BACS 287 Programming Fundamentals 5

BACS 287 Programming Fundamentals This lecture introduces the following topics: – Procedures Built-in Functions User-defined Procedures – Functions – Subroutines Passing data via Arguments

BACS 287 Procedures A procedure is a set of instructions that perform a particular service. (It is a module in structured programming terminology.) There are 2 generic categories of procedures: built-in (intrinsic) and user- defined. There are also 3 types of procedures: Functions, Subroutines, and Property procedures.

BACS 287 Procedures All built-in Visual Basic procedures are of the function type. User-defined procedures can be either the function or subroutine type. Functions return a value. Subroutines do not return a value. Functions and subroutines are capable of performing the same kind of work. They do it in slightly different ways.

Procedures BACS 287

Procedures versus Methods Methods are also a type of procedure They perform work for a specific type of object. All methods are procedures, but not all procedures are methods. Methods differ from procedures in that a method may only be called from a specific object while generic procedures can be called from any object. Ex: Debug.write, intX.ToString, strY.Trim

BACS 287 Built-In Functions Built-in (intrinsic) functions are used anywhere an expression is valid. For example: datX = Date  ’Date’ is an intrinsic function Some functions require additional information (called arguments). sngX = Sqrt(10) strX = MsgBox(“Prompt”,vbYes,”Error”) sngX = Cos(Rnd)

BACS 287 Built-In Functions VB provides many built-in functions. Date - returns current date Format - date or number converted to a text string InputBox - text entered into a dialog box by user MsgBox - text displayed to user at run-time Len - number of characters in a text string Mid - selected portion of a string Rnd - random number InStr – find character(s) in a string Val - string converted to a number and many more.....

BACS 287 Built-In Functions Built-in functions are provided by Visual Basic as a convenience to the programmer. Most could be written as user-defined functions, but there is no need to do this since they are already written and tested. You should be familiar with the built-in functions provided by the language to improve personal productivity. A number of built-in functions are also available as methods in VB.Net.

Built-In Methods Objects in VB have methods associated with them. Methods give VB objects functionality (that is, it allows them to act upon data). Many VB methods do things that can also be done with functions. Using methods is preferred because they are compatible with other.NET languages. BACS 287

Built-In Methods VB provides many methods. Number.toString – converts the number to a string String.toInt32 – converts the string to an integer data type String.toInt16 – converts the string to a short data type String.length – returns the length of the string variable String.split – places each string component into an array element Math.Sqrt(Number) – returns the square root of the number MyArray.count – returns the number of elements in the array MyArray.sort – sorts the elements of an array MyArray.reverse – reverses the order of elements in an array and many more.....

BACS 287 User-Defined Procedures If a built-in function does not exist to perform needed work, you can build your own as a user-defined procedure. User-defined procedures can be either the function type or the subroutine type. They can be defined in form modules and code modules.

BACS 287 User-Defined Functions Function Syntax: [Public | Private] [Friend] Function function-name [(args)] [As type] [statements] [function-name = expression] or [Return expression] [Exit Function] [statements] [function-name = expression] End Function

BACS 287 User-Defined Function Example 1 Private Function Divide_Num (byVal sngX as single, byVal sngY as single) as single If sngY = 0 then Return 0 Exit Function End IF Return (sngX / sngY) End Function You return a value to the calling routine by assigning the return value to the function name.

BACS 287 User-Defined Function Example 2 Public Function Time_Check()as boolean If TimeValue(Now) < #08:00:00# then Return vbFalse Else Return vbTrue End IF End Function Checks the current system time. If its before 8 AM, it returns false, otherwise it returns true.

BACS 287 User-Defined Functions Functions are called by using their name in an expression. The arguments are enclosed in () and are separated by commas. sngX = Divide_Num(15.2, 17.3) If there are no arguments, you can leave off the (). blnX = System_Status

BACS 287 User-Defined Functions User-defined functions have data types. This determines the type of the return value. If not specified, the return type is object. Since functions return values, they can be used to build up larger (more complex) statements. If Check_It(strInput) then...

BACS 287 User-Defined Sub Procedures There are 2 types of sub procedures: General procedures and Event procedures. General procedures are invoked by the programmer specifically calling them in the program. Event procedures are automatically invoked by the system when a specific event occurs. Event procedures have private scope only.

BACS 287 Sub Procedures Sub Procedure Syntax: [Public | Private] [Friend] Sub subroutine-name [(args)] [statements] [Exit Sub] or [Return] [statements] End Sub

BACS 287 Sub Procedure Example 1 Private Sub Compute_Area (byVal sngLen_ as single,byVal sngWidth as single) Dim dblArea as Double If sngLen = 0 or sngWidth = 0 then Exit Sub End If dblArea = sngLen * sngWidth System.Console.WriteLine dblArea End Sub

BACS 287 Sub Procedure Example 2 Public Sub Beep_Bell () Dim intX as integer Dim intCnt as integer intX = InputBox(“How Many Beeps?”) For intCnt = 1 to intX Beep Next intCnt End Sub

BACS 287 Sub Procedures General sub procedures are called with the Call statement. Call Compute_Area(11, 21) Call My_SubProcuedure(intX) Because they do not return a value, subroutines cannot be used to build expressions.

BACS 287 User-Defined Procedures Many times you can use either a function or a subroutine procedure to accomplish the same task. Professional programmers generally use functions for most user-defined procedures because you can test the success of the execution and proceed appropriately.

BACS 287 Arguments There are 2 basic methods of passing data to a user-defined procedure: Increase the variable scope or use arguments. The preferred method is to pass data to the procedure via arguments. This allow you to better control the data that is used by the procedure.

BACS 287 Arguments Passing Arguments to Procedures: – Argument Data Type – Passing Arguments by Value – Passing Arguments by Reference – Optional Arguments – Indefinite Number of Arguments – Named Arguments

BACS 287 Argument Data Type All arguments have a data type. If you do not declare a specific type, it is object by default. Function X (byVal strX as string, byVal objY as object)... Sub Y (byVal datZ as date, byVal sngR as single)... Function Z (byVal intX as integer, byVal objG) as single...

BACS 287 Passing Arguments by Value A copy of the variable is passed to the procedure. Any changes affect only the copy, not the original. This is the default. Sub X (ByVal intY as integer) Function Y (objX, ByVal strY as_ string)as Integer

BACS 287 Passing Arguments by Reference The actual variable is passed to the procedure. Any changes to the variable affect the original. Sub R (byRef intY as integer) Function Z (byRef objX, byRef blnG as boolean)as Integer

BACS 287 byRef Arguments When arguments are passed byRef, any changes made to the value of the variable by the procedure are visible to the entire scope of the variable. This means that a subroutine can return a value to its calling code if variables are passed ‘byRef’. Also, this would be somewhat more efficient if the things being passed were large (i.e., objects). Still better to do this with Functions if you have no overriding reason to use byRef.

BACS 287 Using Optional Arguments You can specify that arguments be optional. All optional arguments must supply a default value. This value cannot be a variable. Once you define one optional argument, all subsequent arguments must be optional also. Function A (byVal intX as integer, Optional byVal intY as integer = 2, Optional byVal strZ as string = “test”)... To call this function: Answer = A(4,,”string”)

BACS 287 Using Indefinite Arguments You can allow an arbitrary number of arguments if the last argument is specified as a ParmArray. Sub mySum (byRef intSum as integer, _ byVal ParmArray intNums() as integer) Dim intX as integer For Each intX in intNums intSum = intSum + intNums(intX) Next intX End Sub

BACS 287 Using Indefinite Arguments There can only be one ParmArray in a procedure and it must be the last thing defined. It must be passed byVal. The code in the procedure must treat it as a one-dimension array. All arguments preceding the ParmArray word must be required (that is, no Optional arguments allowed).

BACS 287 Named Arguments You can specify named arguments by using the := operator in the procedure call. Function X (byVal strX as string, byVal intX as integer) as Boolean... Return_Value = X(intX := 7, strX := “My Name”)

BACS 287 In-Class Example 1 Write a user-defined function called My_Length to accept a string and return the length of that string to the user.

BACS 287 Answer #1 Private Function My_Length (byVal strInput as string) as Integer Return len(strInput) End Function

BACS 287 In-Class Example #2 Modify the previous example to return “a big string” if the string is more than 10 characters long. Return “a small string” if it is 10 or less.

BACS 287 Answer #2 Private Function My_Length (byVal strInput as string) as String Dim intLen as integer intLen = len(strInput) If intLen > 10 then Return “a big string” else Return “a small string” End If End Function

BACS 287 In-Class Example #3 Write a user-defined sub procedure called My_Clear to clear the computer screen when called. (Note, the ‘cls’ function does this.) Next, write code to call the procedure.

BACS 287 Answer #3 Private Sub My_Clear () cls End Sub Call My_Clear()