Question

I have a Dictionary<string, List<int>> in my code which I am using in the following manner:

Key           Values  
2011-07-15    1, 2, 3
2011-07-20    4, 5, 6
2010-02-11    7, 8, 9

My code needs to be able to query for all values matching a particular substring in the key. For example, if I had the substring 2011-07 it should return values {1, 2, 3, 4, 5, 6}. A substring of 11 should return all IDs from 1-9.

Can anyone recommend a concise way to achieve this? Or provide a better data structure for retrieving this information?

Answer 1

I would do an extension method :

public static class DictionaryExt
{
    public static IEnumerable<T> PartialMatch<T>(this Dictionary<string, T> dictionary, string partialKey)
    {
        // This, or use a RegEx or whatever.
        IEnumerable<string> fullMatchingKeys = 
            dictionary.Keys.Where(currentKey => currentKey.Contains(partialKey));

        List<T> returnedValues = new List<T>();

        foreach (string currentKey in fullMatchingKeys)
        {
            returnedValues.Add(dictionary[currentKey]);
        }

        return returnedValues;
    }
}

The “cost” of adding values to the dictionary wouldn’t change, but the cost of retrieval would be higher, but only when you know you’re going with a partial match.

Btw, I’m sure you could transform this in a single Lambda expression, but the concept remains the same.

Edit: In your example, this method would return 2 lists of values, but you can change it to merge the lists. Here is the extension method you could do :

public static IEnumerable<T> PartialMatch<T>(
    this Dictionary<string, IEnumerable<T>> dictionary,
    string partialKey)
{
    // This, or use a RegEx or whatever.
    IEnumerable<string> fullMatchingKeys = 
        dictionary.Keys.Where(currentKey => currentKey.Contains(partialKey));

    List<T> returnedValues = new List<T>();

    foreach (string currentKey in fullMatchingKeys)
    {
        returnedValues.AddRange(dictionary[currentKey]);
    }

    return returnedValues;
}

Edit 2: Come to think of it, you could also make it more generic. With the next extension method, it would work on any dictionary, as long as you provide a comparer that check what you mean by “partial match” :

public static IEnumerable<TValue> PartialMatch<TKey, TValue>(
    this Dictionary<TKey, IEnumerable<TValue>> dictionary,
    TKey partialKey,
    Func<TKey, TKey, bool> comparer)
{
    // This, or use a RegEx or whatever.
    IEnumerable<TKey> fullMatchingKeys = 
        dictionary.Keys.Where(currentKey => comparer(partialKey, currentKey));

    List<TValue> returnedValues = new List<TValue>();

    foreach (TKey currentKey in fullMatchingKeys)
    {
        returnedValues.AddRange(dictionary[currentKey]);
    }

    return returnedValues;
}