[2005] Using Array.Exists?

[blakemckenna]

I'm trying to use the Array.Exists function and having problems with it. I tried using the Help but it only shows examples in C++/C#. Here is what I have and what it's giving me back.

            For Each row As DataRow In dsQVElements.Tables("tblQueueView").Rows
                If arrQueueView Is Nothing Then
                    ReDim Preserve arrQueueView(x)
                    arrQueueView(x).groupID = row("groupID")
                    arrQueueView(x).labelName = row("labelName")
                    arrQueueView(x).sortOrder = row("sortOrder")
                    x += 1
                Else
                    If Array.Exists(arrQueueView, row("labelName")) Then
                    Else
                        ReDim Preserve arrQueueView(x)
                        arrQueueView(x).groupID = row("groupID")
                        arrQueueView(x).labelName = row("labelName")
                        arrQueueView(x).sortOrder = row("sortOrder")
                        x += 1
                    End If
                End If
            Next

When the highlighted code is executed....I get an "Object reference not set to an instance of an object" error.

[Atheist]

Well, I would guess the reason is that row("labelName") returns Nothing. However, the second parameter in the Exists call should be of type Predicate.

[blakemckenna]

what is the Predicate? I've seen this term before but I don't understand how it's used.

[Atheist]

A Predicate is basically a delegate, ie it wraps a method-pointer. Here's how it would be applied in the Array.Exists case:
(This example checks wether the value 4 exists within an integer array)

Private Sub someSubroutine()
        Dim myIntArray() As Integer = {1, 2, 3, 4}
        Dim valueExists As Boolean = Array.Exists(myIntArray, New Predicate(Of Integer)(AddressOf myCompareFunction))
    End Sub
 
    Private Function myCompareFunction(ByVal value As Integer) As Boolean
        Return (value = 4)
    End Function

So basically you create a function that takes one argument which must be the same type as the array you're passing to the Exists function. It must also have the return type Boolean.
The Exists method will iterate through each element in your array and pass them to this function, as soon as any element returns True, Exists will return true.

[dbasnett]

when i searched for 'array.exists' this was fourth from the top

http://msdn.microsoft.com/en-us/libr...be(VS.85).aspx

[blakemckenna]

This totally blows my mind only because the value to be searched seems as though it's hard-coded, which obviously can't be. Does anyone have a real example of this. That might help me if I can see it in use realtime.

Thanks,

[dbasnett]

    Dim lookfor As String = "fubar"

    Dim dinosaurs() As String = {"Compsognathus", _
        "Amargasaurus", "Oviraptor", "Velociraptor", _
        "Deinonychus", "Dilophosaurus", "Gallimimus", _
        "Triceratops"}
    Public Sub Main()
        Stop 'single step from here and see what happens
        dinosaurs(3) = lookfor
        If Array.Exists(dinosaurs, AddressOf EndsWithSaurus) Then
            Debug.WriteLine("found")
        End If
    End Sub
    Dim findThis As String = "fubar"
    Private Function EndsWithSaurus(ByVal s As String) _
            As Boolean
        If findThis = s Then
            Debug.WriteLine("Yes " & s)
            Return True
        Else
            Debug.WriteLine("No " & s)
            Return False
        End If
    End Function