Repetition

1
Repetition
Chapter 7
2
Overview

• For Loop
• Do Loop
• Do While Loop
• Do Until Loop
• Do Loop While
• Do Loop Until
• Nested Loops

3
Repetition

• Computers are good at doing tasks humans find
  tedious
• Examples
• Determine if the letter 'a' appears in a string
• Find the sum of the numbers from 1 to 1000000
• Calculate pi to 1000 digits
• Calculate monthly loan payments

4
Types of Loops

• Using VB, we can tell a computer to repeat a
  specific set of instructions
• A predetermined number of times
• For loop
• While (or until) some conditional statement is
  true
• Do loop

5
For Loop

• A For Loop is a structure that is repeated a
  fixed number of times
• We usually select a For Loop when the loop starts
  at a specific value, increments by a set amount,
  and terminates at a specific value
• The syntax for the For Loop
• For LoopCounter InitialValue to
  TerminatingValue
• Program Statement(s)
• Next LoopCounter

6
For Loop Example (1)Default Increment Value of
1

• Private Sub CmdOutput_Click()

intCounter
intSum
Dim intCounter as Integer
0
?
/
/
Dim intSum as Integer
1
/
1
/
intSum 0
2
3
/
/
For intCounter 1 to 5
intSum intSum intCounter
3
6
/
/
Next intCounter
4
10
/
/
MsgBox intSum
15
5
End Sub
intSum 15 after the loop is executed
7
For Loop Example (2)Incrementing by Values
Other than One

• Private Sub CmdOutput_Click()
• Dim intCounter as Integer
• Dim intSum as Integer
• intSum 0
• For intCounter 1 to 5 Step 2
• intSum intSum intCounter
• Next intSum
• MsgBox intSum
• End Sub

intCounter
intSum
?
0
/
/
1
1
/
/
3
4
/
/
5
9
intSum 9 after the loop is executed
8
For Loop Example (3)Decrementing the Loop
Counter

• Private Sub CmdOutput_Click()
• Dim intCounter as Integer
• Dim intSum as Integer
• intSum 0
• For intCounter 5 to 1 Step -2
• intSum intSum intCounter
• Next intSum
• MsgBox intSum
• End Sub

intCounter
intSum
?
0
/
/
5
5
/
/
3
8
/
/
9
1
intSum 9 after the loop is executed
9
Do Loops

• Do While Loops are used when you are repeating a
  process that is not controlled by counting with a
  fixed-step amount, as in a For Loop
• Use this construct when your loop will continue
  while a certain condition evaluates to True
• There are four formats that you may use Do Loops
  with in VB

10
First Do Loop Construct

• This format is used when you wish to test if a
  condition evaluates to True, before you execute
  the program statements, and to continue to
  execute the statement while the condition
  evaluates to True.
• Do While (Condition)
• Program statement(s)
• Loop

11
Do While Loop

• intAnswer 0
• Do While (intAnswer ltgt vbYes)
• intAnswer Msgbox ("Continue?",_
• vbYesNo)
• Loop
• 'Pretest loop

12
Second Do Loop Construct

• This is used when you wish the loop to execute
  until a given condition evaluates to True
• Do Until (Condition)
• Program statement(s)
• Loop

13
Do Until Loop

• intAnswer 0
• Do Until (intAnswer vbYes)
• intAnswer Msgbox ("Continue?",_
• vbYesNo)
• Loop
• 'Pretest loop

14
Third Do Loop Construct

• Another form of the Do Loop is used when you wish
  to execute the program statements at least once
  and continue to execute the statement while the
  condition evaluates to True.
• Do
• Program statement(s)
• Loop While (Condition)

15
Do Loop While Loop

• Do
• intAnswer Msgbox ("Continue?",_
• vbYesNo)
• Loop While (intAnswer ltgt vbYes)
• 'Post-test loop

16
Fourth Do Loop Construct

• The final looping construct is used when you wish
  to execute the program statements at least once,
  and continue to execute them until the given
  condition evaluates to True.
• Do
• Program statement(s)
• Loop Until (Condition)

17
Do Loop Until Loop

• Do
• intAnswer Msgbox ("Continue?",_
• vbYesNo)
• Loop Until (intAnswer vbYes)
• 'Post-test loop

18
Nested Loops

• When a loop is contained within the body of
  another loop, it is said to be nested
• Like nesting dolls, hollow wooden dolls that fit
  one inside of the other
• The body of the inner loop is executed many times

19
Nested Loops

• intAcc 0
• For intI 1 to 20
• For intJ 1 to 5
• intAcc intAcc 1
• Next intJ
• Next intI

20
Nested Loops

• intAcc 0
• For intI 0 to 20 Step 4
• For intJ 100 to 5 Step -16
• intAcc intAcc 1
• Next intJ
• Next intI