Whenever you access or modify an element in an array, VB validates the index
values of the array to make sure that you aren’t trying to overwrite its
bounds. If you have 10 items in an array, you don’t want to inadvertently
refer to a nonexistent eleventh item, as for instance:
Dim aiMyArray (9) as Integer
This ordinarily gives you 10 items in the array, as Option Base 0 is the VB
default. Because there are 10 items, attempting to access the item at index 10
is a common mistake:
Dim iIndex as integer
Do While iIndex <= 10
iIndex = iIndex + 1
aiMyArray(iIndex) = iIndex
Loop
Because the index is zero-based, this loop never assigns a value to the item
at index 0, and the last pass through the loop attempts to assign a value to a
nonexistent eleventh item in the array. By default, VB saves you from attempts
like this to overstep the bounds of an array. (It doesn’t help you with
the overlooked item 0, however.) Naturally, this takes some processing time. You
can eliminate this checking by using this optimization option.
To make sure nothing goes wrong, you will want to use the
Ubound() and Lbound() functions to ensure that
you aren’t doing anything illegal. The real savings will occur if you process
arrays in loops. Instead of having VB’s automatic checking occur with every
access, you can conduct your tests outside the loop. This way, you can still be
assured that your program is safe and cut down on the arrayprocessing overhead.
The preceding example, for instance, could be rewritten this way:
Dim aiMyArray (9) as Integer
Dim iIndex as Integer, iLimit as Integer
iIndex = LBound(aiMyArray) ‘ assure start at first item
iLimit = UBound(aiMyArray)
Do While iIndex <= iLimit
iIndex = iIndex + 1
aiMyArray(iIndex) = iIndex
Loop
Now that the necessary checks are in place, you can take advantage of this
optimization without fear of writing beyond the bounds of the array.
