Matakuliah
Tahun
: T0063 – Pemrograman Visual
: 2009
Repeating Processing Tasks
Loop Structures
CHAPTER SEVEN
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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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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7.1 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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7.1 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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7.1 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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7.2 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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7.3 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
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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.)
• 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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