Tuesday, October 27, 2009

Arrays in VB.NET


Arrays are programming constructs that store data and allow us to access them by numeric index or subscript. Arrays helps us create shorter and simpler code in many situations. Arrays in Visual Basic .NET inherit from the Array class in the System namespace. All arrays in VB as zero based, meaning, the index of the first element is zero and they are numbered sequentially. You must specify the number of array elements by indicating the upper bound of the array. The upper bound is the numder that specifies the index of the last element of the array. Arrays are declared using Dim, ReDim, Static, Private, Public and Protected keywords. An array can have one dimension (liinear arrays) or more than one (multidimensional arrays). The dimensionality of an array refers to the number of subscripts used to identify an individual element. In Visual Basic we can specify up to 32 dimensions. Arrays do not have fixed size in Visual Basic.

The following code demonstrates arrays.Imports System.Console
Module Module1

Sub Main()
Dim sport(5) As String
'declaring an array
sport(0) = "Soccer"
sport(1) = "Cricket"
sport(2) = "Rugby"
sport(3) = "Aussie Rules"
sport(4) = "BasketBall"
sport(5) = "Hockey"
'storing values in the array
WriteLine("Name of the Sport in the third location" & " " & sport(2))
'displaying value from array
End Sub

End Module

Understanding the Code

The above code declared a sport array of type string like this: Dim sport(5) as String. This sport array has 6 elements starting from sport(0) to sport(5). The first element of an array is always referred by zero index. The image below displays output from above code.

You can also declare an array without specifying the number of elements on one line, you must provide values for each element when initializing the array. The following lines demonstrate that:

Dim Test() as Integer
'declaring a Test array
Test=New Integer(){1,3,5,7,9,}

Reinitializing Arrays

We can change the size of an array after creating them. The ReDim statement assigns a completely new array object to the specified array variable. You use ReDim statement to change the number of elements in an array. The following lines of code demonstrate that. This code reinitializes the Test array declared above.

Dim Test(10) as Integer
ReDim Test(25) as Integer
'Reinitializing the array

When using the Redim statement all the data contained in the array is lost. If you want to preserve existing data when reinitializing an array then you should use the Preserve keyword which looks like this:

Dim Test() as Integer={1,3,5}
'declares an array an initializes it with three members
ReDim Preserve Test(25)
'resizes the array and retains the the data in elements 0 to 2

Multidimensional Arrays

All arrays which were mentioned above are one dimensional or linear arrays. There are two kinds of multidimensional arrays supported by the .NET framework: Rectangular arrays and Jagged arrays.

Rectangular arrays

Rectangular arrays are arrays in which each member of each dimension is extended in each other dimension by the same length. We declare a rectangular array by specifying additional dimensions at declaration. The following lines of code demonstrate the declaration of a multidimensional array.

Dim rectArray(4, 2) As Integer
'declares an array of 5 by 3 members which is a 15 member array
Dim rectArray(,) As Integer = {{1, 2, 3}, {12, 13, 14}, {11, 10, 9}}
'setting initial values

Jagged Arrays

Another type of multidimensional array, Jagged Array, is an array of arrays in which the length of each array can differ. Example where this array can be used is to create a table in which the number of columns differ in each row. Say, if row1 has 3 columns, row2 has 3 columns then row3 can have 4 columns, row4 can have 5 columns and so on. The following code demonstrates jagged arrays.

Dim colors(2)() as String
'declaring an array of 3 arrays
colors(0)=New String(){"Red","blue","Green"}
initializing the first array to 3 members and setting values
colors(1)=New String(){"Yellow","Purple","Green","Violet"}
initializing the second array to 4 members and setting values
colors(2)=New String(){"Red","Black","White","Grey","Aqua"}
initializing the third array to 5 members and setting values

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