Chapter 4 - Visual Basic Schneider General Procedures Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Outline & Objective Sub Procedures Procedure Parameters Function Procedures Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider What is a procedure? A general procedure is a set of commands that is given a name so that it can be invoked by another part of the program Procedures make a program easier to develop, test, and correct Code written to perform one well-defined subtask is referred to as a Sub procedure or function A Sub procedure is a part of a program that performs one or more related tasks, has its own name, To distinguish them from event procedures, Sub procedures and functions are refrred to as a General Procedures Sub procedures allow us to focus on the tasks and later o how to accomplish each task Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider General Procedures In Visual Basic, there are two types of procedures: Sub Function Note: To distinguish procedures from event procedures, Sub and Function procedures are referred to as general procedures. As a rule, a Sub procedure should perform only one task, or several closely related tasks, and should be kept relatively small Chapter 4 - Visual Basic Schneider

Sub Procedures Properties: can be called (invoked) by using call subProcedureName() passing data called arguments Chapter 4 - Visual Basic Schneider

Creating Visual Basic Sub Procedure: Activate the code window Select Add Procedure from the Tools menu Type in the name of the Sub procedure Click on Private in the Scope box Press the Enter key or click the OK button Type the statements of the Sub procedure into the code window Chapter 4 - Visual Basic Schneider

Syntax of a Sub Procedure Private Sub ProcedureName ( ) statement(s) End Sub Procedure Name Example Statements

Components of Sub Procedure: name: used to identify the Sub procedure parameters: a Sub procedure accepts values from the caller through its parameters. It may also send values back to the caller through its parameters. Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Sub Procedure's Name The rules for naming Sub Procedures are the same as naming variables. In this text, Sub procedure names begin with uppercase letters in order to distinguish them from variable names. Chapter 4 - Visual Basic Schneider

Example

Example

Passing Arguments A Sub procedure receives the location of the arguments, and may use and modify the value of the arguments stored at that location. Example 1.4.2 It provides a vehicle for passing values from a Sub procedure back to the place which the Sub procedure was called Different names may be used for an arguments and its corresponding parameter Only one memory location is involved Example 2.4.2 12 12

Passing arguments to parameters Arguments : Variables or expressions placed in parentheses in a Call statement. Go over example 4.1.6 Arguments Chapter 4 - Visual Basic Schneider 13 13

Passing arguments to parameters Parameters : Variables appearing in the sub procedure definition are called paramanters. Go over example 4.1.6 The second CALL statement uses different variable names for the arguments to show that using the same argument names Is not necessary. Parameters 14 14

Passing Arguments to Parameters Call Add (x, y ) Private Sub Add ( num1 As Single, num2 As Single) When the Call Statement is executed , each value of the arguments is assigned to it’s corresponding parameter variable the value of num1 is assigned to x and the value of num2 is assigned to y Arguments Parameters 15 15

Passing arguments to parameters Call Triple(num) Private Sub Triple (num As Single) Argument Go over example 4.1.6 The second CALL statement uses different variable names for the arguments to show that using the same argument names Is not necessary. Parameter Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Sub Procedure Triple: Private Sub cmdCompute_Click() Dim num As Single num = Val(InputBox("Enter a number:")) Call Triple(num) End Sub cmdCompute Triple num par/num Private Sub Triple(num As Single) ' Multiply the value of the number by 3 picResult.Print "The number is"; 3 * num End Sub Private Sub Triple(par As Single) ' Multiply the value of the number by 3 picResult.Print "The number is"; 3 * par End Sub Chapter 4 - Visual Basic Schneider

Example

Example cmdDisplay Triple amt num

Passing by value Sometime you want to pass a variable to a sub procedure, but you want to ensure that the variable will retain its original value after the sub procedure terminates. Such variable is said to be passed by value

Passing by value There are two ways to pass the variable by value 1. In the call statements, enclose the variable in an extra pairs of parenthesis cmdDisplay Triple amt num

2. In the private Sub statement, precede the corresponding parameter with the word ByVal cmdDisplay Triple amt num

Passing by value When the variable is passed by value, the variable will retain its original value cmdDisplay Triple amt num

ByRef and ByVal Examples Private Sub cmdCall_Click() Dim a As Integer Dim b As Integer a = 10 b = 15 Print a; b Call mySub(a, b) End Sub Sub mySub(x As Integer, y As Integer) x = 11 y = 16 Output 10 15 11 16 cmdCall mysub a x b y

ByRef and ByVal Examples Private Sub cmdCall_Click() Dim a As Integer Dim b As Integer a = 10 b = 15 Print a; b Call mySub(a, b) End Sub Sub mySub(ByRef x As Integer, ByRef y As Integer) x = 11 y = 16 Output 10 15 11 16 cmdCall mysub a x b y

ByRef and ByVal Examples Private Sub cmdCall_Click() Dim a As Integer Dim b As Integer a = 10 b = 15 Print a; b Call mySub(a, b) End Sub Sub mySub(ByRef x As Integer, ByVal y As Integer) x = 11 y = 16 Output 10 15 11 15 cmdCall mysub a x b y

ByRef and ByVal Examples Private Sub cmdCall_Click() Dim a As Integer Dim b As Integer a = 10 b = 15 Print a; b Call mySub(a, b) End Sub Sub mySub(ByVal x As Integer, ByVal y As Integer) x = 11 y = 16 Output 10 15 cmdCall mysub a b x y

ByRef and ByVal Examples Private Sub cmdCall_Click() Dim a As Integer Dim b As Integer a = 10 b = 15 Print a; b Call mySub(a, b) End Sub Sub mySub(ByRef y As Integer, ByRef x As Integer) x = 11 y = 16 Output 10 15 16 11 cmdCall mysub a y b x

Important Rules for Passing Arguments to a Sub The number of arguments and parameters must match. The data type of each argument must match its corresponding parameter. Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Local Variables: A variable that is used only in a specific procedure (Sub or Function). The scope of the local variable is the Sub or Function in which that variable has been defined. When the same variable name appears in two different Sub procedure , Visual Basic gives the variables separate identity and treats them as two different variables. Also each time a Sub procedure is called, all declared variables that are not parameters assume their default values. Example 4.2.4 (the value of num is set to default value) Example 4.2.5 (x is treated as different variable Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Local Variables: Declared within a procedure definition Private to a procedure definition Variables in different procedures are totally independent Different procedures can have variables with the same names; however, each variable will have its own memory location Chapter 4 - Visual Basic Schneider

Advantages of Local Variables Extremely useful for team programming Protects against accidental side effects (change of the value of the variable in one procedure that causes an error in another) Chapter 4 - Visual Basic Schneider

Example cmdDisplay pr1 x y The variables x and y are local for the procedure cmdDisplay_Click The variables x and y are local for the procedure pr1

Any change of the value of parameters (x and y) will affect only on the value of its corresponding arguments (a and b) in the caller procedure cmdCall mysub a x b y x y z Local variables

Chapter 4 - Visual Basic Schneider Form-Level Variables Form-level variables are visible to every procedure. Form-level variables appear at the top of the code window. Making a variable visible to every procedure in a form’s code without being passed. When a form-level variable is assigned a value by a procedure, it retains its value when the procedure is exited Chapter 4 - Visual Basic Schneider

How to create Form-Level Variables? Activate the code window Click on the down arrow to the right of the object list box Click on General Click on Declaration in the procedure list box Type in Dim statements for form-level variables Example 4.2.6 Delete Dim statements in Declaration section of General and show the resut The sum of 2 and 3 is 5 num1 = 2 num2 = 3 Chapter 4 - Visual Basic Schneider

The variables x and y are Form-Level variables (Global) cmdCall mysub X Global X Global y Global y Global Note: The values of Form-level variables are visible to all procedures in the program

If the procedure has a local variables with the same name of the form-level variables, the procedure uses its local variable instead of the global variable x y 7 6 Form_level Variables x 5 Local y The procedure uses the global variable if it has no local variable 8 Local

Examples 23 23 23 23 23 23

Form_load In some situation we want to assign the value immediately to the Form-level variable Visual basic has a special event procedure called Form_Load that is executed as soon as the program is run.

Example Command1 pro1 X Global X Global y Global y Global / b x a

Chapter 4 - Visual Basic Schneider What is a function? Similar to a Sub, performs a specific task Unlike a Sub, returns a single value to the calling program Chapter 4 - Visual Basic Schneider

Types of Functions Standard functions (built-in) VBasic has many bulit-in functions (Predefined functions). E.g. (Left, Right, Len, Sqr, Round, …) User-defined functions: In addition to using bulit-in functions, we can define functions of our own. These new function are called function procedures or user-defined functions

Built-in Functions Print Len(“Hello”)  5 Print Left(“Hello”,3)  “Hel” Print Sqr(9)  3 For example, the function Len returns the number of characters in the string. It has One parameter of type String and returns only one value of type Single

User-Defined Function A function returns a single value The value is returned by assigning a value to the name of the function The values of the parameters of a function should not be used for returning values from a function It returns a single value This value is returned in the function itself, not as an argument A function is called by using its name in an A function is called by using its name in an expression the arguments of a function should not be altered in the function If a procedure needs to return more than one value, a SUB procedure should be used instead. Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider The Function Syntax Private Function FunctionName(parameter-list) As dataType Statement(s)…… ….. FunctionName = value End Function Note: value must be of the type dataType specified in the function declaration Chapter 4 - Visual Basic Schneider

Rules for Defining and Calling a Function User-defined function must include a statement that assigns the function name a value. User-defined functions are called in the same way that built-in functions are called. A user-defined function may be called in an expression. Chapter 4 - Visual Basic Schneider

Example of a Function The function Max has 2 parameters of type single What is the data type of the returned value?

Chapter 4 - Visual Basic Schneider Example of a Function Private Sub cmdDetermine_Click() Dim nom As String nom = FirstName(txtFullName.Text) picFirstName.Print "The first name is “; nom End Sub Private Function FirstName(nom As String) As String Dim firstSpace As Integer firstSpace = InStr(nom, " ") FirstName = Left(nom, firstSpace - 1) End Function Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Another Example Private Sub cmdDisplay_Click() Dim num As Single num = 5 picOutput.Print Triple (num) picOutput.Print num End Sub Private Function Triple(x As Single) As Single num = 3 Triple = num * x End Function Output: 15 5 Chapter 4 - Visual Basic Schneider

18 Example Private sub com_click() x=fun(3) + fun( 5) Print x End sub Private function fun(a) fun= a a=a+1 fun=fun + a End function 18

Example Private sub com_click() x=fun(3, fun(2, 1)) Print x End sub Private function fun(a,b) as integer a=a+b End function

Difference between calling sub-procedure and functions To call a sub-procedure Type the word Call followed by the procedure name, with the arguments (if any) in parentheses Call pro1(5,3) After the procedure statements are executed, the execution continues with the line following the call statements.

Difference between calling sub-procedure and functions To call a sub-procedure

Difference between calling sub-procedure and functions To call a function procedure Use its name, with the arguments (if any) in parentheses EX: Fun(5,3) A function is called in an expression. Expression may be part of an assignment statement or an output statement Print Fun(5,6) X = 2*Fun(10,3)+3 Print Fun(x,y)+Fun(x+1,x+2)

After the function procedure statements are executed, the execution returns back to the calling point. And the function call statement (Max(10,2)) is substituted by the returned-value (10). 10

Any function can be rewritten as a sub and vise versa

Chapter 4 - Visual Basic Schneider Common Errors Passing incorrect data types Not specifying the data type of the returned value Not assigning a value to the Function name inside the Function definition Misspelling of the Function name Chapter 4 - Visual Basic Schneider

Arithmetic Operations The VBasic arithmetic operations are addition, subtraction, multiplication, division and Exponentiation Operator Operation Basic expression ^ Exponentiation A ^ B * Multiplication A * B / Division A / B + Addition A + B - Subtraction A - B hhhhhhh 59

More Arithmetic Operators In VB, there are many other arithmetic operators such as: Integer division (Uses the backslash, \) (num1 \ num2) is the Quotient of num1 divided by num2 (the integer part of the result of dividing two integers ) Remainder: (num1 Mod num2) is the whole number remainder of num1 divided by num2 17 / 2 = 8.5 17 \ 2 = 8 17 Mod 2 = 1 16 / 2 = 8 16\ 2 =8 16 Mod 2 = 0 34/7= 4.8571428571286 34 \ 7 = 4 34 Mod 7 = 6

Chapter 4 - Visual Basic Schneider Mod Rules The whole number remainder If the numbers are single they are rounded first If one or both of a and b are negative the result of the Mod will have the sign of “a” num1 Mod 1 = 0 num1 Mod num1 = 0 Num Mod 0 Division by zero Result of Mod should be less than num2 Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider Mod Examples 25 mod 7 = 4 3 mod 1 = 0 18.7mod 3.2 = 1 -3 mod 1 = 0 -35 mod 4 = -3 -35 mod -4 = -3 35 mod -4 = 3 -18.7 mod 3.2 = -1 -18.7 mod -3.2 = -1 18.7 mod -3.2 = 1 3 mod 3 = 0 3 mod -3 = 0 -3 mod 3 = 0 -3 mod -3 = 0 Chapter 4 - Visual Basic Schneider

Order of Precedence All operations inside of () are evaluated first ^ is evaluated next * and /are at the same level of precedence and are evaluated next \ (integer division) is evaluated next Mod is evaluated next + and – have the same level of precedence and are evaluated last

Order of Precedence When operators are on the same level Performed from left to right

What is the output of the following expression? 2+3^2+(2+5*2)/3 Mod 3 + 3 2+3^2+(2+10)/3 Mod 3 + 3 2+3^2+12/3 Mod 3 + 3 2+9+12/3 Mod 3 + 3 2+9+4 Mod 3 + 3 2+9+1 + 3 15 There are 3 arithmetic operators (+ , / and Mod). Which operator has the highest priority?

Chapter 4 - Visual Basic Schneider 1 Private Sub cmd_Click() Dim a As Integer Dim b As Integer a = 14 b = 5 picOutput.Print a / b \ b End Sub Output: 1. 2.8 2. 0 3. 2 4. 4 5. Infinity /Illegal Division Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider 2. Private Sub cmd_Click() Dim a As Integer Dim b As Integer a = 14 b = 5 picOutput.Print a mod b \ b End Sub Output: 1. 2.8 2. 0 3. 2 4. 4 5. Infinity /Illegal Division Chapter 4 - Visual Basic Schneider

Chapter 4 - Visual Basic Schneider More Examples Private Sub cmd_Click() Dim a As Integer Dim b As Integer a = 14 b = 5 Print a / b Print a \ b Print a Mod b End Sub Output: 2.8 2 4 Chapter 4 - Visual Basic Schneider