UBound inside Redim Preserve throws "Subscript out of range"

Question

I want to use a dynamic array containing arbitrary number of strings. The array is filled by if ... then logic instead of a loop. I keep getting Subscript out of range error:

Dim Files() As String

If True Then
    ReDim Preserve Files(UBound(Files) + 1) ' Throws "Subscript out of range" error
    Files(UBound(Files)) = "foo.pdf"
End If

If True Then
    ReDim Preserve Files(UBound(Files) + 1)
    Files(UBound(Files)) = "bar.txt"
End If

If True Then
    ReDim Preserve Files(UBound(Files) + 1)
    Files(UBound(Files)) = "baz.jpg"
End If

I have a function declared like this:

Function SendFiles(Files() As String)

I want to get rid of this error without using variants if possible. I can rewrite the code but I cannot use a loop.

@bigben what would be the cleanest approach to handle this? I don't know in advance how many entries need to be added, there is no loop, and I am asked to use types wherever possible. — Salman A, Nov 08 '19 at 13:26
You could just use `ReDim Files(0)` before your first `If`. If you know the largest possible size at the outset you can redim to that at the outset and then cut out any unnecessary elements at the end using Preserve. — SJR, Nov 08 '19 at 13:29
You know `ReDim Preserve` allocates an entire new array, copies elements one by one, and then updates the original reference, right? It's a pig for performance. If you have an array you expect to append to often, you should look at the [`Collection`](https://learn.microsoft.com/en-us/office/vba/language/reference/user-interface-help/collection-object) type. — Joel Coehoorn, Nov 08 '19 at 14:31
`If True Then` are these lines needed? When will TRUE not be TRUE? — Darren Bartrup-Cook, Nov 08 '19 at 15:06

Erik A · Accepted Answer · 2019-11-08T15:08:36.727

2

Your array is not initialized at the start, and you can't Redim Preserve an uninitialzed array.

If you want a string array to hold a variable amount of items, possibly zero, you can start with initializing it to a zero-length array using Split:

Files = Split(vbNullString)

edited Nov 08 '19 at 15:08

answered Nov 08 '19 at 14:56

Erik A

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This seems to do the job. Is there a language construct to initialize an array declared as `Dim Files() As String` an array having UBound = -1? – Salman A Nov 08 '19 at 18:20
@SalmanA No, unfortunately, there's not. You can either use `Split(vbNullString)` to get a 0-length string array, or a complicated WinAPI call to do it with a little less overhead. See [This Q&A](https://stackoverflow.com/q/55123413/7296893) for more background, getting zero-length array is difficult for anything but strings. – Erik A Nov 08 '19 at 18:36

score 1 · Answer 2 · answered Nov 08 '19 at 15:05

You could also allocate a large enough array and resize to used size afterwards. That way you have only 1 resize. Something like this:

Dim Files(1000) As String, i as long

If True Then
    Files(i) = "foo.pdf": i = i+1
End If

If True Then
    Files(i) = "bar.txt": i = i+1
End If

If True Then
    Files(i) = "baz.jpg": i = i+1
End If

redim preserve Files(i-1)

UBound inside Redim Preserve throws "Subscript out of range"

2 Answers2