CHAPTER SEVEN
Repeating Processing Tasks
Loop Structures
7- 2
Introduction
• Most applications certain processing steps
repeatedly.
• This structure or flow of control is known as a
loop.
• The Do…Loop structure executes a block of
statements until a specified condition occurs.
• The For…Next structure executes a block of
statements a specified number of times.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 3
Objectives
•
•
•
•
Use the Do…Loop and For…Next structure.
Know when each structure is appropriate.
Understand the operation of nested loops.
Construct user interfaces with the ListBox,
ComboBox, and ListView controls.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 4
7.1 The Do…Loop Structure
• Syntax and Action of the Do…Loop
Do
statementblock
Loop
– Run Time: The Effect of the Do…Loop Structure
• Statements in a loop’s interior are executed one at
a time.
• The Do marks the top of the loop.
• The Loop sends the execution back to the top.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 5
7.1 The Do…Loop Structure (cont.)
• Do While…Loop
Do While condition
statementblock
Loop
– Run Time: The Effect of the Do While…Loop
Structure
• Loop continues to execute “while the condition is
true.”
• Loop is exited when the condition is false.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 6
7.1 The Do…Loop Structure (cont.)
•
Coding the Loop Body
– One or more statements in the loop body must
eventually cause the condition to become false.
– Order of statements inside loop affects the
result of the loop.
– Loop may terminate immediately.
– Loop can be viewed as a meta statement.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 7
7.1 The Do…Loop Structure (cont.)
• Do Loop…While
– Works like the Do While…Loop.
– Termination at the bottom of the loop.
– Syntax:
Do
statementblock
Loop While condition
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 8
7.1 The Do…Loop Structure (cont.)
• Do Until…Loop and Do…Loop Until
– Do Until…Loop
• Termination condition at the top of the loop.
• Same syntax and action as the Do While…Loop.
– Except:
» Difference in keywords.
» Loop statements inside the Do Until…Loop are
executed when the conditions are False.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 9
7.1 The Do…Loop Structure (cont.)
– Do…Loop Until
• Termination condition at the bottom of the loop.
• Exit when the condition is True.
• Initialization and Termination
• Loops may start or end at the wrong place.
– The initialization statement may be off.
– The condition operator may need to be changed.
– The order of statements within the loop may need
to be changed.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 10
7.2 The For…Next Loop Structure
• Syntax and Action of For…Next
For counter = start To end
statementblock
Next
– Run Time: The Effect of the For…Next Structure
• Counter variable and start and end expressions
must be evaluated to control the number of times
the loop will be processed.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 11
7.2 The For…Next Loop Structure (cont.)
• For…Next versus Do…Loop Variations
– Use a For…Next loop when the number of loop
iterations can be determined.
– Use a Do…Loop variety only when the number of
iterations depends on the results of statements
inside the loop.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 12
7.2 The For…Next Loop Structure (cont.)
• The Step Amount
– Counter variable may be incremented by a step
amount, such 3 instead of 1.
– Syntax:
For counter = start To end Step increment
statementblock
Next
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 13
7.2 The For…Next Loop Structure (cont.)
• Avoiding For…Next Errors
– Do not place statements inside the loop that
change start, end, and increment expressions.
– Do not change the counter variable inside the
loop.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 14
7.3 Exit Do and Exit For
• Exit Do is used inside a Do…Loop structure to
exit the loop immediately.
• Exit For is used inside a For…Next structure to
exit the loop immediately.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 15
7.4 Nested Loops
• Loops may be embedded inside of other loops.
• Nested Do While…Loops
Do While condition1
statementblock1a
Do While condition2
statementblock2
Loop
statementblock1b
Loop
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 16
7.4 Nested Loops (cont.)
• Nested For…Next Loops
For counter1 = start1 To end1
statementblock1a
For counter2 = start2 To end2
statementblock2
Next
statementblock1b
Next
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 17
7.5 The ListBox and ComboBox Controls
• The ListBox
– Display a list of items from which the user makes
their selection.
– Appearance and Use
• Rectangle that displays rows of text.
• Vertical scroll bar is automatically provided, if
necessary.
• A means to obtain input and output.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7.5 The ListBox and ComboBox Controls
(cont.)
7- 18
– Properties
HorizontalScrollbar
Items
MultiColumn
SelectedIndex
SelectedIndices
SelectedItem
McGraw Hill/Irwin
SelectedItems
SelectionMode
Sorted
Text
UseTabStops
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7.5 The ListBox and ComboBox Controls
(cont.)
7- 19
– Events
•
•
•
•
•
DoubleClick
KeyPress
Enter
Leave
SelectIndexChanged
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7.5 The ListBox and ComboBox Controls
(cont.)
7- 20
– Methods
• Two common used
– FindString and FindStringExact
• Selected methods of the Items property
– Add(text)
– Insert(index, text)
– Remove(text)
– RemoveAt(index)
– Clear()
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7.5 The ListBox and ComboBox Controls
(cont.)
7- 21
• The ComboBox
– Combination TextBox/Listbox
– Properties
• Has many of the TextBox and ListBox properties.
• The three styles of the ComboBox are controlled
by the DropDownStyle property.
– Events
• Similar to the ListBox.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 22
7.6 Collections and the For Each…Next Structure
• A Collection object is an ordered set of items
that can be referred to as a unit.
– The ListBox provides collections.
– The ListBox permits either single or multiple item
selection.
• The For Each…Next Structure
– Repeats a group of statements for each element
in a collection.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7.6 Collections and the For Each…Next
Structure (cont.)
7- 23
– Syntax of the For Each…Next structure
For Each object In collection
statementblock
Next
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 24
7.7 The ListView Control
• Used to display lists of items that can include
icons and/or text.
– Window Explorer uses a ListView control.
– Appearance and Use
• Appears as a rectangular area that can contain
icons, text, or combinations of icons and text.
• Can display textual information in a row and
column format.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 25
7.7 The ListView Control (cont.)
– Properties
CheckBoxes
CheckedIndices
CheckedItems
FullRowSelect
GridLines
Items
McGraw Hill/Irwin
MultiSelect
Scrollable
SelectedIndices
SelectedItems
Sorting
View
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 26
7.7 The ListView Control (cont.)
– Events
• Responds to both the DoubleClick and
SelectedIndexChanged events.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 27
Chapter Summary
• Loop structures are used to execute a group of
statements repeatedly.
• Two basic types of loops
1. Do…Loop
– Number of iterations unknown at the start.
2. For…Next Loop
– Number of iterations known at the beginning.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 28
Chapter Summary (cont.)
• Two variations of the Do…Loop.
– Do While Loop
• Condition evaluated at the beginning of loop.
– Do…Loop While
• Condition evaluated at the end of loop.
• Two other variations of the Do…Loop
– Do Until…Loop
– Do…Loop Until
– Identical to the While except the condition used to
control the loop is negated.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 29
Chapter Summary (cont.)
• Statements inside the loop must change the
values of the variables involved in the condition.
• The For…Next loop uses a counter variable to
control the number of iterations.
• Variations of the Do…Loop can be terminated
using the Exit Do statement.
• The body of the loop can contain another loop.
• The ListBox control presents the user with a list
of items that can be selected.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.
7- 30
Chapter Summary (cont.)
• The ComboBox control combines the features of
a ListBox and a TextBox.
• Both ListBox and ListView components provide
properties to access items selected by the user.
• A collection stores ListBox and ListView
properties.
• Use a For…Each structure to process a
collection.
• The ListView component provides a way to
display a list of icons and text in tabular form.
McGraw Hill/Irwin
©2002 by The McGraw-Hill Companies, Inc. All rights reserved.