Question

Possible Duplicate:
Dictionary returning a default value if the key does not exist

I have a string that contains only digits. I’m interested in generating a frequency table of the digits. Here’s an example string:

var candidate = "424256";

This code works, but it throws a KeyNotFound exception if I look up a digit that’s not in the string:

var frequencyTable = candidate
    .GroupBy(x => x)
    .ToDictionary(g => g.Key, g => g.Count());

Which yields:

Key Count
4   2 
2   2 
5   1 
6   1 

So, I used this code, which works:

var frequencyTable = (candidate + "1234567890")
    .GroupBy(x => x)
    .ToDictionary(g => g.Key, g => g.Count() - 1);

However, in other use cases, I don’t want to have to specify all the possible key values.

Is there an elegant way of inserting 0-count records into the frequencyTable dictionary without resorting to creating a custom collection with this behavior, such as this?

public class FrequencyTable<K> : Dictionary<K, int>
{
  public FrequencyTable(IDictionary<K, int> dictionary) 
    : base(dictionary)
  { }

  public new int this[K index]
  {
    get
    {
        if (ContainsKey(index))
            return base[index];
        return 0;
    }
  }
}

Answer 1

If you do not somehow specify all possible key values, your dictionary will not contain an entry for such keys.

Rather than storing zero counts, you may wish to use

Dictionary.TryGetValue(...)

to test the existence of the key before trying to access it. If TryGetValue returns false, simply return 0.

You could easily wrap that in an extension method (rather than creating a custom collection).

static public class Extensions
{
    static public int GetFrequencyCount<K>(this Dictionary<K, int> counts, K value)
    {
        int result;
        if (counts.TryGetValue(value, out result))
        {
            return result;
        }
        else return 0;
    }
}

Usage:

Dictionary<char, int> counts = new Dictionary<char, int>();
counts.Add('1', 42);
int count = counts.GetFrequencyCount<char>('1');