Building a dictionary of counts of items in a list

Question

I have a List containing a bunch of strings that can occur more than once. I would like to take this list and build a dictionary of the list items as the key and the count of their occurrences as the value.

Example:

List<string> stuff = new List<string>();
stuff.Add( "Peanut Butter" );
stuff.Add( "Jam" );
stuff.Add( "Food" );
stuff.Add( "Snacks" );
stuff.Add( "Philosophy" );
stuff.Add( "Peanut Butter" );
stuff.Add( "Jam" );
stuff.Add( "Food" );

and the result would be a Dictionary containing:

"Peanut Butter", 2
"Jam", 2
"Food", 2
"Snacks", 1
"Philosophy", 1

I have a way to do this, but it doesn't seem like I'm utilizing the good stuff in C# 3.0

public Dictionary<string, int> CountStuff( IList<string> stuffList )
{
    Dictionary<string, int> stuffCount = new Dictionary<string, int>();

    foreach (string stuff in stuffList) {
        //initialize or increment the count for this item
        if (stuffCount.ContainsKey( stuff )) {
            stuffCount[stuff]++;
        } else {
            stuffCount.Add( stuff, 1 );
        }
    }

    return stuffCount;
}

Answer 1

You can use the group clause in C# to do this.

List<string> stuff = new List<string>();
...

var groups = 
    from s in stuff
    group s by s into g
    select new { 
        Stuff = g.Key, 
        Count = g.Count() 
    };

You can call the extension methods directly as well if you want:

var groups = stuff
    .GroupBy(s => s)
    .Select(s => new { 
        Stuff = s.Key, 
        Count = s.Count() 
    });

From here it's a short hop to place it into a Dictionary<string, int>:

var dictionary = groups.ToDictionary(g => g.Stuff, g => g.Count);

Answer 2

I would have made a specialized List, that backed by the Dictionary and the add method would test for membership and increase count if found.

sorta like:

public class CountingList
{
    Dictionary<string, int> countingList = new Dictionary<string, int>();

   void Add( string s )
   {
        if( countingList.ContainsKey( s ))
             countingList[ s ] ++;
        else
            countingList.Add( s, 1 );
   }
}

Answer 3

One idea would be to give the dictionary a default value of zero, so you wouldn't have to special case the first occurrence.