CODEWITHSUNDEEP
linq
b3n
b3n

Reputation: 3865

Creating a dictionary with LINQ and keys from nested list

I have code that looks like this:

class A {
public string Name {get;set;}
public IList<string> Keywords {get;set;}
}

...

IList<A> a = new List<A>{
        new A{Name = "a1", Keywords = new List<string>{"k1", "k2"}},
        new A{Name = "a2", Keywords = new List<string>{"k1", "k3"}},
        new A{Name = "a3", Keywords = new List<string>{"k3", "k4"}},
        new A{Name = "a4", Keywords = new List<string>{"k1", "k3", "k4"}}
    };

Is there any way I can use LINQ to create a dictionary in the form of:

{"k1", List<A> {a1, a2, a4}} 
{"k2", List<A> {a1}}
{"k3", List<A> {a2, a3, a4}}
{"k4", List<A> {a3, a4}}

Basically mapping the objects to the keywords.

Thanks

Upvotes: 0

Views: 437

Answers (2)

vc 74
vc 74

Reputation: 38179

That should do it:

var keywordsAndNames = a.SelectMany(anA => anA.Keywords.Select(kw => 
                           new { KeyWord = kw, Name = anA.Name }));
var lookup = keywordsAndNames.ToLookup(entry => entry.KeyWord, entry => entry.Name);

Note that this code creates a lookup and not a dictionary because for each key there are multiple names.

lookup["k1"] returns an IEnumerable<string> yielding "a1", "a2" and "a4" for instance.

Upvotes: 1

Rawling
Rawling

Reputation: 50114

Something like 

var dict = a
    .SelectMany(myA =>
        myA.Keywords.Select(kw => new { Name = myA.Name, Keyword = kw }))
    .GroupBy(pair => pair.Keyword)
    .ToDictionary(g => g.Key, g => g.Select(pair => pair.Name).ToList());

should do it.

Upvotes: 1

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