Transcript
Microsoft Visual Basic 2005 BASICS
Lesson 10
Do Loops
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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.
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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
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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.
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Using Do While
Do While loop Repeats a block of code as long as a
condition remains True Syntax example
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Using Do While (cont.)
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Using Do Until
Do Until loop Repeats a block of code until a condition is
no longer True Syntax example
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Using Do Until (cont.)
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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
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Choosing Do While or Do Until (cont.)
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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
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Using the InputBox Function (cont.)
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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.
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Using the InputBox Function within a Do Until Loop (cont.)
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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
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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
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Event Procedure Problems (cont.)
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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
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Event Procedure Solutions (cont.)
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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
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Event Procedure Solutions (cont.)
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Using Nested Loops
Like If statements, loops may be nested.
It is not uncommon to need a loop within a loop.
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Summary
Much of the work a computer does is repeated many times. This repetition in programs is accomplished using loops.
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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.
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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.
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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.
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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.
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