Lecture Learning programming using Visual Basic Net – Chapter 7 Repeating processing tasks Loop structures

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.
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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.

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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.
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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.

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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.

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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

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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.

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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.
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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.

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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.

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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

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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.

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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.

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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

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7.4 Nested Loops (cont.)
• Nested For…Next Loops
For counter1 = start1 To end1
statementblock1a

For counter2 = start2 To end2
statementblock2

Next

statementblock1b
Next

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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.

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7.5 The ListBox and ComboBox Controls
(cont.)
– Properties

HorizontalScrollbar
Items
MultiColumn
SelectedIndex
SelectedIndices

SelectedItem

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SelectedItems
SelectionMode
Sorted
Text
UseTabStops

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7.5 The ListBox and ComboBox Controls
(cont.)
– Events

•
•
•
•
•

DoubleClick
KeyPress
Enter
Leave

SelectIndexChanged

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7.5 The ListBox and ComboBox Controls
(cont.)
– Methods

• Two common used

– FindString and FindStringExact
• Selected methods of the Items property

– Add(text)
– Insert(index, text)
– Remove(text)
– RemoveAt(index)
– Clear()
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7.5 The ListBox and ComboBox Controls
(cont.)
• 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.

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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.

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7.6 Collections and the For Each…Next
Structure (cont.)
– Syntax of the For Each…Next structure

For Each object In collection
statementblock
Next

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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.

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7.7 The ListView Control (cont.)

– Properties

CheckBoxes
CheckedIndices
CheckedItems
FullRowSelect
GridLines
Items

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MultiSelect
Scrollable
SelectedIndices
SelectedItems
Sorting
View

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