When using arrays you must be careful not to consume too much memory. For example
Dim MyArray (10000) As Long
Because each element is declared as long, and a long variable occupies 4 bytes of memory, the MyArray requires 40 004 (10 001 x4) bytes of memory. This may not sound like much, but when you have 10 such arrays in your program these arrays consume 400 040 (40 004 x 10) bytes of memory. Therefore, it is wise to always try to set the size of your arrays to the minimum your program requires. Sometimes, however, it is only possible to detemine this during runtime. In these cases you can use the ReDim statement to change the size of an array. An array that changes its size during runtime is called a dynamic array.
When you declare a dynamic array, you do not declare it like a fixed array. When you declare a dynamic array the size is not specified. Instead you use the following syntax:
Dim ArrayName() As DataType
Dim is the scope of the array. If you declare it in a Form, use Dim, if you declare it in a Module and you want every procedure to access it, declare it as Global, otherwise use Public. If you declare the array in a procedure use Dim. ArrayName is the name of the array. DataType is a valid datatype. Normally Integer (-32 768 to 32 767), String (A string of characters), Boolean (True or False).
You then use the ReDim statement in your procedures or functions, using the following syntax:
ReDim ArrayName(LowerValue To HigherValue)
In fact, it is almost identical to a normal declaration for a fixed array, except that 1), it is not a declaration, as it is executed at runtime, 2) it uses ReDim, and 3) there is no datatype declaration (this cannot be changed).
So, the following code declares a dynamic array called gArray, and then sets the size during runtime:
Dim sStringArray() As String
Sub Form1_Load()
' Initialise array
ReDim sStringArray(1 To 10)
End Sub
This code assigns 10 elements to gArray when Form1 loads. Note: When using dynamic arrays, you must set the size of an array using the ReDim statement, before filling the array.
However, when using the ReDim statement, any values already in the array (if it has been resized previously), will be deleted. In some cases, this is not what you would want! So, you use the Preserve keyword:
ReDim Preserve ArrayName(LowerValue To HigherValue)
If the array has grown, there will be a number of blank array spaces at the end of the array. If the array has shrunk, you will lose the end items.
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