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Chapter 8

Chapter 8. Arrays, Timers, and More. Chapter 8 Introduction. Chapter 8 Topics. Arrays are like groups of variables that allow you to store sets of similar data A single dimension array is useful for storing and working with a single set of data

james-french
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Chapter 8

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  1. Chapter 8 Arrays, Timers, and More

  2. Chapter 8Introduction

  3. Chapter 8 Topics • Arrays are like groups of variables that allow you to store sets of similar data • A single dimension array is useful for storing and working with a single set of data • A multidimensional array can be used to store and work with multiple sets of data • Array programming techniques covered • Summing and averaging all the elements in an array • Summing all the columns in a two-dimensional array • Searching an array for a specific value • Using parallel arrays

  4. Section 8.1Arrays An Array Is Like a Group of Variables With One Name You Store and Work With Values in an Array by Using a Subscript

  5. Array Characteristics • An array stores multiple values of same type • Like a group of variables with a single name • For example, the days of the week would be: • a set of 7 string variables • maximum length of 9 characters • All variables within an array are called elementsand must be of the same data type • You access the elements in an array through a subscript

  6. Subscript Characteristics • A subscript, also known as an index, is a number that identifies a specific element within an array • Subscript numbering works like the index for a list box: • Subscript numbering begins at 0 • 1st element in an array is always subscript 0 • Last element is total number of elements - 1

  7. Declaring an Array • ArrayName is the variable name of the array • UpperSubscript is the value of the array's highest subscript • Must be a positive whole number • DataType may be any valid Visual Basic data type • VB knows an array is declared when name is followed by number of elements in parens Dim ArrayName(UpperSubscript) As DataType

  8. Array Declaration Example • Note that 6 elements are available when an array with an uppersubscript of 5 is declared • Each element is an integer type Dim hours(5) As Integer hours(0) hours(1) hours(2) hours(3) hours(4) hours(5)

  9. Default Initialization • All elements of an Integer array are initialized to zero • Same initialization as an integer variable • Each array element is initialized exactly the same as a simple variable of that data type • A Single is initializled to zero (0.0) • A String is initialized to nothing (empty string)

  10. Implicit Array Sizing & Initialization • An array can be initialized when declared • Example: • This array is implicitly sized • Upper subscript value is left blank • Number of elements implied from initialization • Upper subscript of 5 implied by this example • A 6 element array is declared • Elements are assigned the values shown Dim numbers() As Integer = {2, 4, 5, 7, 9, 12}

  11. Named Constant as Upper Subscript • A named constant may be used as an array's highest subscript instead of a number • A common use for named constants • Highest subscript is often used multiple times • If highest subscript changes, use of a named constant allows it to be changed in one place Const UPPER_SUB As Integer = 100 Dim array(UPPER_SUB) As Integer

  12. Working With Array Elements • Simply include the subscript to use an array element like an ordinary variable • numbers(2) refers to 3rd element of array numbers(0) = 100 numbers(1) = 200 numbers(2) = 300 numbers(3) = 400 numbers(4) = 500 numbers(5) = 600 pay = hours(3) * rate tallies(0) += 1 MessageBox.Show(grossPay(5).ToString)

  13. Arrays and Loops • Loops are frequently used to process arrays • Use a loop to prompt for 10 values • Use a loop to reset values in the names array to an empty string Dim count As Integer For count = 0 To 9 series(count) = CInt(InputBox("Enter a number")) Next count Dim count As Integer For count = 0 To 999 names(count) = "" Next count

  14. Array Bounds Checking • Visual Basic checks each subscript value of each array reference used at run time • A subscript is invalid if it is • Negative • Greater than the UpperSubscript declared • An invalid subscript causes VB to throw an exception • Bounds checking is not done at design time

  15. For Each … Next Statement • This statement is similar to a For…Next • Iterates for each element in the array • strName assigned value of next array element at each iteration • Tutorial 8-1 demonstrates array processing Dim employees As String() = {"Jim", "Sally", _ "Henry", "Jean", "Renee"} Dim strName As String For Each strName In employees MessageBox.Show(strName) Next name

  16. Section 8.2More About Array Processing There Are Many Uses of Arrays and Many Programming Techniques That Involve Them For Example, Arrays Are Used to Total Values or Search for Data

  17. Determining Number of Elements • Arrays have a Length property that holds the number of elements in the array • Note that length is number of array elements, not the upper subscript of the array • Length property always 1 greater than the upper subscript Dim values(25) As Integer For count = 0 to (values.Length – 1) MessageBox.Show(values(count).ToString) Next count

  18. Total the Values in an Array • Variable to hold sum (intTotal)is initialized to zero prior to loop • Iterate over all elements, adding each element to intTotal Dim intUnits(24) as Integer ‘Declare array Dim intTotal As Integer = 0 'Initialize accumulator Dim intCount as Integer ‘Declare loop counter ‘Find total of all values held in array For intCount = 0 To (intUnits.Length – 1) intTotal += intUnits(intCount) Next intCount

  19. Average the Values in an Array • Similar to previous example but then divides total by number of elements to get average Dim intUnits(24) as Integer ‘Declare array Dim intTotal As Integer = 0 'Initialize accumulator Dim dblAverage as Double ‘Declare average var Dim intCount as Integer ‘Declare loop counter ‘Find total of all values held in array For intCount = 0 To (intUnits.Length – 1) intTotal += intUnits(intCount) Next intCount ‘Use floating-point division to compute average dblAverage = intTotal / intUnits.Length

  20. Find Highest & Lowest Array Values • Pick first element as the highest • Search rest of array for higher values • If a higher value is located, save the value • Use similar logic to find lowest value Dim intNumbers(24) as Integer Dim intCount as Integer Dim intHighest as Integer = intNumbers(0) For intCount = 1 To (intNumbers.Length - 1) If intNumbers(intCount) > intHighest Then intHighest = intNumbers(intCount) End If Next intCount

  21. Copy Values in One Array to Another • Done by copying elements one at a time • Note that this cannot be done by a simple assignment newValues = oldValues • Causes newValues variable to reference the same memory location as oldValues • Thus any change to oldValues will affect newValues and vice versa For intCount = 0 To 100 newValues(intCount) = oldValues(intCount) Next intCount

  22. Parallel Arrays • Sometimes useful to store related data in two or more arrays called parallel arrays • Causes the ith element of one array to be related to the ith element of another • Allows related data to be accessed using the same subscript on both arrays

  23. Parallel Arrays Example • Assume the following array declarations • Element 1 refers to the same person with name in one array and address in the other Dim names(4) As String Dim addresses(4) As String Names(0) Names(1) Names(2) Names(3) Names(4) Person #1 Person #2 Person #3 Person #4 Person #5 Addresses(0) Addresses(1) Addresses(2) Addresses(3) Addresses(4)

  24. Parallel Arrays Processing • Use parallel array processing to add name and address of each person to a list box • Tutorial 8-2 has an example of parallel arrays Dim names(4) As String Dim addresses(4) As String For intCount = 0 To 4 lstPeople.Items.Add("Name: " & names(intCount) & _ " Address: " & addresses(intCount)) Next intCount

  25. Array & List Box Parallel Processing • A list box selection used as an array index ' Initialize a List Box with names lstPeople.Items.Add("Jean James") ' Subscript 0 lstPeople.Items.Add("Kevin Smith") ' Subscript 1 lstPeople.Items.Add("Joe Harrison") ' Subscript 2 ' Initialize an Array with corresponding phone numbers phoneNumbers(0) = "555-2987" phoneNumbers(1) = "555-5656" phoneNumbers(2) = "555-8897" ' Process a selection If lstPeople.SelectedIndex > -1 And _ lstPeople.SelectedIndex < phoneNumbers.Length Then MessageBox.Show(phoneNumbers(lstPeople.SelectedIndex)) Else MessageBox.Show("That is not a valid selection.") End If

  26. Searching Arrays, The Search • Find 1st instance of value 100 in array scores • intPosition gives position of this value Dim blnFound as Boolean = False Dim intCount as Integer = 0 Dim intPosition as Integer = -1 ‘ Search for a 100 in the array Do While Not blnFound And intCount < scores.Length If scores(intCount) = 100 Then blnFound = True intPosition = intCount End If intCount += 1 Loop

  27. Searching Arrays, The Result • Indicates whether the value was found • If found, position is given as a test number ' Was 100 found in the array? If blnFound Then MessageBox.Show( _ "Congratulations! You made a 100 on test " & _ (intPosition + 1).ToString, "Test Results") Else MessageBox.Show( _ "You didn’t score a 100, but keep trying!", _ "Test Results") End If

  28. Sorting an Array • Arrays have a Sort method • Arranges elements in ascending order (lowest to highest) • Sorted so that numbers = {1, 3, 6, 7, 12} • Sorted so that names = {Alan, Bill, Kim, Sue} Dim numbers() As Integer = { 7, 12, 1, 6, 3 } Array.Sort(numbers) Dim names() As String = { "Sue", "Kim", _ "Alan", "Bill" } Array.Sort(names)

  29. Resizing an Array • ReDim is a new keyword • If Preserve is specified, the existing contents of the array are preserved • Arrayname names the existing array • UpperSubscript specifies the new highest subscript value • Can declare an array with no subscript and state number of elements later with ReDim ReDim [Preserve] Arrayname(UpperSubscript)

  30. Resizing Example • Array scores declared with no elements • User prompted for number of elements • ReDim resizes array based on user input Dim scores() As Single ' Declared with no elements Dim numScores as Integer ' Obtain number of elements from the user numScores = CInt(InputBox("Enter number of test scores")) If numScores > 0 Then ReDim scores(numScores - 1) Else MessageBox.Show("You must enter 1 or greater.") End If

  31. Section 8.3Sub Procedures and Functions That Work With Arrays You May Pass Arrays As Arguments to Sub Procedures and Functions You May Also Return an Array From a Function This Allows You to Write Sub Procedures and Functions That Perform General Operations With Arrays

  32. Passing Arrays as Arguments • Array numbers passed to DisplaySum sub • Sub computes/shows sum of array elements • Can pass any integer array to DisplaySum Dim numbers() As Integer = { 2, 4, 7, 9, 8, 12, 10 } DisplaySum(numbers) Sub DisplaySum(ByVal intArray() As Integer) Dim total As Integer = 0 ' Accumulator Dim count As Integer ' Loop counter For count = 0 To (intArray.Length - 1) total += intArray(count) Next MessageBox.Show(“Total is " & total.ToString) End Sub

  33. Passing Arrays: ByVal and ByRef • When passing an array ByVal • Calling procedure “sees” sub procedure changes to element values • Simple variables don’t work this way • If sub assigns array argument to another array, no effect on array in calling procedure • When passing an array ByRef • Calling procedure “sees” sub procedure changes to element values • If sub assigns array argument to another array, calling procedure array values affected

  34. Passing Arrays Example • After ResetValues procedure executes, numbers array still contains { 1, 2, 3, 4, 5 } • If array passed ByRef, numbers array will contain { 0, 0, 0, 0, 0 } after procedure runs Dim numbers() As Integer = { 1, 2, 3, 4, 5 } ResetValues(numbers) Sub ResetValues(ByVal intArray() As Integer) Dim newArray() As Integer = {0, 0, 0, 0, 0} intArray = newArray End Sub

  35. An Array Returned From a Function • Return type String() indicates an array of strings • Thus the function result must be assigned to an array of strings Function GetNames() As String() ' Get four names from user ' Return them as an array of strings. Dim strNames(3) As String Dim strInput As String Dim count As Integer For count = 0 To 3 strInput = InputBox("Enter name " & _ (count + 1).ToString) strNames(count) = strInput Next Return strNames End Function

  36. Section 8.4Multidimensional Arrays You May Create Arrays With More Than Two Subscripts to Hold Complex Sets of Data

  37. A Two Dimensional Array Picture • Thus far, arrays have been one-dimensional • However, arrays can also be two-dimensional • Picture a two-dimensional array like a spreadsheet with rows and columns Column 0 Column 1 Column 2 Column 3 Row 0 Row 1 Row 2

  38. Two Dimensional Array Syntax • UpperRow and UpperColumn give the highest subscript for the row and column indices of the array • The array on the previous slide could be: • Defines three rows; 0, 1, and 2 • And four columns; 0, 1, 2, and 3 Dim ArrayName(UpperRow, UpperColumn) As DataType Dim array(2,3) As Single

  39. Two Dimensional Array Subscripts Dim array(2,3) As Single Column 0 Column 1 Column 2 Column 3 Row 0 array(0,0) array(0,1) array(0,2) array(0,3) Row 1 array(1,0) array(1,1) array(1,2) array(1,3) Row 2 array(2,0) array(2,1) array(2,2) array(2,3)

  40. Nested Loops And Two Dimensions • Nested loops are often used in processing two-dimensional arrays • In the example, a nested loop is used to insert a value into every element of array scores For row = 0 To 2 For col = 0 To 3 num = Val(InputBox("Enter a score.")) scores(row, col) = num Next col Next row

  41. Implicit Sizing and Initialization • Can be used with multi-dimensional arrays: • Row 0 values • Row 1 values • Row 2 values • Initializes array numberswith the following values Dim numbers(,) As Integer = _ { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }

  42. For Each Loop With Two Dimensions • A For Each Loop will process all elements of an array without requiring nested loops • The example below computes the sum of all elements • Total has the value 45 when loop is complete Dim numbers(,) As Integer = {{1, 2, 3}, _ {4, 5, 6}, _ {7, 8, 9}} For Each element In numbers total += element Next element

  43. Sum Two-Dimensional Array Columns • Outer loop controls column subscript • Inner loop controls row subscript ' Process each column in turn For col = 0 To 2 ' Initialize column accumulator total = 0 ' Sum the column values from each row For row = 0 To 4 total += values(row, col) Next row ' Display the sum of the column. MessageBox.Show("Sum of column " & _ col.ToString & " is " & total.ToString) Next col

  44. Three-Dimensional Arrays & Beyond • VB allows arrays of up to 32 dimensions • Beyond three dimensions, they are difficult to visualize • But, all one needs to do is to be consistent in the use of the different indices

  45. Section 8.5Enabled Property, Timer Control, and Splash Screens You Disable Controls by Setting Their Enabled Property to False The Timer Control Allows Your Application to Execute a Procedure at Regular Time Intervals Splash Screens Are Forms That Appear As an Application Begins Executing

  46. Enabled Property • Most controls have an Enabled property • If this Boolean property is set to false the control is disabled meaning the control: • Cannot receive the focus • Cannot respond to user generated events • Will appear dimmed, or grayed out • Default value for this property is true • May be set in code when needed as shown: btnExample.Enabled = False

  47. Timer Control • A Timer control generates Tick events • Double-click on Timer tool in Toolbox to add • Appears in component tray • Standard prefix is tmr • Can perform a process at regular intervals by coding the process in a Tick event procedure • Two important properties • Enabled must be true to generate Tick events • Interval (milliseconds) decides tick frequency • Interval of 1000 causes 1 timer tick per second • Tutorial 8-5 demonstrates the Timer control

  48. Splash Screens • A form, often with an application logo, that is displayed while an application is loading • A splash screen usually: • Has Topmost property set to True so it is displayed over the forms of other applications • Disappears shortly by using a Timer • Is modeless so the application continues to load while the splash screen is displayed • Tutorial 8-6 demonstrates a splash screen

  49. Section 8.6Anchoring and Docking Controls Controls Have Two Properties, Anchor and Dock, That Allow You to Determine the Control’s Position on the Form When the Form Is Resized at Run Time

  50. Anchor Property • Anchor - a property of various form controls • A control anchored to a form edge keeps the same distance to the form edge • Controls may be anchored to any, all, or none of the right, left, top, or bottom edges • Default is to anchor to top and left edges • Can just as easily anchor to bottom right • If anchoring to opposing sides, control is resized when form is resized • Anchoring to all 4 sides causes control to be resized proportionally to form

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