Introducing Visual Basic

Transcription

1 Introducing Visual Basic September 11, 2006 Chapter 3 - VB 2005 by Schneider 1

2 Today Continuing our intro to VB First some housework Chapter 3 - VB 2005 by Schneider 2

3 Classroom Contract Chapter 3 - VB 2005 by Schneider 3

4 Midterm Two choices: October 16th (week 7) October 23rd (week 8) Chapter 3 - VB 2005 by Schneider 4

5 Assignments First assignment will be this week General plan is to assign on Wednesday and have due the following Monday This will be adjusted to reflect how much work is required for the assignment No late assignments Collaboration? Cheating? Chapter 3 - VB 2005 by Schneider 5

6 CS Submission Server All assignments will be turned in via the submission server submit.cs.sfu.ca Currently down while prepping for semester Chapter 3 - VB 2005 by Schneider 6

7 GradeBook gradebook.cs.sfu.ca Check grades Individual assignments Comparative assignments Not set in stone! More info: Chapter 3 - VB 2005 by Schneider 7

8 Textbook Sold out presently Should be in by Friday at latest Chapter 3 - VB 2005 by Schneider 8

9 Now where were we? Back to chapter 3 So far you ve seen 3.1 Visual Basic Controls 3.2 Visual Basic Events Continuing today 3.2 Continuing with Events 3.3 Numbers 3.4 Strings 3.5 Input and Output Chapter 3 - VB 2005 by Schneider 9

10 Recall Control Boxes and buttons E.g. Text Box Control Button Control Label Control List Box Control Event An action (e.g. user click) Chapter 3 - VB 2005 by Schneider 10

11 3.2 Visual Basic Events An Event Procedure Walkthrough Properties and Event Procedures of the Form The Header of an Event Procedure Chapter 3 - VB 2005 by Schneider 39

12 Examples of Events btnshow.click txtbox.textchanged txtbox.leave General Form: controlname.event Chapter 3 - VB 2005 by Schneider 43

13 The three steps in creating a Visual Basic program: 1. Create the interface; that is, generate, position, and size the objects. 2. Set properties; that is, configure the appearance of the objects. 3. Write the code that executes when events occur. Chapter 3 - VB 2005 by Schneider 44

14 The three steps in creating a Visual Basic program: 1. Create the interface; that is, generate, position, and size the objects. 2. Set properties; that is, configure the appearance of the objects. 3. Write the code that executes when events occur. Chapter 3 - VB 2005 by Schneider 45

15 Code Consists of statements that carry out tasks E.g. change properties of a control or form Chapter 3 - VB 2005 by Schneider 46

16 Code Window Page tab Class Name box Method Name box Chapter 3 - VB 2005 by Schneider 48

17 Structure of an Event Procedure Header Private Sub objectname_event(...) Handles objectname.event statements End Sub (...) is filled automatically with (ByVal sender As System.Object, ByVal e As System.EventArgs) Chapter 3 - VB 2005 by Schneider 49

18 Create an Outline for an Event Procedure; i.e. header and End Sub 1. Double-click on a control or 2. Use the Class Name and Method Name boxes. (We primarily use the first method.) Chapter 3 - VB 2005 by Schneider 50

19 Sample Form txtfirst txtsecond btnred Double Click on txtfirst Chapter 3 - VB 2005 by Schneider 51

20 Code for Walkthrough Public Class frmdemo Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged End Sub End Class Chapter 3 - VB 2005 by Schneider 52

21 Code for Walkthrough Public Class frmdemo Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged txtfirst.forecolor = Color.Blue End Sub End Class Chapter 3 - VB 2005 by Schneider 53

22 IntelliSense Automatically pops up to give the programmer help. Chapter 3 - VB 2005 by Schneider 54

23 Code Window Click tab to return to Form Designer Chapter 3 - VB 2005 by Schneider 55

24 Sample Form txtfirst txtsecond btnred Double-click on btnred Chapter 3 - VB 2005 by Schneider 56

25 Code for Walkthrough Public Class frmdemo Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged txtfirst.forecolor = Color.Blue End Sub Private Sub btnred_click(...) Handles btnred.click End Sub End Class Chapter 3 - VB 2005 by Schneider 57

26 Code for Walkthrough Public Class frmdemo Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged txtfirst.forecolor = Color.Blue End Sub Private Sub btnred_click(...) Handles btnred.click txtfirst.forecolor = Color.Red End Sub End Class Chapter 3 - VB 2005 by Schneider 58

27 Event Procedure txtfirst.leave Select txtfirst from Class Name box drop-down list. Select Leave from Method Name box drop-down list. Chapter 3 - VB 2005 by Schneider 59

28 Code for Walkthrough Private Sub txtfirst_leave(...) Handles txtfirst.leave End Sub Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged txtfirst.forecolor = Color.Blue End Sub Private Sub btnred_click(...) Handles btnred.click txtfirst.forecolor = Color.Red End Sub Chapter 3 - VB 2005 by Schneider 60

29 Code for Walkthrough Private Sub txtfirst_leave(...) Handles txtfirst.leave txtfirst.forecolor = Color.Black End Sub Private Sub txtfirst_textchanged(...) Handles txtfirst.textchanged txtfirst.forecolor = Color.Blue End Sub Private Sub btnred_click(...) Handles btnred.click txtfirst.forecolor = Color.Red End Sub Chapter 3 - VB 2005 by Schneider 61

30 Header of Event Procedure Private Sub btnred_click( ) Handles btnred.click Name, can be changed. Identifies event Private Sub Button_Press( ) Handles btnred.click Chapter 3 - VB 2005 by Schneider 62

31 Handling Multiple Events Event procedure can be invoked by two events. Private Sub Button_Click(...) Handles btnred.click, txtsecond.leave txtfirst.forecolor = Color.Red End Sub Chapter 3 - VB 2005 by Schneider 63

32 Altering Properties of the Form The following won't work: frmdemo.text = "Demonstration" The form is referred to by the keyword Me. Me.Text = "Demonstration" Chapter 3 - VB 2005 by Schneider 64

33 3.3 Numbers Arithmetic Operations Variables Incrementing the Value of a Variable Built-In Functions: Math.Sqrt Int Math.Round Chapter 3 - VB 2005 by Schneider 65

34 Numbers continued The Integer Data Type Multiple Declarations Parentheses Three Types of Errors Chapter 3 - VB 2005 by Schneider 66

35 Arithmetic Operations Numbers are called numeric literals Five arithmetic operations in Visual Basic + addition - subtraction * multiplication / division ^ exponentiation Chapter 3 - VB 2005 by Schneider 67

36 Numeric Expressions * (4 + 5) 2 ^ 3 Chapter 3 - VB 2005 by Schneider 68

37 Displaying Numbers Let n be a number or a numeric expression. The statement lstbox.items.add(n) displays the value of n in the list box. Chapter 3 - VB 2005 by Schneider 69

38 Example 1: Form Chapter 3 - VB 2005 by Schneider 70

39 Example 1: Code and Output Private Sub btncompute_click (...) Handles btncompute.click lstresults.items.add(5) lstresults.items.add(2 * 3) lstresults.items.add((2 ^ 3) 1) End Sub Output 5 in list 6 box 7 Chapter 3 - VB 2005 by Schneider 71

40 Example 1: Code using With Private Sub btncompute_click (...) Handles btncompute.click With lstresults.items.add(5).add(2 * 3).Add((2 ^ 3) 1) End With End Sub Chapter 3 - VB 2005 by Schneider 72

41 Numeric Variable A numeric variable is a name to which a number can be assigned. Examples: speed distance interestrate balance Chapter 3 - VB 2005 by Schneider 73

42 Variables Declaration: Dim speed As Double Variable name Data type Assignment: speed = 50 Chapter 3 - VB 2005 by Schneider 74

43 Initialization Numeric variables are automatically initialized to 0: Dim varname As Double To specify a nonzero initial value Dim varname As Double = 50 Chapter 3 - VB 2005 by Schneider 75

44 Numeric Expressions Numeric variables can be used in numeric expressions. Dim balance As Double = 1000 lstbox.items.add(1.05 * balance) Output: 1050 Chapter 3 - VB 2005 by Schneider 76

45 Assignment Statement Dim numvar1 As Double = 5 Dim numvar2 As Double = 4 numvar1 = 3 * numvar2 lstbox.items.add(numvar1) Output: 12 Chapter 3 - VB 2005 by Schneider 77

46 Incrementing To add 1 to the numeric variable var var = var + 1 Or as a shortcut var += 1 Or as a generalization var += numeric expression Chapter 3 - VB 2005 by Schneider 78

47 Built-in Functions Functions return a value Math.Sqrt(9) returns 3 Int(9.7) returns 9 Math.Round(2.7) is 3 Chapter 3 - VB 2005 by Schneider 79

48 Integer Data Type Variables of type Double can be assigned both whole numbers and numbers with decimals. The statement Dim varname As Integer declares a numeric variable that can only be assigned whole number values between about -2 billion and 2 billion. Chapter 3 - VB 2005 by Schneider 80

49 Multiple Declarations Dim a, b As Double Two other types of multiple-declaration statements are Dim a As Double, b As Integer Dim c As Double = 2, b As Integer = 5 Chapter 3 - VB 2005 by Schneider 81

50 Parentheses Parentheses should be used liberally in numeric expressions. In the absence of parentheses, the operations are carried out in the following order: ^, * and /, + and -. Chapter 3 - VB 2005 by Schneider 82

51 Three Types of Errors Syntax error Run-time error Logic error Chapter 3 - VB 2005 by Schneider 83

52 Some Types of Syntax Errors Misspellings lstbox.itms.add(3) Omissions lstbox.items.add(2 + ) Incorrect punctuation Dim m; n As Integer Chapter 3 - VB 2005 by Schneider 84

53 A Type of Run-time Error Overflow error Dim numvar As Integer = numvar = numvar * numvar Chapter 3 - VB 2005 by Schneider 85

54 A Logical Error Dim average As Double Dim m As Double = 5 Dim n As Double = 10 average = m + n / 2 Value of average will be 10. Should be 7.5. Chapter 3 - VB 2005 by Schneider 86

55 3.4 Strings Variables and Strings Using Text Boxes for Input and Output Concatenation String Properties and Methods: Length Trim IndexOf ToUpper ToLower Substring Chapter 3 - VB 2005 by Schneider 87

56 Strings continued The Empty String Initial Value of a String Option Strict Internal Documentation Line-Continuation Character Chapter 3 - VB 2005 by Schneider 88

57 String Literal A string literal is a sequence of characters surrounded by quotation marks. Examples: "hello" " " "#ab cde?" Chapter 3 - VB 2005 by Schneider 89

58 String Variable A string variable is a name to which a string value can be assigned. Examples: country ssn word firstname Chapter 3 - VB 2005 by Schneider 90

59 String Variable Declaration: Dim firstname As String Variable name Data type Assignment: firstname = "Fred" Chapter 3 - VB 2005 by Schneider 91

60 String Variable You can declare a string variable and assign it a value at the same time. Dim firstname As String = "Fred" Chapter 3 - VB 2005 by Schneider 92

61 Add Method Let str be a string literal or variable. Then, lstbox.items.add(str) displays the value of str in the list box. Chapter 3 - VB 2005 by Schneider 93

62 String Variable You can assign the value of one string variable to another. Dim strvar1 As String = "Hello" Dim strvar2 As String = "Goodbye" strvar2 = strvar1 lstoutput.items.add(strvar2) Output: Hello Chapter 3 - VB 2005 by Schneider 94

63 Variables and Strings Private Sub btndisplay_click(...) Handles btndisplay.click Dim today As String today = "Monday" lstoutput.items.add("hello") lstoutput.items.add(today) End Sub Output: hello Monday Chapter 3 - VB 2005 by Schneider 95

64 Using Text Boxes for Input and Output The contents of a text box is always a string Input example strvar = txtbox.text Output example txtbox.text = strvar Chapter 3 - VB 2005 by Schneider 96

65 Data Conversion Because the contents of a text box is always a string, sometimes you must convert the input or output. dblvar = CDbl(txtBox.Text) Converts a String to a Double txtbox.text = CStr(numVar) Converts a number to a string Chapter 3 - VB 2005 by Schneider 97

66 Concatenation Combining two strings to make a new string quote1 = "We'll always " quote2 = "have Paris." quote = quote1 & quote2 txtoutput.text = quote & " - Humphrey Bogart" Displays We'll always have Paris. - Humphrey Bogart Chapter 3 - VB 2005 by Schneider 98

67 Appending To append str to the string variable var var = var & str Or as a shortcut var &= str Chapter 3 - VB 2005 by Schneider 99

68 Appending Example Dim var As String = "Good" var &= "bye" txtbox.text = var OUTPUT: Goodbye Chapter 3 - VB 2005 by Schneider 100

69 String Properties and Methods "Visual".Length is 6. "Visual".ToUpper is VISUAL. "123 Hike".Length is 8. "123 Hike".ToLower is 123 hike. "a" & " bcd ".Trim & "efg" is abcdefg. Chapter 3 - VB 2005 by Schneider 101

70 Positions in a String Positions of characters in a string are numbered 0, 1, 2,. Consider the string Visual Basic. Position 0: V Position 1: i Position 7: B Substring al begins at position 4 Chapter 3 - VB 2005 by Schneider 102

71 Substring Method Let str be a string. str.substring(m, n) is the substring of length n, beginning at position m in str. Visual Basic.Substring(2, 3) is sua Visual Basic.Substring(0, 1) is V Chapter 3 - VB 2005 by Schneider 103

72 IndexOf Method Let str1 and str2 be strings. str1.indexof(str2) is the position of the first occurrence of str2 in str1. (Note: Has value -1 if str2 is not a substring of str1.) "Visual Basic".IndexOf("is") is 1. "Visual Basic".IndexOf("si") is 9. "Visual Basic".IndexOf("ab") is -1. Chapter 3 - VB 2005 by Schneider 104

73 The Empty String The string "", which contains no characters, is called the empty string or the zero-length string. The statement lstbox.items.add("") skips a line in the list box. The contents of a text box can be cleared with either the statement txtbox.clear() or the statement txtbox.text = "" Chapter 3 - VB 2005 by Schneider 105

74 Initial Value of a String By default the initial value is Nothing Strings can be given a different initial value as follows: Dim today As String = "Monday" Chapter 3 - VB 2005 by Schneider 106

75 Option Strict Visual Basic allows numeric variables to be assigned strings and vice versa, a poor programming practice. To turn this feature off, put the following statement at the very top of the code window Option Strict On Chapter 3 - VB 2005 by Schneider 107

76 Option Strict On for All Programs Select Options from the Tools menu In left pane, expand Projects and Solution Select VB Defaults Set Option Strict to On Chapter 3 - VB 2005 by Schneider 108

77 With Option Strict On Dim dblvar As Double, intvar As Integer Dim strvar As String Not Valid: Replace with: intvar = dblvar intvar = CInt(dblVar) dblvar = strvar dblvar = CDbl(strVar) strvar = intvar strvar = CStr(intVar) Chapter 3 - VB 2005 by Schneider 109

78 Comments Private Sub btncompute_click (...) Handles btncompute.click 'Calculate the balance in an account Dim rate As Double 'Annual rate of interest Dim curbalance As Double 'Current balance Chapter 3 - VB 2005 by Schneider 110

79 Internal Documentation 1. Other people can easily understand the program. 2. You can understand the program when you read it later. 3. Long programs are easier to read because the purposes of individual pieces can be determined at a glance. Chapter 3 - VB 2005 by Schneider 111

80 Automatic Colorization Comments green String literals maroon Keywords blue Note: Keywords are words such as Sub, Handles, Private, With, and End that have special meaning in Visual Basic. They cannot be used as variable names. Chapter 3 - VB 2005 by Schneider 112

81 Line-Continuation Character A long line of code can be continued on another line by using an underscore (_) preceded by a space msg = "I'm going to make " & _ "him an offer he can't refuse." Chapter 3 - VB 2005 by Schneider 113

82 3.5 Input and Output Formatting Output with Format Functions Formatting Output with Zones Reading Data from Files Getting Input from an Input Dialog Box Using a Message Dialog Box for Output Using a Masked Text Box for Input Chapter 3 - VB 2005 by Schneider 114

83 Formatting Output with Format Functions Function FormatNumber( , 1) FormatCurrency( , 2) FormatPercent(0.183, 0) String Value 12,345.6 $12, % Chapter 3 - VB 2005 by Schneider 115

84 Formatting Output with Zones Use a fixed-width font such as Courier New Divide the characters into zones with a format string. Dim fmtstr As String = "{0, 15}{1, 10}{2, 8}" lstoutput.items.add(string.format(fmtstr, _ data0, data1, data2)) Chapter 3 - VB 2005 by Schneider 116

85 Formatting Output with Zones Dim fmtstr As String = "{0, -15}{1, 10}{2, 8}" lstoutput.items.add(string.format(fmtstr, _ data0, data1, data2)) Here, 15 was preceded by a minus sign. This produces left justification in 0 th zone. There will be right justification in the other two zones. Chapter 3 - VB 2005 by Schneider 117

86 Zone Formatting Symbols Symbols: N, C, and P :Nr :Cr :Pr Effect on zone FormatNumber(data, r) FormatCurrency(data, r) FormatPercent(data, r) Dim fmtstr As String = "{0,15:N1}{1,10:C2}{2,8:P0}" Chapter 3 - VB 2005 by Schneider 118

87 Reading Data from Files Data can be stored in files and accessed with a StreamReader object. We assume that the files are text files (that is, have extension.txt) and have one piece of data per line. Chapter 3 - VB 2005 by Schneider 119

88 Sample File: PAYROLL.TXT Mike Jones John Smith Name Hourly wage Number of hours worked Chapter 3 - VB 2005 by Schneider 120

89 Steps to Use StreamReader Execute a statement of the form Dim readervar As IO.StreamReader = _ IO.File.OpenText(filespec) or the pair of statements Dim readervar As IO.StreamReader readervar = IO.File.OpenText(filespec) Chapter 3 - VB 2005 by Schneider 121

90 Steps to Use StreamReader Read items of data in order, one at a time, from the file with the ReadLine method. strvar = readervar.readline After the desired items have been read from the file, terminate the communications link readervar.close() Chapter 3 - VB 2005 by Schneider 122

91 Example using StreamReader Dim name As String Dim wage, hours As Double Dim sr As IO.StreamReader = _ IO.File.OpenText("PAYROLL.TXT") name = sr.readline wage = CDbl(sr.ReadLine) hours = CDbl(sr.ReadLine) lstbox.items.add(name & ": " & wage * hours) OUTPUT: Mike Jones: Chapter 3 - VB 2005 by Schneider 123

92 Comment on Example Consider lstbox.items.add(name & ": " & wage * hours) The ampersand automatically converted wage * hours into a string before concatenating. We didn t have to convert wage * hours with CStr. Chapter 3 - VB 2005 by Schneider 124

93 Getting Input from an Input Dialog Box stringvar = InputBox(prompt, title) filename = InputBox("Enter the name " _ & "of the file containing the " & _ "information.", "Name of File") Title Prompt Chapter 3 - VB 2005 by Schneider 125

94 Using a Message Dialog Box for Output MsgBox(prompt, 0, title) MsgBox("Nice try, but no cigar.", 0, _ "Consolation") Title Prompt Chapter 3 - VB 2005 by Schneider 126

95 Masked Text Box Similar to an ordinary text box, but has a Mask property that restricts what can be typed into the masked text box. Chapter 3 - VB 2005 by Schneider 127

96 Input Mask Dialog Box Chapter 3 - VB 2005 by Schneider 128

97 Mask A Mask setting is a sequence of characters, with 0, L, and & having special meanings. 0 Placeholder for a digit. L Placeholder for a letter. & Placeholder for a character or space. Chapter 3 - VB 2005 by Schneider 129

98 Sample Masks State abbreviation: LL Phone number: Social Security Number: License plate: &&&&&& Chapter 3 - VB 2005 by Schneider 130