You are here:
Visual
Basic > VB6
(Beginners Tutorial)
<< Previous | Contents
| Next >>
Inserting and deleting items using Arrays- Visual Basic 6
Some of the most common operations you perform on arrays are inserting and
deleting items, shifting all the remaining elements toward higher indices to make
room or toward lower indices to fill the "hole" a deletion has left.
You usually do this with a For…Next loop, and you can even write generic
array procedures that work with any type of array (with the usual restrictions
about arrays of UDTs and fixed-length strings that can't be passed to a Variant
parameter):
Sub InsertArrayItem(arr As Variant, index As Long, newValue As Variant)
Dim i As Long
For i = UBound(arr) - 1 To index Step -1
arr(i + 1) = arr(i)
Next
arr(index) = newValue
End Sub
Sub DeleteArrayItem(arr As Variant, index As Long)
Dim i As Long
For i = index To UBound(arr) - 1
arr(i) = arr(i + 1)
Next
' VB will convert this to 0 or to an empty string.
arr(UBound(arr)) = Empty
End Sub
If your application works intensively with arrays, you might find that an approach
based on For…Next loops is too slow. In some cases, you can considerably
speed up these operations by using the RtlMoveMemory API function, which many
Visual Basic programmers know under its popular alias name, CopyMemory.1 This
function lets you move a block of bytes from one memory address to another memory
address and works correctly even if the two areas partially overlap. Here's the
code that inserts a new item in an array of Longs:
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory"
_
(dest As Any, source As Any, ByVal numBytes As Long)
Sub InsertArrayItemLong(arr() As Long, index As Long, newValue As Long)
' We let VB evaluate the size of each item using LenB().
If index < UBound(arr) Then
CopyMemory arr(index + 1), arr(index), _
(UBound(arr) _ index) * LenB(arr(index))
End If
arr(index) = newValue
End Sub
Sub DeleteArrayItemLong(arr() As Long, index As Long)
If index < UBound(arr) Then
CopyMemory arr(index), arr(index + 1), _
(UBound(arr) _ index) * LenB(arr(index))
End If
arr(index) = Empty
End Sub
IMPORTANT NOTE: The prerequisite for using the CopyMemory
API function is that data must be stored in contiguous memory locations, so you
absolutely can't use it to insert or remove elements in String and Object arrays,
nor in arrays of UDTs that contain conventional strings, object references, or
dynamic arrays. (Fixed-length strings and static arrays in UDTs are OK, though.)
Note that while you can't use the preceding routines for arrays other than
Long arrays, the statements in the procedure body can be recycled for another
data type without any change, thanks to the use of the LenB function. Therefore,
you can derive new array functions that work for other data types by simply modifying
the procedure's name and its parameter list. For example, you can create a new
function that deletes an item in a Double array by editing just the first line
of code (shown in boldface):
Sub DeleteArrayItemDouble(arr() As Double, index As Long)
' All the other statements here are the same as in DeleteArrayItemLong
' ...
End Sub
More Topics on Visual Basic 6 Arrays
<< Previous | Contents
| Next >>
|
