Array already dimentioned/subscript out of range - excel

Question

If I set an array as dynamic, I get a subscript out of range error. If I set it as static, I get an array already dimentioned error. What am I missing?
Dim arrTrips() As String 'dynamic
Dim arrTrips(1 To 99) As String 'static
...
For i = 2 To lastrow
If .Cells(i, "C").Value2 = Target.Value2 Then
ReDim Preserve arrTrips(UBound(arrTrips) + 1) <-- error here
arrTrips(UBound(arrTrips)) = .Cells(i, "M").Value2
Debug.Print arrTrips(UBound(arrTrips))
End If
Next
Edit: adding more context. I am adding items to the array inside a loop.
LOL I fixed it. It's so stupid. All I did was add ReDim arrTrips(1) As String right below Dim arrTrips() As String

Answer 1

As mentioned, you're intializing an empty array, the Ubound function fails on that. There is no built-in method to test whether an array is "empty" so you need to either use error-trapping or a UDF that encapsulates error-trapping in order to determine whether the array has no dimensions.
Alternatively, since you know at the onset that the array is empty, you can simply ReDim it to something, and then you can later ReDim Preserve it within your loop.
Sub staticArra()
Dim arrTrips() As String
ReDim arrTrips(1 To 1) As String
Dim i As Integer
For i = 1 To 10
ReDim Preserve arrTrips(LBound(arrTrips) To UBound(arrTrips) + 1) As String
Next
End Sub
The second one, using ReDim arrTrips(1 to 99) fails to compile, by design.
https://learn.microsoft.com/en-us/office/vba/Language/Reference/user-interface-help/redim-statement
The ReDim statement is used to size or resize a dynamic array that has already been formally declared by using a Private, Public, or Dim statement with empty parentheses (without dimension subscripts).
But the ideal approach (in terms of conserving lines & avoiding redundant calls) would be to instantiate your array using ReDim instead of Dim, with a subscript. You can ReDim Preserve this:
Sub f()
ReDim arrTrips(0) As String
Dim i As Integer
For i = 1 To 10
If (i - 1) > UBound(arrTrips) Then
ReDim Preserve arrTrips(i)
End If
Next
End Sub
In short: give your array an upper bound to begin with.

Answer 2

Declaration 1
Dim arrTrips() As String
This is dynamic but its empty. When you do a Ubound on this, compiler will not find a size and will throw an error. Below will work since it is not derived from your arrTrips array
ReDim Preserve arrTrips(10)
Declaration 2:
Dim arrTrips(1 To 99) As String
This being static array, it can only be dimensioned once and will throw an error if you try to redimesion it.

Answer 3

You can use a counter:
Dim arrTrips() As String
...
Dim k As Long
k = 1
For i = 2 To lastrow
If .Cells(i, "C").Value2 = Target.Value2 Then
ReDim Preserve arrTrips(1 to k)
arrTrips(k) = .Cells(i, "M").Value2
Debug.Print arrTrips(k)
k = k + 1
End If
Next
You can also use COUNTIF to set the size of the array before the loop:
Dim arrTrips() As String
...
Dim k As Long
k = Application.CountIf(.Range("C:C"), Target.Value2)
ReDim arrTrips(1 To k)
Dim j As Long
j = 1
For i = 2 To lastrow
If .Cells(i, "C").Value2 = Target.Value2 Then
arrTrips(j) = .Cells(i, "M").Value2
Debug.Print arrTrips(j)
j = j + 1
End If
Next