Chapter 4 - Visual Basic Schneider1 Chapter 4 General Procedures.

Chapter 4 - Visual Basic Schneider 1

Chapter 4

General Procedures


Outline & Objective

Sub Procedures Procedure Parameters Function Procedures


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


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.


Sub Procedures Properties:

can be called (invoked) by using•call subProcedureName()

passing data called arguments


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

Syntax of a Sub Procedure

Private Sub ProcedureName ( ) statement(s)End Sub

Procedure Name

Statements

Example


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.


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.

Example

12

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.


Passing arguments to parameters

Arguments

Arguments : Variables or expressions placed in parentheses in a Call statement.

14


Parameters

Parameters : Variables appearing in the sub procedure definition are called paramanters.

15

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

Parameters

Arguments



Call Triple(num)

Private Sub Triple (num As Single)

Argument

Parameter


Sub Procedure Triple:

Private Sub Triple(num As Single) ' Multiply the value of the number by 3 picResult.Print "The number is"; 3 * numEnd Sub

Private Sub Triple(par As Single) ' Multiply the value of the number by 3 picResult.Print "The number is"; 3 * parEnd Sub

Private Sub cmdCompute_Click() Dim num As Single num = Val(InputBox("Enter a number:")) Call Triple(num)End Sub

cmdCompute Triple

num par/num

Example

ExamplecmdDisplay 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 valuecmdDisplay Triple

amt

num

ByRef and ByVal Examples

Private Sub cmdCall_Click()Dim a As IntegerDim b As Integera = 10b = 15Print a; bCall mySub(a, b)Print a; bEnd Sub

Sub mySub(x As Integer, y As Integer) x = 11 y = 16End Sub

Output10 1511 16

cmdCall mysub

a x

b y



Sub mySub(ByRef x As Integer, ByRef y As Integer) x = 11 y = 16End Sub

Output10 1511 16

cmdCall mysub

a x

b y



Sub mySub(ByRef x As Integer, ByVal y As Integer) x = 11 y = 16End Sub

Output10 1511 15

cmdCall mysub

a x

b

y



Sub mySub(ByVal x As Integer, ByVal y As Integer) x = 11 y = 16End Sub

Output10 1510 15

cmdCall mysub

a

b

x

y



Sub mySub(ByRef y As Integer, ByRef x As Integer) x = 11 y = 16End Sub

Output10 1516 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.


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.


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


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)

Example

The variables x and y are local for the procedure cmdDisplay_Click

The variables x and y are local for the procedure pr1

cmdDisplay pr1

x

y

x

y

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


Form-Level Variables

Form-level variables are visible to every procedure.

Form-level variables appear at the top of the code window.


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

The variables x and y are Form-Level variables (Global)

Note: The values of Form-level variables are visible to all procedures in the program

cmdCall mysub

X Global X Global

y Global y Global

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

yx

x

y

0 0

05

6

0

7

8

Form_level Variables

Local

LocalThe procedure uses the global variable if it has no local variable

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


What is a function?

Similar to a Sub, performs a specific task

Unlike a Sub, returns a single value to the calling program

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


The Function Syntax

Private Function FunctionName(parameter-list) As dataType

Statement(s)…… ….. FunctionName = valueEnd Function Note: value must be of the type dataType specified in

the function declaration


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.

Example of a Function

The function Max has 2 parameters of type single

What is the data type of the returned value?


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


Another Example

Private Sub cmdDisplay_Click() Dim num As Single num = 5 picOutput.Print Triple (num) picOutput.Print numEnd Sub

Private Function Triple(x As Single) As Single Dim num As Single num = 3 Triple = num * xEnd Function

Output: 15 5

Example

Private sub com_click()x=fun(3) + fun( 5)Print x End sub

Private function fun(a)fun= aa=a+1fun=fun + aEnd function

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Example

Private sub com_click()x=fun(3, fun(2, 1)) Print x End sub

Private function fun(a,b) as integera=a+bEnd function

0

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.


To call a sub-procedure


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


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

Arithmetic Operations

Operator Operation Basic expression

^ Exponentiation A ^ B * Multiplication A * B / Division A / B + Addition A + B - Subtraction A - B

The VBasic arithmetic operations are addition, subtraction, multiplication, division and Exponentiation

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

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

Mod Examples


25 mod 718.7mod 3.2-35 mod 4-35 mod -4 35 mod -4-18.7 mod 3.2

= 4= 1= -3= -3= 3

= -1-18.7 mod -3.2 = -118.7 mod -3.2 = 13 mod 3 = 03 mod -3 = 0-3 mod 3 = 0-3 mod -3 = 0

3 mod 1 = 0-3 mod 1 = 0

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?

1


Private Sub cmd_Click() Dim a As Integer Dim b As Integer a = 14 b = 5 picOutput.Print a / b \ bEnd SubOutput:1. 2.82. 03. 24. 45. Infinity /Illegal Division

2.


Private Sub cmd_Click() Dim a As Integer Dim b As Integer a = 14 b = 5 picOutput.Print a mod b \ bEnd SubOutput:1. 2.82. 03. 24. 45. Infinity /Illegal Division


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 bEnd Sub

Output:2.824

Chapter 4 - Visual Basic Schneider1 Chapter 4 General Procedures.

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Transcript of Chapter 4 - Visual Basic Schneider1 Chapter 4 General Procedures.