts on McKay's. This will perform very fast given the standard LINQ methods, provides just the key:
var maxValue = dict.Max(p => p.Value);
var keys = dict.Where(p => p.Value == maxValue).Select(p => p.Key);
Now, if the OP knows that there is always just one key (no duplicate values) then an improvement (very small) would be to use First with this as due to lazy evaluation only the elements up to the one with the maximum value would be evaluated after all were evaluated to first find the maximum value:
var key = dict.Where(p => p.Value == maxValue).First().Key;

dict.OrderBy(v => v.Value).Last().Key;
should do it. Basically you are ordering the KeyValuePair by value and picking the last one which will be the maximum. And within the last one you are only interested in the Key.

Return number of matches from c# dictionary

I have a dictionary with non unique values and I want to count the matches of a string versus the values.
Basically I now do dict.ContainsValue(a) to get a bool telling me if the string a exists in dict, but I want to know not only if it exists but how many times it exists (and maybee even get a list of the keys it exists bound to)
Is there a way to do this using dictionary, or should I look for a different collection?
/Rickard Haake

To get the number of instances of the value you could do something like this:
dict.Values.Count(v => v == a);
To find the keys that have this value you could do this:
dict.Where(kv => kv.Value == a).Select(kv => kv.Key);

To get the count use Values.Count:
int count = dict.Values.Count(x => x == "foo");
To get the keys I prefer the query syntax:
var keys = from kvp in dict
where kvp.Value == "foo"
select kvp.Key;
Note that this will require scanning the entire dictionary. For small dictionaries or infrequent lookups this may not be a problem.
If you are making many lookups you may wish to maintain a second dictionary that maps the values to the keys. Whilst this will speed up lookups, it will slow down modifications as both dictionaries will need updating for each change.

what about using LINQ: if a is the value you're looking for, the the code could be
dict.Values.Where(v => v == a).Count();

Need to look up key using string in a Dictionary<TKey, TValue> Class

I need to use a list pulled from sql, list is built from
Dictionary<Int32, String> measurementTypes = this.GetIndicatorTypes(MeasurementTypeFilter.All);
is ther a way to retrive the key using the string.
Something like
TypeID = measurementTypes.contains("GEN");

Well, it'll be slow (i.e. O(n)), but you can do:
var keys = measurementTypes.Where(pair => pair.Value == "GEN")
.Select(pair => pair.Key);
That will give you a sequence of pairs which have the given value. There could be 0, 1 or many matches. From there you can pick the first matching key etc - whatever you need. Using First or Single would be appropriate if you think there will be at least one or exactly one; FirstOrDefault would return 0 if there were no matches, which may not be appropriate for you if 0 could also be a valid key.

TypeID = measurementTypes.Values.Where(v => v.Equals("GEN")).FirstOrDefault();

You can use LINQ to help you out there.
TypeID = measurementTypes.Where(kvp => kvp.Value == "GEN")
.Select(kvp => kvp.Key)
.FirstOrDefault()