Variable Type Conversions and Data Type Checking in Programming
Learn about converting variable types using methods and functions in programming. Explore the use of `Convert` class, functions like `IsNumeric`, and examples with `TextBox` and `InputBox`. Understand the `Try...Catch` statement for error handling in programming.
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Presentation Transcript
Introduction to Programming Lecture 4 Msury Mahunnah, Department of Informatics, Tallinn University of Technology
Converting Variable Types The methods of the Convert class that perform data-type conversions: ToBoolean, ToByte, ToChar, ToDateTime, ToDecimal, ToDouble, ToInt16, ToInt32, ToInt64, ToSByte, ToShort, ToSingle, ToString, ToUInt16, ToUInt32, ToUInt64 Example: Dim x$ = 21.03.2011", y As Date y = Convert.ToDateTime(x) y = #3/21/2011#
Converting Variable Types 2 Functions that perform data-type conversions: CBool, CByte, CChar, CDate, CDec, CDbl, CInt, CLng, CSByte, CShort, CSng, CStr, CUInt, CULng, CUShort Example: Dim x$ = 21.03.2011", y As Date y = CDate(x) y = #3/21/2011#
Functions for checking data type IsNumeric() Returns True if its argument is a number (Short, Integer, Long, Single, Double, Decimal). IsDate() Returns True if its argument is a valid date (or time). IsArray() Returns True if its argument is an array.
An example with IsNumeric (TextBox) Private Sub Button1_Click(ByVal sender As System.Object, ...) Dim x% If Not IsNumeric(TBox_Age.Text) Then MsgBox("A number is expected!") TBox_Age.Text = "" TBox_Age.Focus() Exit Sub End If x = Convert.ToInt32(TBox_Age.Text) floating-point number??? MsgBox("You are " & x & " years old!") End Sub Three objects on the form are needed: TextBox with a Name TBox_Age Button with a Name Button_1 Label with a Text Age
A example with IsNumeric (InputBox) Private Sub Button1_Click(ByVal sender As System.Object, ...) Dim strAge As String = "" Dim Age As Integer Do While Not IsNumeric(strAge) strAge = InputBox( Please enter your age") Loop Age = Convert.ToInt32(strAge) if strAge is a floating-point number? MsgBox("You are " & Age & " years old!") End Sub A object on the form is needed: Button with a Name Button_1
Try ... Catch Try ... Catch statement At first, execute sentence(s)1, but if the error appear, execute sentence(s)2 Try sentence(s)1 Catch [ex As Exception] sentence(s)2 [Finally sentence(s)3] End Try [ ] this part of structure is not necessary
Try ... Catch and Textbox Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim Age% Try Age = Convert.ToInt32(TBox_Age.Text) MsgBox("You are " & Age & " years old!") Catch ex As Exception MsgBox(ex.Message) 'MsgBox("A number is expected!") Finally TBox_Age.Text = "" TBox_Age.Focus() End Try if an error appears go there End Sub
Try ... Catch and Inputbox Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim strAge As String = "" Dim Age As Integer Do strAge = InputBox("Please enter your age") Try Age = Convert.ToInt32(strAge) if error here, go to there Exit Do Exit Do when previous sentence doesn t give error Catch ex As Exception MsgBox(ex.Message) End Try Loop MsgBox("You are " & Age & " years old!") End Sub
