Part 3.4Arrays and lists in VB.NET

Collections

A collection can be one of many different type or structures in VB.NET. The main structures or types that are used are arrays and list structures. Other structures include hash maps and linked lists. All of these have their advantages, but tend to be increasingly more complex than the previous.

Arrays

An array is a structure that stores multiple sub-variables inside a much larger single entity variable. It is a fixed size sequential collection of elements that are all of the same type. A string is actually an array of characters in most programming languages.

One of the notable things about arrays is that unless declared they start at 0. Arrays can be any size but it must be declared first.

An array with some values of type Integer where n is the index of the element.

There are two particular ways of declaring an array in VB.NET. The first consists of giving the array values and the second consists of giving the array a fixed length. The first option is shown in the next sample.

Dim array() As String = {"Hello", "World"}
		

The previous example demonstrates a simple array consisting of 2 elements: "Hello" and "World". Elements within this array can be redeclared with new values. The following sample provides a method to output the array showing the value of each element before a change.

Private Sub OutputArray(ByVal arr() As String)
	For Each element As String In arr
		MsgBox(element)
	Next
End Sub

Sub Main()
	Dim array() As String = {"Hello", "World"}
	OutputArray(array)
	array(0) = "Hi"
	OutputArray(array)
End Sub
		

The other option for creating an array is to declare the length of the array and then add elements.

Dim array(1) As String
array(0) = "Hello"
array(1) = "World"
		

The same method can be used to read out the second array.

Private Sub OutputArray(ByVal arr() As String)
	For Each element As String In arr
		MsgBox(element)
	Next
End Sub

Sub Main()
	Dim array(0)(1) As String
	array(0) = "Hello"
	array(0)(1) = "World"
	OutputArray(array(0))
	array(0)(0) = "Hi"
	OutputArray(array(0))
End Sub
		

Loops work very much inline with arrays, as shown with the For Each loop in the examples. In the same way a character in a string can be accessed via an index, an array element can be accessed via its index. For a string if the second character was required it would look something like myString(1). The code for an array is very similar to this code. The following sample demonstrates getting a specific index from an array, noting that the array begins at the index 0.

Sub Main()
	Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
	MsgBox(array(4))
End Sub
		

The sample should output "Adam".

The previous sample shows how to get the value of one specific element given an index, but this sample leaves room for a looping program that can iterate through the whole array.

Sub Main()
	Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
	For pos = 0 To 5
		MsgBox(array(pos))
	Next
End Sub
		

Of course, this is not a very accurate method as it is limited to an array of length 5. The following sample shows how to obtain the length of an array.

Sub Main()
	Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
	For pos = 0 To array.Length()
		MsgBox(array(pos))
	Next
End Sub
		

System.Array methods

Arrays can do so much more than just iterate through. VB.NET has a wealth of additional classes that support the framework core. One such class is the System.Array class.

Reverse

Arrays can be reversed using the Reverse method of the System.Array class.

Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
System.Array.Reverse(array)
		

Sort

In the same way the Reverse method is called, the Sort method can be used. The Sort method sorts the array using the VB.NET sort method.

Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
System.Array.Sort(array)
		

IndexOf

The IndexOf function can be used to find the index of a specific element within an array.

Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
MsgBox(System.Array.IndexOf(array, "Jamie"))
		

Other System.Array methods

There are quite a few other System.Array methods such as Copy, BinarySearch, LastIndexOf, Find, Exist, ForEach, Resize and more that are not being discussed in this article.

Resizing an array

Although resizing an array is not difficult, there are other options to having to resize an array constantly such as using a list structure.

Resizing an array is done using the ReDim method. This is demonstrated below:

Dim array() As String = {"James", "Jack", "John", "Jamie", "Adam", "Frank"}
ReDim Preserve array(6)
array(6) = "Jim"
MsgBox(array(6))
		

Without the Preserve keyword the array would be empty and all previous elements removed.

Lists

An alternative more flexible way to store multiple sub-variables in the one variable is to use lists. Lists are a generic class of flexible arrays that do not need a length applied. Lists can even be converted to arrays if the need arises.

A list structure is very similar to an array, but instead of specifying an index to insert a value into, the Add method is called. All the work for resizing the array is done behind the scenes.

Add

Dim list As New List(Of String)
list.Add("Hello")
list.Add("World")
		

Items can also be inserted at specific indexes as shown in the next sample.

Insert

Dim list As New List(Of String)
list.Add("Hello")
list.Insert(0, "World")
		

Remove

As well as both of these methods there exists a method to remove an item. The method relies on an equality test between the specified item and the item in the list. This means that "Hello" ≠ "hello".

Dim list As New List(Of String)
list.Add("Hello")
list.Add("World")
list.Remove("Hello")
		

Because of the inherent flaws of the Remove method, it is often the case that the next method is used instead.

RemoveAt

The RemoveAt method will remove an element at a specified index from a list. This can be particularly useful when coupled with a loop.

Dim list As New List(Of String)
list.Add("Hello")
list.Add("World")
list.Add("My")
list.Add("name")
list.Add("is")
list.Add("Jamie")

Dim search As String = "Name"

Dim pos As Integer = 0
For Each s As String In list

	If LCase(s) = LCase(search) Then
		Exit For
	End If
	pos = pos + 1
Next

list.RemoveAt(pos)
		

The Exit For line is used to leave a For loop at any time.

Using the LCase function to get the lowercase of each string and comparing it is more efficient than performing multiple Remove methods.

Clear

Lists may also need to be cleared every once in a while. This is done by performing the Clear method on the list.

Dim list As New List(Of String)
list.Add("Hello")
list.Add("World")
list.Add("My")
list.Add("name")
list.Add("is")
list.Add("Jamie")

list.Clear()
		

A list can also be redeclared to clear it.

Dim list As New List(Of String)
list.Add("Hello")
list.Add("World")
list.Add("My")
list.Add("name")
list.Add("is")
list.Add("Jamie")

list = New List(Of String)
		

Convert a list to an array

Lists can be converted to arrays because of the similarities between the two types. This is a useful way of avoiding having to resize an array constantly but maintaining it's original array structure.

To convert a list to an array type the ToArray method is used.

Dim list As New List(Of String)
Dim s() As String
s = list.ToArray()
		

Other list methods

There are a variety of different list methods that are not being included in this article. Most of the methods are identical to those found in the System.Array class because of the similar structure link between lists and arrays.