BACS 287

1. BACS 287 Programming Fundamentals 5 BACS 287

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

3. 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

4. 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. BACS 287

5. Procedures BACS 287

6. 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

7. 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

8. 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

9. 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. BACS 287

10. 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

11. 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

12. User-Defined Function Example 1 Private Function Divide_Num (byValsngX as single, byValsngY 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

13. User-Defined Function Example 2 Public Function System_Status()as boolean #If Win16 then Return vbFalse #Else Return vbTrue #End IF End Function • Checks the current run-time environment and returns true for a 32 bit Windows and false otherwise. BACS 287

14. 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

15. 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

16. 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

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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Argument Data Type • All arguments have a data type. If you do not declare a specific type, it is object by default. Function X (byValstrX as string, byValobjY as object) ... Sub Y (byValdatZ as date, byValsngR as single) ... Function Z (byValintX as integer, byValobjG) as single ... BACS 287

25. 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

26. 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

27. 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

28. 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 (byValintX as integer, Optional byValintY as integer = 2, Optional byValstrZ as string = “test”) ... To call this function: Answer = A(4,,”string”) BACS 287

29. 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

30. 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

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

32. 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

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

34. 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

35. Answer #2 Private Function My_Length (byValstrInput 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

36. 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

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