I'm looking for a way to search the values in a .net hashtable using wildcards

Question

I've got a whole host of values stored in a .net 2.0 hashtable. What I would really like to find is a way to, essentially, do a SQL select statement on the table.

Meaning, I'd like to get a list of keys whose associated values match a very simple text pattern (along the lines of "starts with a number".)

The final goal will be to remove these records from the hashtable for further processing.

I've been beating my head against this for a while now, and I can't seem to come up with anything.

Any ideas?

(On the off chance this matters: due to the realities of this project, any 3rd party widgets or upgrading to a more recent version of .net are off the table.)

Answer 1

You could use a regex against every key in the hashtable. This is very dirty but it works:

    static void Main(string[] args)
    {
        Hashtable myhashtable = new Hashtable();
        myhashtable.Add("Teststring", "Hello");
        myhashtable.Add("1TestString1", "World");
        myhashtable.Add("2TestString2", "Test");

        List<String> newht = new List<String>;

        //match all strings with a number at the front
        Regex rx = new Regex("^[1-9]");
        foreach (string key in myhashtable.Keys)
        {
            if (rx.IsMatch(key) == true)
            {
                newht.Add(key);
            }
        }

        //Loop through all the keys in the new collection and remove them from
        //them from the main hashtable.
        foreach (string key in newht)
        {
            myhashtable.Remove(key);
        }
    }

EDIT: And just for fun, here is the LINQ version (sorry I just had too).

            Hashtable myhashtable = new Hashtable();
            myhashtable.Add("Teststring", "Hello");
            myhashtable.Add("1TestString1", "World");
            myhashtable.Add("2TestString2", "Test");

            Regex rx = new Regex("^[1-9]");
            var k = (from string key in myhashtable.Keys
                     where rx.IsMatch(key)
                     select key).ToList();

            k.ForEach(s => myhashtable.Remove(s));

EDIT: I have just added list of sting rather then a hashtable, I couldn't remember which .net version had generic lists in it ***slaps forehead

Answer 2

If you are truly looking for things that start with a number, then you can do it much faster than with a Regex. Just look at the first character of each key and determine if it is a digit. Store the keys you want to remove in a List since you only need to keep the key.

    List<string> keysToRemove = new List<string>( myhashtable.Count );
    foreach (string key in myhashtable.Keys)
    {
        if (char.IsDigit(key[0])
        {
            keysToRemove.Add(key);
        }
    }

    foreach (string key in keysToRemove)
    {
        myhashtable.Remove(key);
    }

Answer 3

Using LINQ:

Dim myhashtable As New Hashtable
    myhashtable.Add("Teststring", "Hello")
    myhashtable.Add("1TestString1", "World")
    myhashtable.Add("2TestString2", "Test")

For Each i As String In From Element In myhashtable.Cast(Of DictionaryEntry)() Let k = DirectCast(Element.Value, String) Where k.StartsWith("W") Select DirectCast(Element.Key, String)
        MsgBox("This key has a matching value:" & i)
    Next

But better of using Dictionary if using LINQ:

    Dim d = New Dictionary(Of String, String)()
    d.Add("Teststring", "Hello")
    d.Add("1TestString1", "World")
    d.Add("2TestString2", "Test")

    For Each i As String In From element In d Where element.Value.StartsWith("W") Select element.Key
        MsgBox("This key has a matching value:" & i)
    Next

And instead of .StartsWith("W") you can of course do any other filtering you want.