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Arrays of arrays in VB6 (Visual Basic 6)

While you can create two-dimensional arrays in Visual Basic, their structure isn't really flexible for at least two reasons: All rows in the array must have the same number of elements, and you can use ReDim Preserve to change the number of columns but you can't add new rows. The first point is especially important because it often leads you to declare an array that's far too large for your needs, thus allocating a lot of memory that in most cases remains largely unused. You can solve both problems using a structure known as an array of arrays.

The technique is conceptually simple: Since you can store an array in a Variant variable, you can build an array of Variants, where each item holds an array. Each subarray—a row of this pseudo-array—can hold a different number of elements, and you don't need to use more memory than is strictly necessary.

Here's an example, based on an imaginary PIM (Personal Information Manager) program. In this program, you need to keep track of a list of appointments for each day of the year. The simplest solution would be to use an array in which each row corresponds to a day in the year and each column to a possible appointment. (For the sake of simplicity, let's assume that each appointment's data can be held in a string.)

ReDim apps(1 To 366, 1 To MAX_APPOINTMENTS) As String

Of course, you now have the problem of setting a reasonable value for the MAX_APPOINTMENTS symbolic constant. It should be high enough to account for all possible appointments in a day but not too high because you might be wasting a lot of memory without any real reason. Let's see how the array of arrays technique can help us save memory without posing any artificial limit to your application:

' A module-level variable
Dim apps(1 To 366) As Variant

' Add an appointment for a given day.
Sub AddNewAppointment(day As Integer, description As String)
Dim arr As Variant
If IsEmpty(apps(day)) Then
' This is the first appointment for this day.
apps(day) = Array(description)
' Add the appointment to those already scheduled.
arr = apps(day)
ReDim Preserve arr(0 To UBound(arr) + 1) As Variant
arr(UBound(arr)) = description
apps(day) = arr
End If
End Sub

' Extract all the appointments for a given day.
Sub ListAppointments(day As Integer, lst As ListBox)
Dim i As Long
For i = 0 To UBound(apps(1))
lst.AddItem apps(1)(i)
End Sub

In this example, I kept the code as simple as possible and used an array of Variant arrays. You could save even more memory if each row of this array were built using an array of a more specific data type (String, in this case). Note the special syntax used to address an item in an array of arrays:

' Change the description for the Nth appointment.
apps(day)(n) = newDescription

Nothing keeps you from extending this concept further, introducing an array of arrays of arrays, and so on. If you're dealing with arrays in which each row can vary considerably in length, this approach is going to save you a lot of memory and, in most cases, improve your overall performance too. A key feature of an array of arrays is that you can process entire rows of your pseudo-array as if they were single entities. For example, you can swap them, replace them, add and delete them, and so on.

' Move the January 1st appointments to January 2nd.
apps(2) = apps(1)
apps(1) = Empty

Finally, an important advantage of this technique is that you can add new rows without losing the current contents of the array. (Remember that you can use ReDim Preserve on regular arrays only to modify the number of columns, not the number of rows.)

' Extend the appointment book for another nonleap year.
ReDim Preserve apps(1 to UBound(apps) + 365) As Variant

More Topics on Visual Basic 6 Arrays

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