1. IS 350 Arrays

2. Objectives
Understand the concept of random numbers and how to generate them
Describe the similarities and differences between arrays and collections
Declare arrays and use Do loops and For loops to examine array elements
Sort, reverse, and search for elements in an array
Find and diagnose common array errors
Work with multidimensional arrays and arrays of objects 2

3. Introduction to Random Numbers
Many applications require the use of random numbers
Gaming applications
Computer simulations
Creating test data 3

4. Random Number Generation
A random number generator is initialized based on a seed value
Given the same seed value, the same random number sequence will be generated
Different seed values will generate different random number sequences
The time of day is often used to create random seed values

5. The System.Random Class
The System.Random class is used to create a random number generator and get random values
The constructor creates the random number generator
Without arguments, a random seed value is used
The seed value is based on the time of day
Dim RandomSeed As New Random()
With an argument, a constant seed value is used
Dim FixedSeed As New Random(10)

6. The Next Method
Without arguments, the Next method gets a positive random Integer value
Dim RandomValue As Integer
RandomValue = RandomSeed.Next()
With two arguments, a value within a range is returned
The first argument contains the lower bound and the second argument contains the upper bound (exclusive)
RandomValue = RandomSeed.Next(1, 10)

7. The NextDouble Method
The NextDouble method returns a random value between 0.0 and 1.0
Example:
Dim RandomDouble As Double
RandomDouble = _ RandomSeed.NextDouble()

8. Generating many Random Values
Most often, a sequence of random numbers is generated rather than just one
Example to add random values to a list box:
Dim CurrentRandom As New System.Random()
Dim CurrentIndex As Integer
Dim CurrentRandomValue As Double
For CurrentIndex = 1 To 100
CurrentRandomValue = _
CurrentRandom.NextDouble()
lstRandomList.Items.Add( _
CurrentRandomValue.ToString())
Next

9. Introduction to Arrays
Chapter 8 introduced the concept of a collection (list)
This chapter introduces the concept of an array
Lists and arrays have similarities and differences
Both arrays and instances of the List class store multiple items having the same data type
The size of both lists and arrays can be changed at run time
The Add and RemoveAt methods work with lists
An array's size is changed through a process called redimensioning

10. Characteristics of Arrays
An array stores multiple data items, each having the same data type
An array can store a list of values having primary data types or other data types
An array of integers or text boxes, for example
An array has one or more dimensions
A one-dimensional array can be thought of as a list
A two-dimensional array can be thought of as a grid or table
A three-dimensional array can be thought of as a cube

11. Characteristics of Arrays (continued)
The number of dimensions in an array is called the rank
A one-dimensional array has a rank of 1
A two-dimensional array has a rank of 2
Each data item stored in an array is called an element
An array element is referenced by a unique index value called a subscript
One subscript is used to reference elements in a one-dimensional array
Two subscripts are used to reference elements in a two-dimensional array

12. The Array Class (members)
The Length property gets the number of elements in the array
The Length property is 1-based
The Rank property gets the number of dimensions
A one-dimensional array has a Rank of 1
The GetLength method gets the number of array elements in a particular dimension
The 0-based array dimension is passed as an argument
The GetUpperBound and GetLowerBound methods get the largest and smallest subscript for a dimension
These methods are 0-based
The array dimension is passed as an argument
The Sort method sorts an array or part of an array

13. Common Array Tasks
Arrays must be declared just as any variable must be declared
It is possible to declare and initialize an array in the same statement
It is possible to determine the bounds of an array
An array's size can be changed at run time
Data must be stored and retrieved to and from array elements

14. Declaring Arrays (syntax)
[Public | Friend | Private | Dim] arrayName([size]) As dataType = initExpr
The access modifier (Public, Friend, Private) defines the array's visibility
The identifier (name) is defined by arrayName
The optional size contains the value of the largest subscript
The size argument is 0-based
Omit the size to declare a dynamic uninitialized array
dataType defines the data type of the array’s elements
initExpr is used to assign initial values to the array's elements

15. Declaring Arrays (examples)
Declare an uninitialized one-dimensional array
Dim EmptyArray() As Integer
Declare a one-dimensional array with one element
Dim OneElementArray(0) As Integer
Declare a one-dimensional array with four elements
Dim SingleArray(3) As Single

16. Implementation of Arrays
The Array class is a reference type
Thus, any array variable stores the memory address of the actual array
An uninitialized array has a value of Nothing

17. How memory is allocated to an array

18. Initializing an Array An array can be initialized when it is declared
The initialization list appears in braces ({})
A comma separates each initialization value
The array must be dynamic (declared without an initial subscript value)

19. Initializing an Array (example)
Declare and initialize a Double array with four elements:
Dim QuarterlySalesAmounts() As Double = _ {240.92, , , }

20. Declaring and initializing an array

21. Declaring and Initializing Boolean and String Arrays
Declare and initialize a Boolean array
Dim WeekDayArray() As Boolean = _
{False, True, True, True, True, True, False}
Declare and initialize a String array
Dim MonthNames() As String = _
{"January", "February", "March", "April", "May", _
"June", "July", "August", "September", _
"October", "November", "December"}

22. Declaring and initializing an array of strings

23. Determining an Array's Bounds
The GetUpperBound method gets the largest subscript for a particular dimension
It returns the largest subscript not the number of elements
The GetLowerBound method gets the smallest subscript for a particular dimension
The 0-based dimension is passed as an argument to each method
The methods will throw an exception if the array dimension does not exist

24. Determining an Array's Bounds (Examples)
Get the lower and upper bound of the first array dimension for the array named MonthNames
Dim SmallestSubscript, _ LargestSubscript As Integer
SmallestSubscript = _ MonthNames.GetLowerBound(0) ' 0
LargestSubscript = _
MonthNames.GetUpperBound(0) ' 12

25. Redimensioning an Array
An array's size is determined by the number of dimensions in the array and the number of elements in each dimension
An array is redimensioned with the ReDim statement
Arrays must be redimensioned when the initial size is not known

26. The ReDim Statement (Syntax)
ReDim [Preserve] varname(size)
The ReDim statement redimensions an array
The ReDim statement is an executable statement
It must appear inside of a procedure
The optional Preserve keyword preserves the array's contents
There are restrictions on the use of the Preserve keyword
varname contains the name of the array to redimension
size contains the new array size

27. Redimensioning an Array (Example)
Declare an array
Dim IntegerList() As Integer = _ {24, 12, 34, 42}
Redimension the array (increasing the size by one to five elements)
ReDim IntegerList(4)
Redimension the array and preserve its contents
ReDim Preserve IntegerList(4)

28. Redimensioning an array

29. Performing Assignment Statements with Arrays
Assignment statements using arrays require a subscript
The subscript appears in parentheses following the array name
The data types of the left and right side of the assignment statement must be compatible

30. Performing Assignment Statements with Arrays (examples)
Declare an array with 13 elements (subscript values from 0 to 12)
Dim MonthlySalesAmounts(12) As Double
Store and retrieve a value from the second array element (a subscript value of 1)
Dim CurrentMonthlySalesAmount As Double
MonthlySalesAmounts(1) =
CurrentMonthlySalesAmount = _
MonthlySalesAmounts(1)

31. Storing and retrieving array values

32. Type Conversion and Arrays
Explicit type conversion of array elements is possible
Call methods of the System.Convert class or call the ToString method to convert an element to a string
Convert an array element (Double) to a string
Dim OutputString As String
OutputString = MonthlySalesAmounts(1).ToString()
Convert a String to a Double and store the result in an array element
MonthlySalesAmounts(1) = _
ToDouble(txtSalesAmount.Text)

33. Type Conversion and Arrays (Errors)
Use care to apply the subscript when working with arrays
The following statement is missing a subscript and will return the data type of the array's elements:
OutputString = SalesList.ToString()
Example output:
System.Double[]

34. Variables and Array Subscripts
Variables are often used as array subscripts
Especially when using loops to examine all the elements in an array
Variables used as subscripts must have an integral data type

35. Variables and Array Subscripts (example)
Reference an array element with a variable as a subscript:
Dim SubscriptValue As Integer = 1
Dim SalesListItem As Double
SalesListItem = _
MonthlySalesAmounts(SubscriptValue)

36. Using Loops to Examine Array Elements
Common array tasks involving loops
Calculate the total value of all array elements using an accumulator
Calculate the minimum value stored in an array
Calculate the maximum value stored in an array
Calculate the average value of the elements in an array

37. Using a loop to examine array elements and calculate their total value

38. Examine an Array's Elements (example)
Tally the values of an array's elements. AnnualSalesTotal contains the total value
Dim AnnualSalesTotal As Double = 0.0
Dim Counter As Integer = 1
Do While Counter <= _
MonthlySalesAmounts.GetUpperBound(0)
AnnualSalesTotal += _
MonthlySalesAmounts(Counter)
Counter += 1
Loop

39. Determining the Minimum Value of an Array’s Elements
First, initialize the current minimum value
Use the value of the first array element or
Use the largest value for the underlying data type
Use a loop to find the current minimum value comparing the current minimum value with the current array element’s value
Replace the current minimum value, if necessary

40. Determining the Minimum Value of an Array Element (example)
Find the minimum value in the array named MonthlySalesAmounts
The first element is not used in the example
Dim Counter As Integer = 1
Dim CurrentMinimum As Double = _
MonthlySalesAmounts(Counter)
For Counter = 1 To _
MonthlySalesAmounts.GetUpperBound(0)
If MonthlySalesAmounts(Counter) < _
CurrentMinimum Then
CurrentMinimum = _
End If
Next

41. Determining the Maximum Value of an Array Element
The process is nearly the same as finding the minimum value
The condition in the decision-making statement is reversed
Initialize the current maximum value to the value of the first element or the maximum value for the data type

42. Determining the Maximum Value of an Array Element (example)
Find the maximum value in the array named MonthlySalesAmounts
Note the first array element is not used in the example
Dim Counter As Integer = 1
Dim CurrentMaximum As Double = _
MonthlySalesAmounts(Counter)
For Counter = 1 To _
MonthlySalesAmounts.GetUpperBound(0)
If MonthlySalesAmounts(Counter) > _
CurrentMaximum Then
CurrentMaximum = _
End If
Next

43. Determining the Average Value of an Array's Elements
Using an accumulator, tally the value of all array elements
Divide the total value by the number of array elements to determine the average

44. Arrays as Function Arguments
An array can be passed to a procedure as an argument
In the Function declaration, declare the argument with parentheses in the same way a dynamic array is declared
Example:
Public Shared Function Total( _
ByVal argList() As Double) As Double
End Function

45. Sorting Array Elements
Use the Sort method of the System.Array class to sort array elements
Example:
Private SampleArray() As Double = _
{6.11, 4.12, 5.88, -6.44}
System.Array.Sort(SampleArray)
Example output:
-6.44, 4.12, 5.88, 6.11

46. Sorting Array Elements (continued)
It's possible to sort part of an array
Call the Sort method with additional arguments
The first argument contains the array to sort
The second argument contains the starting array subscript
The third argument contains the subscript of the last element to sort

47. Sorting Array Elements (continued)
Sort the array named SampleArray excluding the first element
Private SampleArray() As Double = _ {0, -6.44, 4.12, 5.88, 6.11}
System.Array.Sort(SampleArray, 1, _
SampleArray.GetUpperBound(0))

48. Sorting part of an array

49. Reversing Array Elements
Calling the Reverse method of the System.Array class reverses the order of an array’s elements
It's possible to reverse all array elements or a range of elements
The arguments to the Reverse method are the same as the arguments to the Sort method
Example:
System.Array.Reverse(SampleArray)

50. Introduction to Searching
It's often necessary to search for an array element
Searching can be performed in different ways
A sequential search examines each element, in order, until the element is found or the entire array has been searched
A sorted array can be efficiently searched using a binary search

51. Using a Sequential Search
A sequential search works on an unordered (unsorted) array
Each element is examined sequentially using a loop
If the item is found, the loop exits
If all elements are examined, the item is not found and the loop exits
Use the IndexOf method to search for an array element
First argument contains the array name
Second argument contains the search value

52. Using a Binary Search
If an array is sorted, a binary search can be used to find an array element
The performance is much better than a sequential search
Call the BinarySearch method instead of the IndexOf method
First argument contains the array name
Second argument contains the search value

53. Using a Binary Search (example)
Private SampleArray() As Double = _ {-6.44, 4.12, 5.88, 6.11} Dim SearchValue As Double = 5.88 Dim ArrayElement As Integer ArrayElement = System.Array.BinarySearch( _ SampleArray, _ SearchValue)

54. Comparisons using a binary search

55. Common Array Errors Errors are commonly made when processing arrays
Forgetting to include a subscript
Dim DemoArray(12) As Integer
DemoArray = 100
Subscript out of range errors
Dim DemoArray(12) As Double
For Counter = 0 To 24
Total += DemoArray(Counter)
Next

56. Introduction to Two-dimensional Arrays
A two-dimensional array has rows and columns
Conceptually, it's similar to a grid or table
Two-dimensional arrays have two subscripts instead of one
Declare a dynamic two-dimensional array
Dim Table(,) As Integer
Declare a two-dimensional array with 4 rows and 5 columns (20 elements)
Dim SalesArray(3, 4) As Integer

57. Initializing Two-dimensional Arrays
Two-dimensional arrays can be initialized just as one-dimensional arrays can be initialized
The array must not be given an initial size
The initialization list is nested as follows:
Private SalesArray(,) As Integer = { _
{150, 140, 170, 178}, _
{155, 148, 182, 190}, _
{162, 153, 191, 184}, _
{181, 176, 201, 203} _ }

58. Table 9-1 Sample sales data

59. Referencing Elements in a Two-dimensional Array
Two-dimensional arrays require two subscripts instead of one
A comma separates the two subscripts
Reference the third row and fourth column in the array named SalesArray
Dim Cell As Integer
Cell = SalesArray(2, 3)

60. Referencing an element in a two-dimensional array

61. Introduction to Three-dimensional Arrays
It's possible to create arrays with three dimensions
A three-dimensional array has three subscripts instead of two
Declare a dynamic three-dimensional array
Dim Cube(,,) As Integer
Declare a 3 by 3 by 3 array (9 elements)
Dim TicTacToeBoard(2, 2, 2) As String

62. Referencing an element in a three-dimensional array

63. Working with Arrays of Objects
Arrays can store object references in addition to storing primary data types
Example to store text box references
Dim TextBoxList(2) As TextBox
TextBoxList(0) = txtLogID
TextBoxList(1) = txtLogStartTime
TextBoxList(2) = txtLogEndTime

64. Memory allocation to store an array of text boxes

65. Chapter Summary Simulations require the use of random numbers
The System.Random class implements random numbers
The Next method gets a random integral value
The NextDouble method gets a random floating-point value
Arrays store a list of values instead of a single (scalar) value
Arrays have one or more dimensions
Reference an element using a subscript
Call GetLowerBound and GetUpperBound to get the largest subscript for a particular dimension
Multidimensional arrays have one or more dimensions