Ppt lesson 10

Microsoft Visual Basic 2005 BASICS

Lesson 10

Do Loops

Microsoft Visual Basic 2005 BASICS 2

Objectives

Explain what a loop is. Use the Do While and Do Until

loops. Use the InputBox function. Use the Application.DoEvents

statement. Use nested loops.


What Are Loops?

Statement repetition is accomplished by a loop.

Iteration structure The code required to create a loop

Loops Do While Do Until For


Using the Do Loops

Knowing which of the Do loops to use is just a matter of experience.

Both types of Do loops rely on a condition to be either True or False.


Using Do While

Do While loop Repeats a block of code as long as a

condition remains True Syntax example


Using Do While (cont.)


Using Do Until

Do Until loop Repeats a block of code until a condition is

no longer True Syntax example


Using Do Until (cont.)


Choosing Do While or Do Until

Primary difference A Do While loop tests the condition at the

top of the loop. The Do Until loop tests the condition at

the bottom of the loop. Do While loop

Should only be used if you are sure that you want the loop to execute at least once


Choosing Do While or Do Until (cont.)


Using the InputBox Function

InputBox function The opposite of the MsgBox function Displays a window to ask the user for input

Using the InputBox function Supply two strings

The text that will prompt the user The title for the window’s title bar


Using the InputBox Function (cont.)


Using the InputBox Function within a Do Until Loop

The InputBox function can be used inside a Do Until loop. To repeatedly ask the user for data until a

specified condition is met Place a call to the InputBox function

inside a Do Until loop.


Using the InputBox Function within a Do Until Loop (cont.)


Potential Event Procedure Problems and Solutions

When an event is triggered An event procedure is executed to handle

the event Executing an event procedure takes some

time. Can cause problems


Event Procedure Problems

Long event procedures Includes a loop that processes thousands

of instructions Program is unresponsive to other events

Endless loops Condition to stop the loop never becomes

True Usually a programming error


Event Procedure Problems (cont.)


Event Procedure Solutions

Application.DoEvents subroutine Allows computer to process other events

even though the current event procedure is not yet complete

Add it to a loop that may occupy a lot of the computer’s time.

Introduces the potential for repetitive Click event procedures


Event Procedure Solutions (cont.)



Preventing repetitive Click event procedures Disable the button at the beginning of the

event procedure. Prevents the user from clicking the button

again until the event processing is completed




Using Nested Loops

Like If statements, loops may be nested.

It is not uncommon to need a loop within a loop.


Summary

Much of the work a computer does is repeated many times. This repetition in programs is accomplished using loops.


Summary (cont.)

A Do loop condition applies a test to achieve either a True or False result. A Do While loop tests the condition at the top of the loop and repeats a group of statements while a certain condition is True. A Do Until loop tests the condition at the bottom of the loop and repeats a group of statements until a certain condition becomes True. The code in a Do Until loop is always executed at least once.


Summary (cont.)

The InputBox function creates a window that prompts the user for input. To use the InputBox function, you supply the text for the prompt, the title for the window’s title bar, and the optional default text for the text box.

Sometimes, long event procedures can make a program unresponsive to other events.


Summary (cont.)

An endless loop is a loop in which the condition that stops the loop is never met. Clicking the Break All button in the IDE will pause a program with an endless loop and will highlight the code where the program stopped.


Summary (cont.)

An Application.DoEvents subroutine allows the program to process other events while an event procedure is executing.

Loops can be nested in the same way that If statements are nested.

Technology

Ppt lesson 10