Reverse many-to-many Dictionary>

Question

Actually my previous question got me thinking and I realized that reversing a Dictionary is not trivial. What is the most elegant and readable way to do it?

Answer 1

Slightly different way (a bit more comprehensible to my brain anyway :) ...

var newDict = new Dictionary<int, List<int>>();
var dict = new Dictionary<int, List<int>>();
dict.Add( 1, new List<int>() { 1, 2, 3, 4, 5 } );
dict.Add( 2, new List<int>() { 1, 2, 3, 4, 5 } );
dict.Add( 3, new List<int>() { 1, 2, 6 } );
dict.Add( 4, new List<int>() { 1, 6, 7 } );
dict.Add( 5, new List<int>() { 8 } );

var newKeys = dict.Values.SelectMany( v => v ).Distinct();

foreach( var nk in newKeys )
{
   var vals = dict.Keys.Where( k => dict[k].Contains(nk) );
   newDict.Add( nk, vals.ToList() );
}

Answer 2

I'm not sure exactly how this differs from your previous question.

If you're just asking how to return a Dictionary<int, List<int>> rather than Dictionary<int, IEnumerable<int>> then all you need is a call to the ToList method.

Stealing and amending Mehrdad's answer to your other question:

var classToStudent = studentToClass
    .SelectMany(
        pair => pair.Value.Select(val => new { Key = val, Value = pair.Key }))
    .GroupBy(item => item.Key)
    .ToDictionary(gr => gr.Key, gr => gr.Select(item => item.Value).ToList());

Answer 3

To reverse a dictionary is very easy:

var newDic = oldDic.ToDictionary(x => x.Value, x => x.Key);

That's all.

Now, your question is different. It is about reversing a many-to-many relationship, established on a dictionary.

So, let's say you have Dictionary<TEntity1, IEnumerable<TEntity2>>. The idea is to extract from this the "middle table" of the many-to-many relationship. Then you can regroup it by the other side, and retransform into a dictionary.

For the first part, we will use the overload of SelectMany that

"Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein."

var table =
    dict.SelectMany(
        x => x.Value,
        (dictEntry, entryElement) => new
               {
                      Entity1 = dictEntry.Key,
                      Entity2 = entryElement
               }
    );

So, now you just have to regroup this table the way you want, and then convert it to a dictionary.

 var newDict =
     table
         .GroupBy(x => x.Entity2,
                  x => x.Entity1,
                  (entity2, entity1) => new {entity1, entity2})
         .ToDictionary(x => x.entity2, x => x.entity1);