Lecture Learning programming using Visual Basic Net – Chapter 6 Reducing program complexity General sub procedures and developerdefined functions

Chapter six Reducing program complexity General sub procedures and developerdefined functions. After completing this unit, you should be able to Share code by creating general sub procedures and functions; use parameters to share data between procedures and functions; use code modules to organize code for reusability; use the KeyPress, Enter, and Leave events; use the concept of form modality; create and program main menu.

CHAPTER SIX

Reducing Program Complexity General Sub Procedures and Developer-defined Functions

• Three important consideration help us design,

construct, and maintain complex programs:

1 Break complex tasks into smaller “subtasks.”

2 Give each subtask a descriptive name.

3 Find processing tasks that have subtasks

in common.

• Share code by creating general sub procedures and functions

• Use parameters to share data between

procedures and functions

• Use code modules to organize code for

reusability

• Use the KeyPress, Enter, and Leave events

• Use the concept of form modality

• Create and program main menus

6.1 General Sub Procedures

• We must be very precise in writing the criterion

and alternative actions for decisions

• In a program,

– A condition is represented as an expression.

– An outcome is the result of an evaluated

condition.

– An appropriate action follows the outcome.

6.1 General Sub Procedures (cont.)

– Eliminate inconsistencies by placing common

statements in a general sub procedure.

– Apply a descriptive name for the sub procedure.

6.1 General Sub Procedures (cont.)

• Execution of General Sub Procedures

– Names of event procedures always end with an

underscore followed by the type of event.

– Names of general sub procedures do not.

– A procedure call invokes a procedure.

6.1 General Sub Procedures (cont.)

• Local Variables in General Sub Procedures

– General sub procedures can access module-level and global variables.

– They have their own variables.

– Procedure-level variables are not related to other procedures.

6.1 General Sub Procedures (cont.)

• General Sub Procedures and Project Structure

– Locating a General Sub Procedure in the Code

6.1 General Sub Procedures (cont.)

– Procedure Scope

• Determines which procedure’s can invoke it.

• Private and Public.

– Code Modules

• Can contain general sub procedures.

• Help to organize a project.

• Can be included in many different projects.

– Project Structure

• See Figure 6.13 of textbook.

6.1 General Sub Procedures (cont.)

• Creating General Sub Procedures

– Start from the Code window of a form or code

module.

– Enter header for the sub procedure.

– Enter general sub procedure statements.

6.2 Procedures with Parameters

• Drawbacks of Module-Level and Global

Variables

– Public access can mean trouble for some

variables.

– Procedures wanting to share data using the

global variable have to “know” its name.

6.2 Procedures with Parameters (cont.)

– Procedure Calls with Parameters

• See Figure 6.16 in textbook.

6.2 Procedures with Parameters (cont.)

– Analyzing Procedures That Use Parameters

• Hand-Check parameter passing.

6.2 Procedures with Parameters (cont.)

• Multiple Parameters

– A parameter list is key.

– Every parameter list should contain:

• Number of arguments.

• Types of arguments.

• Sequence of arguments.

• Names arguments are referred to.

• What the parameters and arguments represents?

6.2 Procedures with Parameters (cont.)

• Passing by Reference and Passing by Value

– ByRef keyword is short for “By Reference.”

• Parameter and argument refer to the same variable.

• Only way a sub procedure can change the value it

is passed.

– ByVal keyword is short for “By Value.”

• Specifies that the called procedure cannot change the value stored in a variable passed to it.

• Parameter is a local copy of the passed argument.

6.2 Procedures with Parameters (cont.)

• Passing Expressions

– Calling procedure can pass an expression.

• Correcting Common Mistakes in Parameter

Passing

– Arguments Not Specified

• Number of arguments must match parameters.

– Invalid Cast Exception

• Parameter and argument type mismatch.

6.2 Procedures with Parameters (cont.)

– Arguments Out of Order

• Arguments and parameters must be associated by position, not by name.

– Conflict between Parameter Name and Local

Variable Name

• Parameter name and local variable names must be different

• General Sub Procedures versus Event

Procedures

– An event procedure is always associated with a

control.

– Developers create the procedure heading for a

general sub procedure.

– Visual Basic NET creates the procedure heading for an event procedure.

6.2 Procedures with Parameters (cont.)

• General Sub Procedures versus Event

Procedures and the Object Paradigm

– We have created a form class.

– We also created two methods.

• One was event procedure.

• The other was a general sub procedure.

6.3 Developer-Defined Functions

• Perform calculations or string manipulations

• Return values

• Use Return statements

• Function headings include type specifications

6.3 Developer-Defined Functions (cont.)

6.4 Code Modules

• Sub Main

– Use to begin execution of the program by

executing a general sub procedure.

– Select in the Misc Property Pages dialog box

under the Project menu.

– You must create a public general sub procedure

named Main in a code module.

6.4 Code Modules (cont.)

6.5 The KeyPress Event

• Enables your programs to respond to keystrokes made by the user

• Any control that can have the focus is able to

respond to this event

6.5 The KeyPress Event (cont.)

• The KeyPress Event for TextBox Controls

– User presses a key.

– Visual Basic NET stores ANSI character of the

6.5 The KeyPress Event (cont.)

– The SendKeys Class

• Used to send any keystroke to an application

• Keystrokes are represented by codes

6.6 The Enter and Leave Events

• Enter event occurs for a control when the control receives the focus

• Leave event occurs for a control when the

control loses the focus

• Both events are triggered by the user or by

code

• Help to make an application more intuitive for

the user

6.7 Modal versus Modeless Forms

• Most real-world applications use multiple forms

• Form modality controls how a form is displayed

– When a modal form is shown, other forms

become inactive.

– When a modeless form is displayed, both its

controls and the controls on other forms are

active.

6.7 Modal versus Modeless Forms (cont.)

• Modal Forms

– A message box is the simplest example.

– User must click OK to return control to the

procedure.

• Presenting a Sequence of Forms

– Using modeless forms

• Control code must be written within each form.

– Using modal forms

• Control of all forms can be in a one procedure.

6.8 The MainMenu Control

• Start by right-clicking on the MainMenu control in the component tray

• Then select Edit Menu from the pop-up menu

• Enter both main and sub menu items

Chapter Summary

• General sub procedures and developer-defined

functions break programming into small parts

• A general sub procedure performs a specific

processing task

• Developers can use a library of procedures

performing common tasks

• General sub procedures are not linked to any

control on a form

Chapter Summary (cont.)

• General sub procedures can be called from

other general sub procedures or from event

procedures

• A developer-defined function returns a single

value when it executes

• Procedures and functions can use parameters to share data

• An argument can be passed to a procedure or

Chapter Summary (cont.)

• The KeyPress event procedure has a parameter that determines the key that the user has

pressed

• The Enter event occurs when the user gives the focus to a control

• Form modality refers to the way forms react

when two or more forms are displayed at one

time

• Form modality is specified by the type of Show

method used

Chapter Summary (cont.)

• Applications can incorporate a main menu bar

by using the MainMenu control