Visual Basic Dynamic Array Introduction

A dynamic array is an array that changes size while the program is running. Unlike the static array in the example above, dynamic arrays do not specify their size when they are declared. Unless you are certain the size of an array won't change, (like 7 days in a week), it's best to use dynamic arrays, so your program can take full benefit of any enhancements added to the system the program is running on. Also, dynamic arrays can shrink in size when your program doesn't need so much memory, allowing other applications to utilize the system's resources.

Visual Basic Dynamic Array Declaration

General Syntax

(Dim, Private or Public) ArrayName() As DataType(i.e. int, double, or object)

For example, the following statement declares a dynamic array to store an unknown number of names. The declaration informs the compiler that there is a dynamic array with a specified name in the program to store String data. No memory is allocated at declaration time:

Dim EmployeeName () As String

The Keyword Dim is used to declare a local array, Keyword Private is used to declare module-scope arrays, and Keyword Public is used to declare global-scope arrays.

Visual Basic Resizing a Dynamic Array - Resetting Data

At declaration time, no memory is allocated for a dynamic array. In order to store values in a dynamic array, the array must be resized to prepare the array to accept data. There are two ways to resize a Visual Basic dynamic array.

ReDim: The keyword ReDim is used to resize a dynamic array to the desired size, while resetting the existing values in an array to their default values (0 for numeric arrays, and False for Boolean Arrays). An array can be resized as many times as necessary during the program execution to a bigger or smaller size. The general syntax for ReDim is:

ReDim ArrayName(NewArraySize)

Where ArrayName is the name of the Visual Basic dynamic array, and NewArraySize is a whole number greater than or equal to 0, specifying the size of the array. After ReDim, the dynamic array will have NewArraySize + 1 elements.

Create the Form_Load method by double-clicking on the blank form as indicated in the diagram below:

Figure Array-14: Create Form_Load Method

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Enter the code in the code window as indicated in the diagram below:

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, _
		ByVal e As System.EventArgs) Handles MyBase.Load
        Dim Data() As Short
        ReDim Data(2)
        Dim I As Integer

        Data(0) = 10
        Data(1) = 11
        Data(2) = 12

        ReDim Data(3)
        Data(3) = 13


        I = I

    End Sub
End Class

Figure Array-15: Visual Basic Code to Create Dynamic Array

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Let's take a look at the code before we run the program:

Private Sub Form1_Load: This is the first access point, as the program begins to execute.
Dim Data() As Short: Declare a dynamic array of type short (Numeric values between -32767 to 32768)
ReDim Data(2): Give the Array of size 2 - and causing all elements to become 0.
Dim I As Integer: Dummy variable so we can enable the Debugger.
Data(0) = 10: Set element 0 to 10.
Data(1) = 11: Set element 1 to 11.
Data(2) = 12: Set element 2 to 12.
ReDim Data(3): Resize Array to be 4 elements long.
Data(3) = 13: Set element 3 to 13.
I = I: The debugger will start at this point before the line is executed.

Set a breakpoint as indicated in the diagram below:

Figure Array-16: Set a Visual Basic Breakpoint to observe array values

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Make sure to point to the column just left of the code window. This is the Set/Clear Breakpoint column. The RED dot indicates that a breakpoint has been set. When the code parser comes to the red dot program execution will be suspended while the program developer observes the variable status.

Start the debugger by pressing the right triangle as indicated in the diagram below:

Figure Array-17: Visual Basic Start Debugging

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The program should execute until the parser reaches our Red Dot in the Breakpoint Column. The Red Dot should have a yellow right arrow (→) pointing to the line the program is ready to execute. The line waiting to be executed will be highlighted as indicated in the diagram below: