1. Do … Loop Until (condition is true)
3.3 Iteration Loops
Do … Loop Until (condition is true)
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2. Learning Objectives
State when the Do … Loop Until (condition is true) should be used.
State the general form of the Do … Loop Until (condition is true).
State the minimum number of times the Do … Loop Until (condition is true) will be executed and why.
State when Do … Loop Until (condition is true) will be exited.
Use Boolean variables.
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3. The code above will produce:
Dim Number As Integer
For Number = 5 To 9
Console.WriteLine(Number*Number)
Next Number
The code above will produce: 25 36 49 64 81

4. Do … Loop Until (condition is true)
Is a “post-condition” loop.

5. Differences Between “For …. To …. Next” & “Do …. Loop …
Differences Between “For …. To …. Next” & “Do …. Loop …. Until” Iteration Loops

6. Manually Incrementing a Variable:
Number = Number + 1 Or
Number += 1
Both will increment a variable by one from the previous value.
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7. Console.WriteLine(Number*Number) Next Number To 9
Note that as “Do … Loop Until” loops have their end point at the end of the
loop (instead of at the beginning, as they are in “For …To … Next” loops) the end
point has to be raised to ensure that the loop ends the same way.
Dim Number As Integer
Number = 5 Do
Console.WriteLine(Number*Number)
Number = Number + 1
Loop Until Number = 10
Dim Number As Integer
For Number = 5
Console.WriteLine(Number*Number)
Next Number
To 9
Start
End
Number is
automatically incremented in a “For …To … Next” loop
but not in a “Do … Loop Until” loop.

8. Start / End Point Issue Note that the line to increment:
Number = Number + 1
Could be placed at the beginning of a “Do … Loop Until” loop. This would avoid the end point having to be raised but then the start condition would have to be lowered:
Dim Number As Integer
Number = 4 Do
Console.WriteLine(Number*Number)
Loop Until Number = 9

9. Disadvantages of “Do …. Loop …. Until” Iteration Loops
Complexity:
Start point needs to be manually set before the loop starts:
Number = 5 Do
Incrementing is automatic, so extra manual incrementing line needed:
Number = Number + 1
Start / End point issue:
Start point needs to be lowered or the End point needs to be raised.
Depending on whether you place the manual incrementing line above after the beginning or before the end of the loop.

10. Advantages of “Do …. Loop …. Until” Iteration Loops
More flexibility to incorporate strings (text) conditions in the start and end points of the loop.
Dim Number As Integer
Dim UserStop As String
Number = 5 Do
Console.WriteLine(Number*Number)
Console.WriteLine("Do you want to stop? Enter Yes or No.")
UserStop = Console.ReadLine
Number = Number + 1
Loop Until Number = 10 or UserStop = “Yes”

11. Strings (text) end points in “For …. To …. Next” iteration loops
The only way to do this in a “For …. To …. Next” loop is with an If condition inside the loop and exit the loop if the string end point is met.
Dim Number As Integer
Dim UserStop As String
For Number = 5 To 9
Console.WriteLine(Number*Number)
Console.WriteLine("Do you want to stop? Enter Yes or No.")
UserStop = Console.ReadLine
If UserStop = “Yes” Then
Exit For ‘Exit the loop if the user wants to stop.
End If
Next Number

12. Do … Loop Until (condition is true)
If you omitted the line:
Number = Number + 1
The loop would never end as Number would never equal 10.
This is called an infinite loop.
If you find your program in an infinite loop then you will have to switch to VB and click the Debug menu and click the Stop Debugging option.
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13. General Form of a Do … Loop Until (condition is true)
Loop body statements
Loop Until (condition is true)
Since the condition is set at the end of the loop the loop body must be executed at least once.
The loop will be exited when the condition becomes true.
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14. Program 3.3a Password Entry
Specification:
Allow the user up to three attempts at entering a password.
Inform the user which attempt they are currently on.
Inform the user that the password is correct.
Otherwise inform them that their password is invalid and then asks them if they wish to continue.
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15. Program 3.3a Password Entry
Create a new project named ‘Password Entry’.
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16. Program 3.3a Password Entry
‘Declare variables.
Dim Password As String = "Secret“ ‘Set Password.
Dim PasswordAttempt As String
Dim Attempt As Integer
Dim Cancel As Boolean = False
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Continued on next slide.

17. Program 3.3a Password Entry
Do ‘Start of loop.
Attempt = Attempt + 1 ‘Increase Attempt by 1.
‘Ask user for password.
Console.WriteLine("Enter Password. " & “Attempt: " & Attempt)
PasswordAttempt = Console.ReadLine
If PasswordAttempt = Password Then ‘Is password valid?
Console.WriteLine("This password is valid.") ‘Password valid.
Else ‘Password invalid.
Console.WriteLine("This password is invalid.") ‘Inform user of invalid password.
Console.WriteLine(“Do you wish to stop/cancel? (True/False)”)
Cancel = Console.ReadLine
End If ‘End of If statement.
‘Exit if password is valid or if user has had 3 attempts.
Loop Until PasswordAttempt = Password Or Attempt = 3 Or Cancel = True
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Continued on next slide.

18. Program 3.3a Password Entry
‘Has the user had 3 attempts with an invalid password?
If Attempt = 3 And PasswordAttempt <> Password Then
‘If so inform the user that they have had 3 attempts.
Console.WriteLine("You have had 3 attempts.Your account is blocked")
End If ‘End of If statement.
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19. Program 3.3a Password Entry
Run the program and test it.
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20. Infinite Loops If we removed the line
Attempt = Attempt + 1
If the user does not enter the correct password then the loop would never end as the condition Attempt = 3 would never come true.
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21. Given pseudocode will use the following structure:
REPEAT
<statement(s)>
UNTIL <condition>

22. “Commenting Out”
When you are asked change previous code from previous programs it is helpful and even necessary for to keep the original code to be changed visible, without VB executing it.
This can be done by “commenting out” the original code so that VB treats the lines as comments (ignores them) but you as a human can still see and read them.

23. Extension 3.3b
Change any of the “For …. To …. Next” loop programs you wrote in 3.1 Loops or 3.2 Loops to use a:
Do … Loop Until (condition is true)
instead.
With the incrementing line at the end of the loop, immediately before Loop Until …. e.g. Do …
Index = Index + 1
Loop Until Index = …..
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Do … Loop Until version).

24. Extension 3.3c
Change any of the “For …. To …. Next” loop programs you wrote in 3.1 Loops or 3.2 Loops to use a:
Do … Loop Until (condition is true)
instead.
With the incrementing line at the beginning of the loop, immediately after Do e.g. Do
Index = Index + 1 …
Loop Until Index = …..
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Do … Loop Until version).

25. Extension 3.3d Change this Program 3.3 Password Entry program to use a
“For …. To …. Next” Loop
Hints:
The “Do Loop …. Until” Loop’s end condition of
PasswordAttempt = Password cannot be used as part of the “For …. To …. Next” Loop’s end condition
Only the end condition Attempt = 3.
As “For …. To …. Next” Loops count and cannot work with strings.
So you will need Exit For to get out of the loop after the password has been entered correctly, this means after the line:
MsgBox("This password is valid.")
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add
(For …. To …. Next Version).
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26. Plenary When should the Do … Loop Until (condition is true) be used?
Used if you do not know exactly how many times the loop should be repeated.
What is the general form of the Do … Loop Until (condition is true)? Do
Loop body statements
Loop Until (condition is true)
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27. Plenary
What is the minimum number of times the Do … Loop Until (condition is true) will be executed and why?
Once, since the condition is set at the end of the loop
When will Do … Loop Until (condition is true) be exited?
The loop will be exited when the condition becomes true.
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