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Unit 7: Arrays in Visual Basic

7.2 ReDim and Preserve Function

After an array has been declared, its size (but not its type) can be changed with a statement of
the form
ReDim arrayName(m)

where arrayName is the name of the already declared array and m is an Integer literal, variable,
or expression. Note: Since the type cannot be changed, there is no need for an “As dataType”
clause at the end of the ReDim statement.

Visual Basic allows you to declare an array without specifying an upper bound with a statement
of the form

Dim arrayName() As varType
Later, the size of the array can be stipulated with a ReDim statement.

The ReDim statement has one shortcoming: It causes the array to lose its current contents. That
is, it resets all String values to Nothing and resets all numeric values to 0. This situation can be
remedied by following ReDim with the word Preserve. The general form of a ReDim Preserve
statement is
ReDim Preserve arrayName(m)

Of course, if you make an array smaller than it was, data in the eliminated elements will be lost.

We can use the ReDim statement repeatedly to change the number of elements
and dimensions in an array. However, we can’t declare an array of one data
type and later use ReDim to change the array to another data type, unless the
array is contained in a Variant.

1: The following program reads the names of the winners of Super Bowl games
from a file and places them into an array. The user can type a team’s name
into a text box and then display the numbers of the Super Bowl games won
by that team. The user has the option of adding winners of subsequent
games to the array of winners. The program uses the text file
SBWINNERS.TXT whose lines contain the names of the winners in order.
That is, the first four lines of the file contain the names Packers, Packers,
Jets, and Chiefs.
Dim teamName() AsString
Dim upperBound As Integer
Private Sub frmBowl_Load(...) HandlesMyBase.Load
Dim sr As IO.StreamReader = IO.File.OpenText(“SBWINNERS.TXT”)
Dim name As String, numLines As Integer
‘Count number of lines in the file and assign it minus one to upperBound
numLines = 0
Do While (sr.Peek <> -1)

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