1. Prepared By: Deborah Becker
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Looping
4/25/2019
Prepared By: Deborah Becker

2. Prepared By: Deborah Becker
Topics of Discussion
Do Loops
For Next
Nesting Looping Structures
Visual Basic like other programming languages executes lines of code from the top of the program downwards. However, often a programmer does not wish all lines to be executed every time the program is run. Instead of writing several different programs for this job, branching and repeating commands can be used. Repeating Structures (or Loops)
Branching commands are used to determine when to execute a small section of code and when not to run it. One example is the ‘If’ command. If something is true then the code will be run, if not it will not be executed and VB will continue further down the program.
Repeating commands are useful for running small sections of code several times. If a programmer needs to read 200 lines of text from a file he could use 200 lines of code, one for every read statement, or alternatively a ‘For Next’ loop could do a similar job using just three or four lines!
4/25/2019
Prepared By: Deborah Becker

3. Prepared By: Deborah Becker
Looping Structures
Do...Loop Until
Do...Loop While
Do While...Loop
Do Until...Loop
For…Next
While…Wend
The easiest way to understand repetition is with the aid of a practical example. Consider the steps a Chip Delivery Driver must perform as he delivers his chips to all the stores on his route. There are four basic steps: 1) check number of bags of chips requested by the store, 2) fetch required bags of chips from his truck, 3) carry back stales from store, and 4) travel to next store. If the route has 20 stores then the salesman has to carry out 80 individual steps to deliver all the chips. A computer simulation of this could be written in Visual Basic using 80 lines of code, one line for each step. However, this is very inefficient. A much better way is to ‘wrap’ the four basic steps in some kind of repeating structure. Visual Basic supports the following types:
4/25/2019
Prepared By: Deborah Becker

4. Information Flow in a Do Loop
l l l l l l l Do
statement1
statement2
...
Loop
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Prepared By: Deborah Becker

5. Prepared By: Deborah Becker
Action of Do...Loop Until
Condition False
Condition True Do
statement1
statement2
. . . Do
statement1
statement2
. . .
Loop Until condition
Loop Until condition
statement
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Prepared By: Deborah Becker

6. Prepared By: Deborah Becker
The 'Do ... Loop Until
Dim intStoreNumber as Integer
‘Initialize variable
intStoreNumber = 1 Do
Count Number of Chips delivered
Count Number of Stales Removed
‘Increment variable intStoreNumber
intStoreNumber = intStoreNumber + 1
Loop until intStoreNumber > 20
This type of repeating structure will always be executed at least once because the loop exit statement is at the end. The Chip Delivery problem could be solved in the following way:
intStoreNumber = 1 Do
' Check the order
' Fetch correct order
' Carry back stales
intStoreNumber = intStoreNumber + 1
Loop Until intStoreNumber > 20
4/25/2019
Prepared By: Deborah Becker

7. Prepared By: Deborah Becker
?’s
When is the condition checked?
How many times will the…..
Do Loop Until execute?
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
4/25/2019
Prepared By: Deborah Becker

8. Prepared By: Deborah Becker
Action of Do...Loop While
Condition True
Condition False Do
statement1
statement2
. . . Do
statement1
statement2
. . .
Loop While condition
statement
Loop While condition
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Prepared By: Deborah Becker

9. Prepared By: Deborah Becker
?’s
When is the condition checked?
How many times will the…..
Do While Loop execute?
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Prepared By: Deborah Becker

10. Prepared By: Deborah Becker
So the Do…Loop While….
is very similar to the previous ‘Do...Loop Until’. The difference is in the logic of the exit condition. For example, the ‘Loop While’ will repeat a block of instructions while a condition remains true, but the ‘Loop Until’ loop will exit when the exit condition turns true.
A ‘Do...Loop While’ loop is very similar to the previous ‘Do...Loop Until’. The difference is in the logic of the exit condition. For example, the ‘Loop While’ will repeat a block of instructions while a condition remains true, but the ‘Loop Until’ loop will exit when the exit condition turns true.
4/25/2019
Prepared By: Deborah Becker

11. Action of Do While…Loop
Condition False
Condition True
Do While condition
statement1
statement2
. . .
Do While condition
statement1
statement2
. . .
Loop
statement
Loop
4/25/2019
Prepared By: Deborah Becker

12. Prepared By: Deborah Becker
Do While...Loop
Do While...Loop (Will this loop ever execute?):
Dim a as Integer
a = 1
Do While a > 10
a = a - 1
Loop
This loop is functionally equivalent to the ‘Do...Loop While’ except that the loop test condition is tested at the start of the loop. In practice this means that the ‘Do While...Loop’ might not be executed at all (if condition is false), whereas the ‘Do...Loop While’ will always be executed at least once.
4/25/2019
Prepared By: Deborah Becker

13. How Many Times Will This Do While…Loop Execute
Dim a as Byte
a = 20
Do While a > 10
a = a - 1
Loop
20-1 = = 18
18-1 = = 16
16-1 = = 14
14-1 = = 12
12-1 = = 10
10-1 = = 8
8-1 = = 6
6-1 = = 4
4-1 = = 2
2-1 = = 0
20, 19, 18, 17, 16, 15, 14, 13, 12, 11 = 10
4/25/2019
Prepared By: Deborah Becker

14. Prepared By: Deborah Becker
Action of Do Until Loop
Condition False
Condition True
Do Until condition
statement1
statement2
. . .
Do Until condition
statement1
statement2
. . .
Loop
Loop
statement
4/25/2019
Prepared By: Deborah Becker

15. Prepared By: Deborah Becker
Do Until…Loop
intStoreNumber = 1
Do Until intStoreNumber > 20
‘Check the Order
‘Fetch the Order
‘Carry Back the Stales
intStoreNumber = intStoreNumber + 1
Loop
This loop, like the previous ‘Do While…Loop’ type, tests the condition at the beginning of the loop. However, whereas the previous loop repeats while the condition is true, the ‘Do Until…Loop’ repeats until the condition is true.
4/25/2019
Prepared By: Deborah Becker

16. Equivalent Do While and Do Until loops
Private Sub DoWhileTest_Click()
Dim X As Integer
Do While X < 3
MsgBox X
X = X + 1
Loop
End Sub
Private Sub DoUntilTest_Click()
Dim X As Integer
Do Until X > = 3
MsgBox X
X = X + 1
Loop
End Sub
0, 1, 2
0, 1, 2
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Prepared By: Deborah Becker

17. Prepared By: Deborah Becker
The 'While ... Wend' CONTROL
Executes a series of statements as long as the condition is True.
Private Sub Form_Activate()
Dim strPassword As String
Const strSysPassW = help2003
While strPassword <> strSysPassW
strPassword = InputBox("Please enter _
your Password:", “Password", "")
Wend
End Sub
The ‘While...Wend’ loop is functionally equivalent to the ‘Do While...Loop’ discussed above. Once again this type of repeating structure should be used in situations where the loop will execute at least once. For example, a program which checks ban PIN numbers would always execute at least once:
Note: In reality an InputBox would not be used since the typed PIN can be seen on the screen and also the correct PIN would not be held in a constant but instead retrieved from an encrypted file, but as an example it demonstrates the potential of ‘While...Wend’ loops.
4/25/2019
Prepared By: Deborah Becker

18. Prepared By: Deborah Becker
The Endless Program
An infinite loop is a loop created where the condition will never be reached.
1.) Closing the Window or
2.) Pressing Ctrl-Break.
*Make sure that the terminating condition can be reached…………..
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Prepared By: Deborah Becker

19. Prepared By: Deborah Becker
Terminating the Loops
Do...Loop Until—repeats until the condition turns true
Do...Loop While—repeats while condition remains true
Do While...Loop—repeats while condition is true
Do Until...Loop—repeats until the condition is true
For...Next –used when the # of iterations is known
Terminates when the upper limit of the loop is reached
While...Wend—equivalent to Do While
Should be used in situations where the loop must execute at least once
4/25/2019
Prepared By: Deborah Becker

20. Prepared By: Deborah Becker
Nesting loops
Dim intCupsFilled , I as Integer
intCupsFilled = 0
Do ‘Post condition loop
' boil water
' add tea bags to pot
' fill pot with boiling water
For i = 1 to 5
' pour tea into cup
' move to next cup
intCupsFilled += 1
Next ‘(i is increased here)
Loop Until intCupsFilled = 100
In the Delivery Man example above the delivery man performs the basic 4 steps at each of the 20 stores he serves. However, consider the more complex problem of simulating the pouring of cups of tea at a Church meeting. The problem is more complex because the tea pot holds only 5 cups of tea and there are 100 to serve. To solve this problem two loops can be used, one inside another (nested):
Note: Different types of loops can be nested inside one another. In the above example the ‘For...Next’ loop is nested within a ‘Do...Loop Until’ structure.
4/25/2019
Prepared By: Deborah Becker