Array variables can be static or dynamic. That’s determined by the Dim statement. If you specify dimensions when you declare the variable, it’s static and always will be. If you leave the dimensions blank in the Dim statement, it’s dynamic and can be changed.

Dim Array1(1 To 10) As String ‘static array
Dim Array2() As String ‘dynamic array

Dynamic arrays can be changed using the Redim statement.

Dim Arr1() As Double

ReDim Arr1(Selection.Columns.Count, Selection.Rows.Count)

If you use Redim, all the data in your array is lost, unless you use the Preserve keyword. This keeps the data in tact, but limits what you can change with a Redim. For instance, when you use Preserve, you can only change the last dimension of the array. Sometimes you have to organize your array horizontally to accomodate this restriction.

Dim Arr1() As Double
Dim cell As Range
Dim i As Long

For Each cell In Range(“A1:A100?).Cells
    If cell.Value < 0.5 Then
        i = i + 1
        ReDim Preserve Arr1(1 To 2, 1 To i)
        Arr1(1, i) = cell.Value
        Arr1(2, i) = cell.Row
    End If
Next cell

Preserve is an expensive keyword, so you use it sparingly. Many people will Redim their arrays in blocks to avoid having to do it in every iteration of a loop.

ReDim Preserve Arr1(1 To 2, 1 To 10)

For Each cell In Range(“A1:A100?).Cells
    If cell.Value < 0.5 Then
        i = i + 1
        If i Mod 10 = 0 Then
            ReDim Preserve Arr1(1 To 2, 1 To i + 10)
        End If
        Arr1(1, i) = cell.Value
        Arr1(2, i) = cell.Row
    End If
Next cell

I’m not a big fan of the block Redim, but if you have a really time intensive procedure and this shaves some valuable milliseconds, then go for it. If there’s a way to figure out the upper bounds of the array before you add data, then you may save time there also.

Dim SmallCells As Long

SmallCells = Application.Evaluate(“=sumproduct(–(a1:A100<.5))”)

ReDim Arr1(1 To 2, 1 To SmallCells)

For Each cell In Range(“A1:A100?).Cells
    If cell.Value < 0.5 Then
        i = i + 1
        Arr1(1, i) = cell.Value
        Arr1(2, i) = cell.Row
    End If
Next cell