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VB.Net Decisions

VB.Net Decisions. The If … Then Statement. If condition Then Statements End If If condition Then Statements Else Statements End If Condition: Simple condition: Comparison of two expressions formed with relational operators:>, <, =, < >, >=, <= Boolean variable Complex condition:

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VB.Net Decisions

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  1. VB.Net Decisions

  2. The If … Then Statement • If condition Then Statements • End If • If condition Then Statements Else Statements • End If • Condition: • Simple condition: • Comparison of two expressions formed with relational operators:>, <, =, < >, >=, <= • Boolean variable • Complex condition: • Formed with logical operators: ( ), Not, And, Or, Xor

  3. Complex Condition • If 12<=Age<=65 Then Fee = 20, else Fee = 5 • Admission rules: Applicants will be admitted if meet one of the following conditions: • GPA>3.0 • GPA>2.5 AND SAT >700 • Scholarship rules: Meet all the conditions: • GPA>3.2 • Must be Accounting or CIS • Admission rules: Applicants will be admitted if meet all the following conditions: • SAT>700 Or Income > 40000 • Not GPA < 2.5

  4. Xor • Truth table • T, T, F • T, F, T • F, T, T • F, F, F • Example: Degree = “MBA”, Experience>20 years • Under qualified • Overqualified: Meet both conditions

  5. Order of Evaluation • ( ), Not, And, Or, Xor • Example: A=5, B=10, C=20, D=30 • A>B OR C<B+20 AND Not(A<0 And D>10) • A>B XOR C<B+20 AND D>10 OR B>A+C

  6. Using Boolean Variables as Flags • A flag is a Boolean variable that signals when some condition exists in the program. Dim goodStudent as Boolean=True If daysAbsent > 10 Then goodStudent = False End if If goodStudent Then TextBox1.text=“Good Student” Else textBox1.text=“Not good student” End If

  7. IF … ELSEIF Statement • IF condition THEN statements [ELSEIF condition-n THEN [elseifstatements] [ELSE [elsestatements]]] End If

  8. Select Case Structure • SELECT CASE testexpression [CASE expressionlist-n [Statements] [CASE ELSE [elsestatements] END SELECT

  9. Select Case Example • SELECT CASE temperature CASE <40 TextBox1.text=“cold” CASE 40 to 60 Text Box1.text=“cool” CASE 60 to 80 TextBox1.text=“warm” CASE ELSE TextBox1.text=“Hot” End Select

  10. The Expression list can contain multiple expressions, separated by commas. Select Case number Case 1, 3, 5, 7, 9 textBox1.text=“Odd number” Case 2, 4, 6, 8, 10 textBox1.text=“Even number” Case Else End Select

  11. Nested If • Decision tree: • Example: Tuition rules based on student’s status and number of units: • Undergraduate student • Graduate student

  12. MessageBox MessageBox.Show(message) MessageBox.Show(message, Caption) MessageBox.Show(message, Caption, Buttons) Note: 1. In each format, arguments are positional and required. 2. This object returns a DialogResult data type. To test the return value: Dim ReturnVal as DialogResult ReturnVal=MessageBox(“hello”, …..) If ReturnVal=DialogResult.OK…

  13. InputBox InputBox(Prompt [,Title] [, Default] [, Xpos] [, Ypos]) Xpos is the distance from the left edge of the screen, and Ypos is the distance from the top of the screen. Both are measured in twips (1/1440th of an inch). Note: The arguments are positional and optional. Enter a comma to skip an argument. cityName = InputBox("Please enter city name:“, , “SF”) If cityName = vbNullString Then MessageBox.Show ("customer click cancel") Else Text1.Text = cityName End If

  14. Group Box and Panel • Controls in a Group Box should move with the boxs. • A panel control can display scrollbars by setting the AutoScroll property to true.

  15. Radio Button • Radio buttons must be grouped together inside a container such as a GroupBox or a form. • When the user selects an option all other options in the same group are deselected. • Checked property value: True/False. • Default button: Set the Checked property to true at the design time.

  16. RadioButton Example 1 Private Sub RadioButton1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton1.CheckedChanged If RadioButton1.Checked = True Then MsgBox("Check RadioButton1") Else MsgBox("uncheck RadioButton1") End If End Sub

  17. RadioButton Example 2 If radioButton1.Checked=true then textbox1.text=“You select radio button 1” ElseIf radioButton2.Checked=true then textbox1.text=“You select radio button 2” Else textbox1.text=“You select radio button 3” End If

  18. Check Box • Check boxes do not belong to a group even when they are grouped in a Group Box. • Checked property and checkedChangedevent

  19. Check Box Example 1 Private Sub CheckBox1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged If CheckBox1.Checked = True Then MsgBox(“check chk1") Else MsgBox("uncheck chk1") End If End Sub

  20. Check Box Example 2 Dim msg as String Msg=“You choose “ If checkBox1.checked=true then msg=msg & “check box 1” End If If checkBox2.checked=true then msg=msg & “check box 2” End If If checkBox3.checked=true then msg=msg & “check box 3” End If

  21. Input Validation • Numbers are checked to ensure they are: • Within a range of possible values • Reasonableness • Not causing problems such as division by 0. • Containing only digits • IsNumeric • Textbox: • Set CauseValidation property to true. • Use the Validating event: • Triggered just before the focus shifts to other control.

  22. TextBox Validating Event Private Sub TextBox1_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating If Not IsNumeric(TextBox1.Text) Then e.Cancel = True MsgBox("enter digits only") Else MsgBox("good") End If End Sub

  23. Working with Strings • String comparison is case-sensitive: • Blank • Digits in numerical order • Uppercase letters in alphabetical order • Lowercase letters in alphabetical order • “A” <> “a” • “a” > “Z” • “alan”, “Chao”, “Smith”

  24. String Methods • ToUpper, ToLower • Length – Number of characters • TrimStart, TrimEnd, Trim • Substring(Start), Substring(Start, length) • IndexOf(SearchString), IndexOf(SearchString, Start) • 0 based index • Case-sensitive • eName=“David” • Position=eName.IndexOf(“d”) • Return –1 if the searchString is not found.

  25. Example: Validate SSN Format Private Sub TextBox1_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating Dim correct As Boolean = True If Not IsNumeric(TextBox1.Text.Substring(0, 3)) Or _ Not IsNumeric(TextBox1.Text.Substring(4, 2)) Or _ Not IsNumeric(TextBox1.Text.Substring(7, 4)) Then correct = False End If If TextBox1.Text.Substring(3, 1) <> "-" Or TextBox1.Text.Substring(6, 1) <> "-" Then correct = False End If If correct Then MsgBox("perfect format") Else e.Cancel = True MsgBox("not correct format") End If End Sub

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