1 INF110 Visual Basic Programming AUBG Spring semester 2011 Reference books: Schneider D., An Introduction to Programming Using Visual Basic, Prentice Hall, Pearson Education Inc., 7th Ed. 2008, 6th Ed Liberty J., Learning Visual Basic.NET, O’Reilly, 2002 Any Visual Basic book available in AUBG library Course lecturer: Assoc. Prof. Svetla Boytcheva, PhD

2 INF110 Visual Basic Programming AUBG Spring semester 2011 Lecture 7 Title: Visual Basic (Repetition/Iteration statements 1)

3 Lecture Contents: §Repetition/Iteration statements: §Pre Test Loops §Post Test Loops §Logically Controlled Loops §Counter Controlled Loops §Nested Loops

4 Quiz 2 on Decision Operators and Expressions

5 Control Structures II Repetition (aka Iteration) INF110 Visual Basic Programming

6 An iterative statement is one which executes one or more other statements a certain number of times i.e. the execution of one or more statements is repeated The iterative statement comes in a variety of “flavours”. They are used for a number of purposes, such as: searching a collection of data items for one with a specific value, or performing a calculation involving a collection of data items (e.g. adding them all together to get a total) or performing some action repeatedly until an event occurs which tells it to stop (imagine a robot placing boxes on a conveyor belt...)

7 Repetition statements (also iterative statements) may classify in two sub groups: Do…Loop statements For…Next statements

8 Do Loops A loop is one of the most important structures in programming. Used to repeat a sequence of statements a number of times. The Do loop repeats a sequence of statements either: as long as, a certain condition is True. until, a certain condition becomes True. It has a few variants …

9 Comments Be careful to avoid infinite loops – loops that never end. VB.NET allows for the use of either the While keyword or the Until keyword at the top or the bottom of a loop. We will mostly use While at the top of a loop (pretest loop) and Until at the bottom of a loop (posttest loop). Semantics: “Do these actions while condition is true” or “Do these actions until a condition becomes true”

10 Variations of the Do…Loop control structure: Do … Loop 1/ Pre test loops with While and Until 2/ Post test loops with While and Until

11 Do Loops Do While boolean expression statements Loop Do Until boolean expression statements Loop Do statements Loop While boolean expression Do statements Loop Until boolean expression Do statements Loop

12 Do Loops in VBasic Pretest loops Do While boolean expression statements Loop Do Until boolean expression statements Loop

13 Do Loops in VBasic Posttest loops Do statements Loop While boolean expression Do statements Loop Until boolean expression

14 Do Loops in VBasic Infinite /endless/ loops Do statements Loop

15 Do While … Loop Statement

16 Do While … Loop Syntax Do While condition statement(s) Loop Condition is tested. If it is True, the loop is run. If it is False, the statements following the Loop statement are executed. These statements are inside the body of the loop and are run if the condition above is True. condition is a Boolean expression

17 Pseudocode and Flow Chart for a Do Loop

18 Dim Counter As Integer = 20 Do While Counter <= 100 Console.Writeline(Counter) Counter = Counter + 10 Loop The above example will keep on adding until Counter ’s value becomes > 100

19 Example Dim PassWord As String PassWord = “” Do While PassWord <> “SHAZAM” Console.Writeline(“Enter password:") PassWord = Console.Readline() Loop PassWord is the loop control variable because the value stored in PassWord is what is tested to determine if the loop should continue or stop.

20 ‘Accumulate sum of the numbers from 1 to 7 Dim Num As Integer = 1 Dim Sum As Integer = 0 Do While Num <= 7 Sum = Sum + Num Num = Num + 1 'Add 1 to the value of Num Loop

21 Do … Loop Until Statement

22 Do … Loop Until Do statement(s) Loop Until condition Loop is executed once and then the condition is tested. If it is False, the loop is run again. If it is True, the statements following the Loop statement are executed. Since the condition is tested at the bottom the loop, the loop will execute at least once.

23 Pseudocode and Flowchart for a Do … Loop Until

24 Dim Counter As Integer = 20 Do Console.Writeline(Counter) Counter = Counter + 10 Loop Until Counter > 100

25 Example Dim PassWord As String Do Console.Writeline(“Enter password:") PassWord = Console.Readline() Loop Until PassWord = "SHAZAM"

26 Nested Loops Statements inside a loop can contain another loop. (Just like If statements). Example: What is the output of this code? i = 0 Do While i < 6 j = 0 Do While j < 6 Console.Write("*") j = j + 1 Loop Console.WriteLine() i = i + 1 Loop Nested loops are quite common, especially for processing tables of data. Assume initially that i = 0

27 Nested Loops Statements inside a loop can contain another loop. (Just like If statements). Example: What is the output of this code? i = 0 Do While i < 6 j = 0 Do While j < i Console.Write("*") j = j + 1 Loop Console.WriteLine() i = i + 1 Loop Nested loops are quite common, especially for processing tables of data. Assume initially that i = 0

28 When condition is a variable of Boolean type, the statements like If flagVar = True Then... can be replaced by If flagVar Then... and the statements like If flagVar = False Then... can be replaced by If Not flagVar Then...

29 The statements like Do While flagVar = True can be replaced by Do While flagVar and The statements like Do While flagVar = False can be replaced by Do While Not flagVar

30 The iterative statement Do While condition statement(s) Loop Has an alternative while control structure: The While repetition structure

31 While Repetition Structure Repetition structure –Allows the programmer to specify that an action should be repeated, depending on the value of a condition. Example (pseudocode) While there are more items on my shopping list Purchase next item Cross it off my list Just like Do While... loop

32 Dim Product As Integer = 2 While Product <= 1000 Console.Writeline(Product) Product = Product * 2 End While The decision is tested each time the loop iterates product <= 1000 true product = product * 2 false

33 ‘ Modern VB 2005/2008 version Dim Product As Integer = 2 While Product <= 1000 Console.Writeline(Product) Product = Product * 2 End While VB6 >>> VB 2005/2008 The End While statement replaces the Wend statement of VB6. ‘ Obsolete VB 6 version Dim Product As Integer = 2 While Product <= 1000 Console.Writeline(Product) Product = Product * 2 WEnd

34 Nested Loops using While … End While control structure Statements inside a loop can contain another loop. Example: What is the output of this code? i=0 While i < 6 j = 0 While j < 6 Console.Write("*") j = j + 1 End While Console.WriteLine() i = i + 1 End While Nested loops are quite common, especially for processing tables of data. Assume initially that i = 0

35 Nested Loops using While … End While control structure Statements inside a loop can contain another loop. Example: What is the output of this code? i=0 While i < 6 j = 0 While j < i Console.Write("*") j = j + 1 End While Console.WriteLine() i = i + 1 End While Nested loops are quite common, especially for processing tables of data. Assume initially that i = 0

36 Do Loops All the variations use the same basic model. A pretest loop is executed either –While the condition is True. Uses While reserved word Do While condition statement-block Loop –Until the condition becomes True. Uses Until reserved word Do Until condition statement-block Loop

37 Examples I = 1 Do While (I <= 10) Console.WriteLine(I) I = I + 1 Loop How many iterations are to be run? (10) Try it

38 Examples I = 1 Do Until I <= 10 Console.WriteLine(I) I = I + 1 Loop How many iterations are to be run? (0) Try it

39 Examples to achieve the same effect I = 1 Do While (I <= 10) Console.WriteLine(I) I = I + 1 Loop ======================= same effect I = 1 Do Until Not(I <= 10) Console.WriteLine(I) I = I + 1 Loop How many iterations are to be run? (10) Try it

40 Examples to achieve the same effect I = 1 Do While Not(I <= 10) Console.WriteLine(I) I = I + 1 Loop ======================= same effect I = 1 Do Until (I <= 10) Console.WriteLine(I) I = I + 1 Loop How many iterations are to be run? (0) Try it

41 Post test loops with While and Until

42 Do Loops All the variations use the same basic model. A posttest loop is executed either –While the condition is True. Uses While reserved word Do statement-block Loop While condition –Until the condition becomes True. Uses Until reserved word Do statement-block Loop Until condition

43 Examples I = 1 Do Console.WriteLine(I) I = I + 1 Loop While (I <= 10) How many iterations are to be run? (10) Try it

44 Examples I = 1 Do Console.WriteLine(I) I = I + 1 Loop Until (I <= 10) How many iterations are to be run? (1) Try it

45 Examples to achieve the same effect I = 1 Do Console.WriteLine(I) I = I + 1 Loop While (I <= 10) =================== same effect I = 1 Do Console.WriteLine(I) I = I + 1 Loop Until Not(I <= 10) How many iterations are to be run? (10) Try it

46 Examples to achieve the same effect I = 1 Do Console.WriteLine(I) I = I + 1 Loop While Not(I <= 10) =================== same effect I = 1 Do Console.WriteLine(I) I = I + 1 Loop Until (I <= 10) How many iterations are to be run? (1) Try it

47 Endless loops with While … End While and Do … Loop

48 Endless loops - examples While True statement-block End While

49 Endless loops - comments The endless loops should be avoided. They must be terminated from within the own loop body with an Exit statement While True statement-block if condition Then Exit While statement-block End While

50 Endless loops - examples Do statement-block Loop

51 Endless loops - comments The endless loops should be avoided. They must be terminated from within the own loop body with an Exit statement Do statement-block If condition Then Exit Do statement-block Loop

52 From Logically controlled loop To Counter controlled loop Introduction to For … Next Loop statement

53 You use a Do loop when you do not know how many times the statements should be run. If you know that they should be run for a specific number of times, then use a For … Next loop. - coming on next slides! This loop variant uses a counter which increases or decreases on each repetition of the loop and the looping ends when this counter reaches a set value.

54 For … Next Loops Used when we know how many times we want the loop to execute. A counter-controlled loop

55 For … Next Loop Syntax The format is: For counter = startNumber To endNumber Step increment One or more VB statements Next starts with counter = startNumber and increments counter each time through the loop by an amount equal to increment until it reaches endNumber. The bounds ( startNumber, endNumber ) are integer quantities, and Step controls the increments (which may be negative).

56 For … Next Loop Syntax One more format is: For counter = startNumber To endNumber [Step increment] One or more VB statements Next [counter] The brackets [ … ] are meta characters and indicate that the keywords in the square brackets are optional.

57 For index = 0 To n Step s... Next Control variable Start value Stop value Amount to add to index

58 For … Next Loop Syntax The most common use of the FOR loop omits Step increment – the default increment is +1, i.e. the counter is increased by 1 each time through the loop.

59 Sample For i = 1 To 5... Next The loop control variable (i.e. counter), i, is –Initialized to 1 –Tested against the stop value, 5 –Incremented by 1 at the Next statement

60 For i = 1 To 5 [Step 1]... Next And its corresponding Do While equivalent i = 1 Do While i <= 5... i = i + 1 Loop

61 (a) For counter=1 to 10 Console.Writeline(counter) Next (b) For counter=1 to 100 Step 10 Console.Writeline(counter) Next (c) For counter=100 to 5 Step -5 Console.Writeline(counter) Next Examples

62 For Num = 1 To 10 NumSquared = Num*Num Console.Writeline(NumSquared) Next Num The statements between the " For Num = 1 To 10 " line and the " Next Num " line are executed ten times. The first time, the variable Num contains the value 1. The second time, Num contains the value 2, the third time, 3, and so on. On the tenth time around (the tenth "iteration") num contains the value 10. After that there are no further iterations - the computer moves on to the line following the " Next num " line.

63 Start, Stop, and Step values Consider a loop beginning with For i = m To n Step s The loop will be executed exactly once if m equals n no matter what value s has. The loop will not be executed at all if m is greater than n and s is positive. The loop will not be executed at all if m is less than n and s is negative.

64 Altering the Control Variable The value of the control variable should not be altered within the body of the loop Doing so might cause the loop to repeat indefinitely Or have an unpredictable number of repetitions. The value of the control variable should not be referenced after the Next statement

65 Altering the Control Variable For I = 0 to 10 Console.WriteLine(i) I = I – 1 Next I This is a loop that never ends. Why?

66 Altering the Stop Value The stop value may be specified as literal For I = 0 to 10 The stop value may be specified as constant Const MyConst As Integer = 10 For I = 0 to MyConst The stop value may be specified as variable Dim endValue As Integer endValue = 10 For I = 0 to endValue

67 Altering the Stop Value Attention: keep in mind when working with For…Next loops that the stop value is set at the beginning of the loop. Changing the value of the EnDVariable in the loop’s body won’t have any effect. For example, The following loop will execute 11 times, not 51 Dim endValue As Integer = 10 For I = 0 to endValue endValue = 50 Console.WriteLine(endValue) Next I

68 Non-integer Step Values Can lead to round-off errors with the result that the loop is not executed the intended number of times. The loop counter should be Integer.

69 Non-integer Step Example A loop beginning with For i = 1 To 2 Step 0.1 will be executed only 10 times instead of the intended 11 times. It should be replaced with For i = 1 To 2.01 Step 0.1

70 Nested For … Next Loops

71 Questions?

72 Thank You For Your Attention!