1. Lecture Roger Sutton c.r.sutton@kent.ac.uk CO331 Visual Programming 12: One-dimensional Arrays 1

2. CO331 Visual Programming 2 Declaration An array is a consecutive group of memory locations that have the same name and type. Their declaration will include an explicit indication of the size of the array or the means to infer its size. E.g.Dim marks(5) As integer This indicates to the compiler that 6 memory locations should be reserved for an integer array called marks. The first element of an array has a position number 0. On declaration, the upper bound (i.e. the highest index number) of the array is specified (not the actual size). Several arrays (or variables) may be declared in the same line: E.g.Dim x(6) As Integer, s(24) As String

3. CO331 Visual Programming 3 Array data types An array can hold any data type Primitive data type, e.g.  Integer  Double  Boolean ADTs, e.g.  Controls- Button,TextBox  VB library- Random  User-defined- Circle,Square,Account The only constraint is that all the elements in an array must be of the same type

4. CO331 Visual Programming 4 Initialisation Arrays may be initialised on declaration, E.g. Dim age( ) As Integer = { 23, 54, 96, 13, 7, 32} Here the size of the array is not specified but is inferred by the number of initial values. By default numeric array elements are initialised to zero String to “” Objects to Nothing. Alternatively a programmer may explicitly initialise the array using assignment statements. Dim age(5) As Integer age(0) = 23 age(1) = 54 age(2) = 96 age(3) = 13 age(4) = 7 age(5) = 32

5. CO331 Visual Programming 5 Referencing To reference a particular element of the array, its name and position number are specified. E.g. the fourth element of marks array is referred to by: marks(3) The position number contained within brackets is more formally called an index. Sometimes it is useful to use a variable value as an index. Such variables are usually declared to be Integer. The Functions LBound and Ubound take an array as their argument and return the lowest and highest numbered index value respectively. These are useful to determine the length of an array and ensure an index remains within its declared range.

6. CO331 Visual Programming 6 E.g. Loops involving arrays The following accumulates a set of marks: This is equivalent to the following which uses a while loop: Dim std As integer Dim total As integer = 0 For std = LBound(marks) to UBound(marks) total += marks(std) Next Dim total As integer = 0 Dim std As Integer = 0 Do while std < UBound(marks) total += marks(std) std += 1 Loop

7. CO331 Visual Programming 7 Example: program

8. CO331 Visual Programming 8 Methods and properties All arrays have access to the methods and properties of System.Array class. E.g.  Length – returns the total number of elements in the array  getUpperBound( 0 ) - returns the upper bound of the one-dimensional array  Sort - sorts the array in an ascending order

9. CO331 Visual Programming 9 Methods and properties – cont’d E.g. Dim size, highestIndexNo As Integer Dim myArray() As Integer = {2, 3, 4, 1} size = myArray.Length highestIndexNo = myArray.GetUpperBound(0) Array.Sort(myArray) 2341 myArray size highestIndexNo myArray 4 3 1234

10. CO331 Visual Programming 10 Example: using arrays Listing days of the week: cmdShow method Private Sub cmdShow_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles cmdShow.Click Dim weekDays() As String = {"Monday", "Tuesday", "Wednesday", _ "Thursday", "Friday"} Dim i As Integer lstBox.Items.Add("Index Day") For i = 0 To weekDays.getUpperBound(0) lstBox.Items.Add( CStr(i) & Tab & weekDays(i) ) Next End Sub

11. CO331 Visual Programming 11 Change array size Once an array is created its length/size is fixed its index ranges from 0 to its upper bound When a program is executed, Visual Studio automatically performs bounds checking to ensure the program does not attempt to access data outside the bounds of an array. Referencing array elements outside the array bounds causes a runtime error. ReDim – is used to change the length/size of an array at run- time, i.e. as a program executes.

12. CO331 Visual Programming 12 Changing array size - cont’d E.g. ReDim Preserve weekDays(6) weekDays(5) = "Saturday" weekDays(6) = "Sunday " Dim weekDays() As String = { "Monday", "Tuesday", "Wednesday", _ "Thursday", "Friday" } weekDays.Length is 5 weekDays.GetUpperBound(0) is 4 weekDays.Length is 7 weekDays.GetUpperBound(0) is 6 The keyword Preserve ensures that data stored in the array are retained.

13. CO331 Visual Programming 13 Passing Arrays An array is passed to a method or function by including the array name in the argument list. E.g.Dim marks(5) As Integer CalculateStats(marks) A single array element may also be passed by simply specifying the array element in the procedure’s argument list. E.g.TopMark(marks(3)) For a method to receive an array through a call, the parameter list must indicate that an array will be received. E.g.Private Sub useArray( ByVal x( ) As Integer ) Note the size of the array is not specified between the brackets.

14. CO331 Visual Programming 14 Passing Arrays - cont’d E.g. Calculate the average course mark: Dim courseMarks() As Integer = {85, 70, 90, 65} Dim mean As Double mean = Average(courseMarks) MessageBox.Show(" Mean: " & CStr(mean) ) Private Function Average(ByVal arr() As Integer) As Double Dim i As Integer Dim totalValue As Integer = 0 Dim totalNo As Integer = arr.Length For i = 0 To totalNo - 1 totalValue += arr(i) Next Return Math.Round( totalValue / totalNo, 1) End Function The size of the array is not specified.

15. CO331 Visual Programming 15 Summary An array is a collection of data with a single name All the elements in an array are of the same type The index of an array starts at 0 All arrays have access to methods and properties of System.Array class, e.g.  Length  getUpperBound( 0 )  Sort Referencing array elements outside the array bounds causes a runtime error The size of an can be changed at run-time using ReDim