1. Microsoft Visual Basic 2005
BASICS
Lesson 10
Do Loops

2. 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.

3. Microsoft Visual Basic 2005 BASICS 3
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

4. Microsoft Visual Basic 2005 BASICS 4
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.

5. Microsoft Visual Basic 2005 BASICS 5
Using Do While
 Do While loop
 Repeats a block of code as long as a
condition remains True
 Syntax example

6. Microsoft Visual Basic 2005 BASICS 6
Using Do While (cont.)

7. Microsoft Visual Basic 2005 BASICS 7
Using Do Until
 Do Until loop
 Repeats a block of code until a condition is
no longer True
 Syntax example

8. Microsoft Visual Basic 2005 BASICS 8
Using Do Until (cont.)

9. Microsoft Visual Basic 2005 BASICS 9
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

10. Microsoft Visual Basic 2005 BASICS 10
Choosing Do While or Do
Until (cont.)

11. Microsoft Visual Basic 2005 BASICS 11
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

12. Microsoft Visual Basic 2005 BASICS 12
Using the InputBox Function
(cont.)

13. Microsoft Visual Basic 2005 BASICS 13
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.

14. Microsoft Visual Basic 2005 BASICS 14
Using the InputBox Function
within a Do Until Loop (cont.)

15. Microsoft Visual Basic 2005 BASICS 15
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

16. Microsoft Visual Basic 2005 BASICS 16
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

17. Microsoft Visual Basic 2005 BASICS 17
Event Procedure Problems
(cont.)

18. Microsoft Visual Basic 2005 BASICS 18
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

19. Microsoft Visual Basic 2005 BASICS 19
Event Procedure Solutions
(cont.)

20. Microsoft Visual Basic 2005 BASICS 20
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

21. Microsoft Visual Basic 2005 BASICS 21
Event Procedure Solutions
(cont.)

22. Microsoft Visual Basic 2005 BASICS 22
Using Nested Loops
 Like If statements, loops may be
nested.
 It is not uncommon to need a loop within
a loop.

23. Microsoft Visual Basic 2005 BASICS 23
Summary
 Much of the work a computer does is
repeated many times. This repetition in
programs is accomplished using loops.

24. Microsoft Visual Basic 2005 BASICS 24
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.

25. Microsoft Visual Basic 2005 BASICS 25
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.

26. Microsoft Visual Basic 2005 BASICS 26
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.

27. Microsoft Visual Basic 2005 BASICS 27
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.