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Tip of the Day
Language: VB7
Expertise: Intermediate
Aug 10, 2002

Create zero-elements arrays

The .NET framework lets you create two types of "empty" arrays: unitialized arrays and arrays that are initialized with zero elements. Uninitialized arrays are actually array variables that are set to Nothing, whereas zero-element arrays are non-Nothing variables that point to arrays with zero elements. Here is the (undocumented) method for creating zero-element arrays:

Dim arr(-1) As Integer   ' or whatever type you need
If you have a routine that returns an array, you can decide whether you want to return Nothing or a zero-element array when an empty array should be returned. In general, returning a zero-element array makes for a more linear code in the caller. Consider this routine, that returns all the items in a string array that contain a given substring:

Function Matches(ByVal arr() As String, ByVal Search As String) As String()
    Dim al As New ArrayList()
    Dim s As String
    For Each s In arr
        If s.IndexOf(Search) >= 0 Then al.Add
    Next
    ' return Nothing if no matches
    If al.Count Then Return Nothing
    ' else move the elements into a string array
    Dim res(al.Count - 1) As String
    Dim i As Integer
    For i = 0 To al.Count - 1
        res(i) = al(i)
    Next
    ' return the array
    Return res
End Function
The caller of the above routine must discern the Nothing case from regular case:

Dim res() As String = Matches( arr, "Find this")
If res Is Nothing Then
    Console.WriteLine("Found 0 matches")
Else
    Console.WriteLine("Found {0} matches", res.Length)
End If
Now, consider what happens if you delete these two lines in the Matches routine:

    ' return Nothing if no matches
    If al.Count Then Return Nothing
Not only is the Matches routine simpler, also the caller requires less code, because it doesn't have to check for Nothing first:

Dim res() As String = Matches( arr, "Find this")
Console.WriteLine("Found {0} matches", res.Length)
Francesco Balena
 
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