Introduction to Programming Lecture 3 Msury Mahunnah, Department of Informatics, Tallinn University of Technology

Two type of procedures Subroutines (Sub... End Sub) perform actions and they don’t return any result Functions (Function... End Function) perform some calculations and return a value A simple and short program does not need them. It is enough to use automatically appearing object’s event procedures (subroutines), For instance: Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click … End Sub

Subroutine general form Sub subroutine_name({param As varType}) Statement(s) {Exit Sub} End Sub {} – this part of the structure may miss or appear discretionary times varType – discretionary type (Integer, Long, Single, Double, Char, Date,...) param = parameter - a special kind of variable, that is used for passing information between procedures There are two kinds of parameters - input (ByVal) and output (ByRef) parameters

ByVal and ByRef ByVal is short form for “By Value”... it means that you are passing a copy of a variable to your Subroutine. You can make changes to the copy and the original will not be altered. ByRef is short form for “By Reference”... this means that you are not handing over a copy of the original variable but pointing to the original variable Declaring a parameter with ByVal, we call it input parameter Declaring a parameter with ByRef, we call it output parameter

Function general form Function function_name({param As varType}) As varType Statement(s) {Exit Function} {Return return_value} {function_name = return_value} End Function

Subroutine in use Format style Sub ShowDate() MsgBox(Format(Now(), "dd.mm.yyyy")) End Sub To call (execute) this subroutine: ShowDate() or Call ShowDate() Now() – built-in function that returns Date value containing the current date and time according to your system Format – built-in function that returns a string formatted according to instructions contained in a format String expression.

Subroutine in use 2 Format style Using a parameter Sub ShowDate(ByVal myDate As Date) MsgBox(Format(myDate(), "dd.mm.yyyy")) End Sub To call (execute) this subroutine: ShowDate(#3/21/2011#) or Call ShowDate(#3/21/2011#) myDate – parameter Format – built-in function that returns a string formatted according to instructions contained in a format String expression.

Function in use Function Fact(ByVal n As Int) As Int Dim i As Int, F As Int F = 1 For i = 1 To n F = F * i Next Return F End Function To call (execute) this function: Dim factorial As Int = Fact(5) ByVal define that n is input parameter Function Fact(ByVal n As Int) As Int Dim i As Int, F As Int F = 1 For i = 1 To n F = F * i Next Fact = F End Function difference

Function in use 2 Function NextDay() As Date Dim theNextDay As Date theNextDay = Now.AddDays(1) Return theNextDay End Function To call (execute) this function: Dim tomorrow As Date = NextDay()

Repetition without Loop-statement Repetition with a recursion - a function calls (executes) itself. Function FactRec&(ByVal n&) If n = 1 Then Return 1 FactRec = n * FactRec(n - 1) End Function

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$ = “ ", 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$ = “ ", 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 Try ‘sentence(s)1 Catch [ex As Exception] ‘ sentence(s)2 [Finally ‘sentence(s)3] End Try At first, execute sentence(s)1, but if the error appear, execute sentence(s)2 [ ] – 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 End Sub if an error appears go there

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