CODEWITHSUNDEEP
linq
user200814
user200814

Reputation: 275

How to combine dictionaries in Linq?

I'm new to Linq. I have code like this:

public class Data
{
    public Dictionary<string,int> WordFrequency; 
}
List<Data> dataList;

What I want is one aggregated dictionary that does a combined WordFrequency for the whole list of Data objects. I know how to do this using loops (iterate over the List, then iterate over each Dictionary), my question is, what is the Linq syntax for this? Thank you.

EDIT: here is my (untested) looping approach, so you can see what I mean.

    public static Dictionary<string, int> Combine()
    {
        Dictionary<string, int> result;
        foreach (Data data in DataList)
        {
            foreach (string key in data.WordFrequencies.Keys)
            {
                if(!result.ContainsKey(key))
                    result[key] = 0;

                result[key] += data.WordFrequencies[key];
            }
        }
   }

Upvotes: 1

Views: 121

Answers (2)

devgeezer
devgeezer

Reputation: 4189

Here is a query-based solution identical in most regards to Tim's:

Dictionary<string, int> allWordFrequency =
    (from d in dataList
    from kvp in d.WordFrequency
    group kvp.Value by d.Key)
       // ^^^^^^^^^ this grouping projection...
    .ToDictionary(g => g.Key, g => g.Sum());
  // ...eliminates need for lambda here ^^

I appreciate how the two from clauses mimic the nested foreach loops in the looping-based approach of the post. Like Tim's solution, the query iterates the KeyValuePair's of the Dictionary rather than iterate the Keys collection - this way the query doesn't need to invoke the indexer to get the corresponding integer count value.

Upvotes: 0

Tim Schmelter
Tim Schmelter

Reputation: 460380

So you want to flatten all dictionaries into a single one, which has no duplicate keys - of course?

You can use Enumerable.SelectMany to flatten all and Enumerable.GroupBy to group the keys.

Dictionary<string, int> allWordFrequency = dataList
            .SelectMany(d => d.WordFrequency)
            .GroupBy(d => d.Key)
            .ToDictionary(g => g.Key, g => g.Sum(d => d.Value));

I have presumed that you want to sum all frequencies.

Upvotes: 6

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