Chapter 4 General Procedures 4 1 Sub Procedures

Chapter 4 - General Procedures • • 4. 1 Sub Procedures, Part I 4. 2 Sub Procedures, Part II 4. 3 Function Procedures 4. 4 Modular Design Chapter 4 - VB 2005 by Schneider 1

4. 1 Sub Procedures, Part I • Sub Procedures • Variables and Expressions as Arguments • Calling Other Sub Procedures Chapter 4 - VB 2005 by Schneider 2

Devices for modularity • Visual Basic has two devices for breaking problems into smaller pieces: • Sub procedures • Function procedures Chapter 4 - VB 2005 by Schneider 3

Sub Procedures • Perform one or more related tasks • General syntax Sub Procedure. Name() statements End Sub Chapter 4 - VB 2005 by Schneider 4

Calling a Sub procedure • The statement that invokes a Sub procedure is also referred to as a call statement. • A call statement looks like this: Procedure. Name() Chapter 4 - VB 2005 by Schneider 5

Naming Sub procedures • The rules for naming Sub procedures are the same as the rules for naming variables. Chapter 4 - VB 2005 by Schneider 6

Example lst. Box. Items. Clear() Explain. Purpose() lst. Box. Items. Add("") Sub Explain. Purpose() lst. Box. Items. Add("Program displays a sentence") lst. Box. Items. Add("identifying a sum. ") End Sub Chapter 4 - VB 2005 by Schneider 7

Passing Values • You can send values to a Sub procedure Sum(2, 3) Sub Sum(By. Val num 1 As Double, By. Val num 2 As Double) lst. Box. Items. Add("The sum of " & num 1 & " and " _ & num 2 & " is " & (num 1 + num 2) & ". " End Sub • In the Sum Sub procedure, 2 will be stored in num 1 and 3 will be stored in num 2 Chapter 4 - VB 2005 by Schneider 8

Arguments and Parameters • Sum(2, 3) arguments parameters Sub Sum(By. Val num 1 As Double, By. Val num 2 As Double) displayed automatically Chapter 4 - VB 2005 by Schneider 9

Several Calling Statements Explain. Purpose() Sum(2, 3) Sum(4, 6) Sum(7, 8) Output: Program The sum displays of 2 and of 4 and of 7 and a 3 6 8 sentence identifying a sum. is 5. is 10 is 15. Chapter 4 - VB 2005 by Schneider 10

Passing Strings and Numbers • Demo("CA", 34) Sub Demo(By. Val state As String, By. Val pop As Double) txt. Box, Text = state & " has population " & pop & _ " million. " End Sub • Note: The statement Demo(34, "CA") would not be valid. The types of the arguments must be in the same order as the types of the parameters. Chapter 4 - VB 2005 by Schneider 11

Variables and Expressions as Arguments Dim s As String = "CA" Dim p As Double = 17 Demo(s, 2 * p) Sub Demo(By. Val state As String, By. Val pop As Double) txt. Box. Text = state & " has population " & pop & _ " million. " End Sub • Note: The variable names in the arguments need not match the parameter names. For instance, s versus state. . Chapter 4 - VB 2005 by Schneider 12

Calling A Sub procedure can call another Sub procedure. Private Sub btn. Add_Click(. . . ) Handles btn. Add. Click Sum(2, 3) End Sub Sum(By. Val num 1 As Double, By. Val num 2 As Double) Display. Purpose() lst. Box. Items. Add("The sum of " & num 1 & " and " _ & num 2 & " is " & (num 1 + num 2) & ". " End Sub Chapter 4 - VB 2005 by Schneider 13

4. 2 Sub Procedures, Part II • • • Passing by Value Passing by Reference Local Variables Class-Level Variables Debugging Chapter 4 - VB 2005 by Schneider 14

By. Val and By. Ref • Parameters in Sub procedure headers are proceeded by By. Val or By. Ref • By. Val stands for By Value • By. Ref stands for By Reference Chapter 4 - VB 2005 by Schneider 15

Passing by Value • When a variable argument is passed to a By. Val parameter, just the value of the argument is passed. • After the Sub procedure terminates, the variable has its original value. Chapter 4 - VB 2005 by Schneider 16

Example Dim n As Double = 4 Triple(n) txt. Box. Text = CStr(n) End Sub Triple(By. Val num As Double) num = 3 * num End Sub Output: 4 Chapter 4 - VB 2005 by Schneider 17

Same Example: n num Dim num As Double = 4 Triple(num) txt. Box. Text = CStr(num) End Sub Triple(By. Val num As Double) num = 3 * num End Sub Output: 4 Chapter 4 - VB 2005 by Schneider 18

Passing by Reference • When a variable argument is passed to a By. Ref parameter, the parameter is given the same memory location as the argument. • After the Sub procedure terminates, the variable has the value of the parameter. Chapter 4 - VB 2005 by Schneider 19

Example Public Sub btn. One_Click (. . . ) Handles _ btn. One. Click Dim num As Double = 4 Triple(num) txt. Box. Text = CStr(num) End Sub Triple(By. Ref num As Double) num = 3 * num End Sub Output: 12 Chapter 4 - VB 2005 by Schneider 20

Example: num n Private Sub btn. One_Click(. . . ) Handles _ btn. One_Click Dim n As Double = 4 Triple(n) txt. Box. Text = CStr(n) End Sub Triple(By. Ref num As Double) num = 3 * num End Sub Output: 12 Chapter 4 - VB 2005 by Schneider 21

Local Variable • A variable declared inside a Sub procedure with a Dim statement • Space reserved in memory for that variable until the End Sub – then the variable ceases to exist Chapter 4 - VB 2005 by Schneider 22

Local Variables Private Sub btn. One_Click(. . . ) Handles btn. One_Click Dim num As Integer = 4 Set. Five() txt. Box. Text = CStr(num) End Sub Set. Five() Dim num As Integer = 5 End Sub Output: 4 Chapter 4 - VB 2005 by Schneider 23

Class-Level Variables • Visible to every procedure in a form’s code without being passed • Dim statements for class-level variables are placed • Outside all procedures • At the top of the program region Chapter 4 - VB 2005 by Schneider 24

Class-Level Variables Dim num As Integer = 4 Private Sub btn. One_Click(. . . ) Handles btn. One_Click txt. Box. Text = CStr(num) Set. Five() txt. Box. Text &= CStr(num) End Sub Set. Five() num = 5 End Sub Output: 45 Chapter 4 - VB 2005 by Schneider 25

Scope • The scope of a variable is the portion of the program that can refer to it. Chapter 4 - VB 2005 by Schneider 26

Scope • Class-level variables have class-level scope and are available to all procedures in the class. • Variables declared inside a procedure have local scope and are only available to the procedure in which they are declared. Chapter 4 - VB 2005 by Schneider 27

Debugging • Programs with Sub procedures are easier to debug • Each Sub procedure can be checked individually before being placed into the program Chapter 4 - VB 2005 by Schneider 28

4. 3 Function Procedures • User-Defined Functions Having Several Parameters • Comparing Function Procedures with Sub Procedures • Collapsing a Procedure with a Region Directive Chapter 4 - VB 2005 by Schneider 29

Some Built-In Functions Function Example Input Output Int(2. 6) is 2 number Math. Round(1. 23, 1) is 1. 2 number, number Format. Percent(. 12) is 12. 00% number string Format. Number(123 45. 628, 1) is 12, 345. 6 number, number string Chapter 4 - VB 2005 by Schneider 30

Function Procedures • Function procedures (aka user-defined functions) always return one value • Syntax: Function. Name(By. Val var 1 As Type 1, _ By. Val var 2 As Type 2, _ …) As data. Type statement(s) Return expression End Function Chapter 4 - VB 2005 by Schneider 31

Example: Form txt. Full. Name txt. First. Name Chapter 4 - VB 2005 by Schneider 32

Example: Code Private Sub btn. Determine_Click(. . . ) _ Handles btn. Determine. Click Dim name As String name = txt. Full. Name. Text Function txt. First. Name. Text = First. Name(name) call End Sub Function First. Name(By. Val name As String) As String Dim first. Space As Integer first. Space = name. Index. Of(" ") Return statement Return name. Substring(0, first. Space) End Function Chapter 4 - VB 2005 by Schneider 33

Example: Form txt. Side. One txt. Side. Two txt. Hyp Chapter 4 - VB 2005 by Schneider 34

Example: Code Private Sub btn. Calculate_Click(. . . ) _ Handles btn. Calculate. Click Dim a, b As Double a = CDbl(txt. Side. One. Text) b = CDbl(txt. Side. Two. Text) txt. Hyp. Text = CStr(Hypotenuse(a, b)) End Sub Function Hypotenuse(By. Val a As Double, _ By. Val b As Double) As Double Return Math. Sqrt(a ^ 2 + b ^ 2) End Function Chapter 4 - VB 2005 by Schneider 35

User-Defined Function Having No Parameters Private Sub btn. Display_Click(. . . ) _ Handles btn. Display. Click txt. Box. Text = Saying() End Sub Function Saying() As String Dim str. Var As String str. Var = Input. Box("What is your" _ & " favorite saying? ") Return str. Var End Function Chapter 4 - VB 2005 by Schneider 36

Comparing Function Procedures with Sub Procedures • Subs are accessed using a call statement • Functions are called where you would expect to find a literal or expression • For example: • result = function. Call • lst. Box. Items. Add (function. Call) Chapter 4 - VB 2005 by Schneider 37

Functions vs. Procedures • Both can perform similar tasks • Both can call other subs and functions • Use a function when you want to return one and only one value Chapter 4 - VB 2005 by Schneider 38

Collapsing a Procedure with a Region Directive • A procedure can be collapsed behind a captioned rectangle • This task is carried out with a Region directive. • To specify a region, precede the code to be collapsed with a line of the form #Region "Text to be displayed in the box. " • and follow the code with the line #End Region Chapter 4 - VB 2005 by Schneider 39

Region Directives Chapter 4 - VB 2005 by Schneider 40

Collapsed Regions Chapter 4 - VB 2005 by Schneider 41

4. 4 Modular Design • Top-Down Design • Structured Programming • Advantages of Structured Programming Chapter 4 - VB 2005 by Schneider 42

Design Terminology • Large programs can be broken down into smaller problems • "divide-and-conquer" approach called "stepwise refinement" • Stepwise refinement is part of top-down design methodology Chapter 4 - VB 2005 by Schneider 43

Top-Down Design • General problems are at the top of the design • Specific tasks are near the end of the design • Top-down design and structured programming are techniques to enhance programmers' productivity Chapter 4 - VB 2005 by Schneider 44

Top-Down Design Criteria 1. The design should be easily readable and emphasize small module size. 2. Modules proceed from general to specific as you read down the chart. 3. The modules, as much as possible, should be single minded. That is, they should only perform a single well-defined task. 4. Modules should be as independent of each other as possible, and any relationships among modules should be specified. Chapter 4 - VB 2005 by Schneider 45

Top-Level Design HIPO Chart Chapter 4 - VB 2005 by Schneider 46

Detailed HIPO Chart Chapter 4 - VB 2005 by Schneider 47

Structured Programming • Control structures in structured programming: • Sequences: Statements are executed one after another. • Decisions: One of two blocks of program code is executed based on a test for some condition. • Loops (iteration): One or more statements are executed repeatedly as long as a specified condition is true. Chapter 4 - VB 2005 by Schneider 48

Advantages of Structured Programming • Goal to create correct programs that are easier to • write • understand • modify • "GOTO –less" programming Chapter 4 - VB 2005 by Schneider 49

Comparison of Flow Charts Chapter 4 - VB 2005 by Schneider 50

Easy to Write • Allows programmer to first focus on the big picture and take care of the details later • Several programmers can work on the same program at the same time • Code that can be used in many programs is said to be reusable Chapter 4 - VB 2005 by Schneider 51

Easy to Debug • Procedures can be checked individually • A driver program can be set up to test modules individually before the complete program is ready • Using a driver program to test modules (or stubs) is known as stub testing Chapter 4 - VB 2005 by Schneider 52

Easy to Understand • Interconnections of the procedures reveal the modular design of the program. • The meaningful procedure names, along with relevant comments, identify the tasks performed by the modules. • The meaningful variable names help the programmer to recall the purpose of each variable. Chapter 4 - VB 2005 by Schneider 53

Easy to Change • Because a structured program is selfdocumenting, it can easily be deciphered by another programmer. Chapter 4 - VB 2005 by Schneider 54

Object-Oriented Programming • an encapsulation of data and code that operates on the data • objects have properties, respond to methods, and raise events. Chapter 4 - VB 2005 by Schneider 55