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Chapter 6 Repetition Chapter 6 - Visual Basic Schneider 1 Outline & Objectives Loop Structure Elements of a Loop Structure Processing Lists of Data with Do Loops Chapter 6 - Visual Basic Schneider 2 Types of LOOP Structures Do While ……. Loop Do Until …… Loop For …… Next loop Chapter 6 - Visual Basic Schneider 3 Basic Definition Looping: the process of repeating a series of statements multiple times until a criteria is met Chapter 6 - Visual Basic Schneider 4 Basic Components of Loops Loop control variable: A variable used to determine whether a loop will be executed Loop body: The statement (s) that are executed each time a loop repeats Chapter 6 - Visual Basic Schneider 5 The Do While ……. Loop Do While condition is true statement(s) Loop Chapter 6 - Visual Basic Schneider 6 Flowchart for a Do While Loop No Is the condition true Yes Execute statements within the loop Execute statements that follow the loop Chapter 6 - Visual Basic Schneider 7 Example (Displays the numbers from 1 through 10) Private Sub cmdDisplay_Click() Dim num As Integer ' Display the numbers from 1 to 10 num = 1 Do While num <= 10 picNumbers.Print num; num = num + 1 Loop End Sub Output: 1 2 3 4 5 6 7 8 9 10 Chapter 6 - Visual Basic Schneider 8 The Do While ……. Loop Is executed as long as the condition is True. If condition is False then the next statement after the Loop is executed. Chapter 6 - Visual Basic Schneider 9 Controlling Loops Methods of controlling loops: • Counter-controlled loops • repeat a specific number of times • Event-controlled loops • repeat until something happens in the loop body to change the value of loop control variable. Chapter 6 - Visual Basic Schneider 10 Example of event-controlled loops passWord = "" Do While passWord <> “ADMIN" passWord = UCase(InputBox("What is the password?")) Loop Chapter 6 - Visual Basic Schneider 11 Counter-controlled Loops Is useful when the programmer knows how many times the loop should be executed. Initialize the counter by setting it to a beginning value before entering the loop. The counter is incremented (or decremented) by the same value during each repetition. Chapter 6 - Visual Basic Schneider 12 Example for Counter-controlled loop num = 1 Do While num <= 10 picOutput.Print num; num = num + 1 Loop Output: 1 2 3 4 5 6 7 8 9 10 Chapter 6 - Visual Basic Schneider 13 Example Example Dim x As Integer Dim s As Integer Do While x >= 2 x = x + 1 s = s + x Loop picOutput.Print x; s Example Example Dim x As Integer Dim s As Integer Do x = x + 1 s = s + x Loop While x >= 2 picOutput.Print x; s Do Until ……. Loop Is executed until the condition becomes True Any Do While…. Loop can be rewritten as a Do Until ….. Loop Chapter 6 - Visual Basic Schneider 18 Example Dim x As Integer Dim s As Integer Do x = x + 1 s = s + x Loop Until x >= 2 picOutput.Print x; s Example Dim x As Integer Dim s As Integer Do x = x + 1 s = s + x Loop Until x <= 2 picOutput.Print x; s Example Dim x As Integer Dim s As Integer Do Until x >= 2 x = x + 1 s = s + x Loop picOutput.Print x; s Comparing While… and Until Loops The Do While … Loop executes while the condition is true The Do Until….. Loop executes until the condition is true Both can be used to create any type of loop Chapter 6 - Visual Basic Schneider 22 Converting While loop to Until loop Do While Condition Action(s) Equivalents to Loop Do While x <= y Action(s) Loop Equivalents to Do Until Not(Condition) Action(s) Loop Do Until x > y Action(s) Loop Do While x <=3 and x >=10 Do Until Not(x<=3 and x>=10) Action(s) Action(s) Loop Equivalents to Loop Review How many times will the following loops execute? num = 11 Do While num <= 10 picOutput.Print num; num = num + 1 Loop num = 11 Do picOutput.Print num; num = num + 1 Loop until num <= 10 0 Infinite loop Chapter 6 - Visual Basic Schneider 24 Review Which loop is infinite? i = 1 Do While i < 10 i = i + 1 Loop i = 0 Do i = i + 1 Loop While i < 10 NO NO i = 11 Do Until i < 10 Loop Yes i = 0 Do i = i + 10 Loop Until i < 10 Yes EOF Function EOF(n) is True if the end of the file having reference number n has been reached. Otherwise, it is False Chapter 6 - Visual Basic Schneider 26 Example Dim count As Integer Open “DATA.TXT” For Input As #1 Do While NOT EOF(1) Input #1, x count= count + 1 s=s+x Loop Print count;s;x will be true if the end of file has been reached, and false otherwise 5 150 50 27 Counters and Accumulators A counter is a numeric variable that keeps track of the number of items that have been processed in a loop. An accumulator is a numeric variable that holds a sub-total during multiple passes through a loop. Chapter 6 - Visual Basic Schneider 28 Example: Counter & Accumulator Private Sub cmdAnalyze_Click() Dim numCoins As Integer, sum As Single, value As Single COINS.TXT Open "COINS.TXT" For Input As #1 numCoins = 0 50 sum = 0 10 Do While Not EOF(1) 5 Input #1, value numCoins = numCoins + 1 25 sum = sum + value Loop picValue.Print "The value of the"; numCoins; "coins is"; sum; "cents." End Sub Chapter 6 - Visual Basic Schneider 29 Compare Do While ……. Loop Do ……. Loop While Do ……. Loop Until Do Until ……. Loop Chapter 6 - Visual Basic Schneider 30 For … Next Loop A loop where the number of iterations is determined by a range of values for a numeric variable Syntax: For controlVariable = initial To terminal statement(s) Next controlVariable Chapter 6 - Visual Basic Schneider 31 Example Private Sub cmdDisplayTable_Click() Dim i As Integer ‘Display a table of the first 5 numbers and their squares Control variable For i = 1 To 5 Terminating value picTable.Print i; i ^ 2 Initial Value Next i End Sub Chapter 6 - Visual Basic Schneider 32 Example Dim numVar As Integer For numVar = 1 To 5 Step 2 picOutput.Print numVar; Next numVar Output: 1 3 5 Chapter 6 - Visual Basic Schneider 33 When a For statement is encountered 1. 2. 3. The control variable is assigned the initial value. After each loop iteration, the step value is added to the value of the control variable. (If there is no step value, 1 is added.) Iteration continues until the terminating value is exceeded. Chapter 6 - Visual Basic Schneider 34 Rules for Using For ... Next loop You should never modify the value of the loop control variable in the loop body. Each For loop must end with a Next statement. Chapter 6 - Visual Basic Schneider 35 Example Private Sub cmdDisplay_Click() Dim i As Integer For i = 1 To 10 picOutput.Print "*"; Next i End Sub Output: ********** Chapter 6 - Visual Basic Schneider 36 Example Private Sub cmdDisplay_Click() Dim i As Integer, stars As Integer stars = Val(InputBox("Row length (1-20) :")) For i = 1 To stars picOutput.Print "*"; Next i End Sub Chapter 6 - Visual Basic Schneider 37 Example Dim numVar As Integer For numVar = 8 To 1 Step -2 picOutput.Print numVar; Next numVar Output: 8 6 4 2 Chapter 6 - Visual Basic Schneider 38 which of the following statements is true about the following code For i = 1 To 5.8 Step 1 pic1.Print i; Next I This code will print numbers from 1 to 6 This code will print numbers from 1 to 5 Run-time error This code will print numbers from 1 to 5.8 What is the output of the following code? Dim i As Integer For i = 9 To 7 Print i; Next i Nothing will be printed What is the output of the following code? Dim i as integer i=4 for i=3 To 5 print 2*i; Next i print i 6 8 10 6 Example Dim x as integer x=0 for x=3 To 1 print x; Next x print x 3 How many stars (*) are printed when the following code is executed? Dim i as Integer for i = 1 To 6 Step 1 print "*" i=i+2 next i 2 stars Private Sub Command1_Click() For x = 1 To 3 Step 0 Print x; Next Print " finally "; x End Sub Infinite loop Nested Loops For outer = 1 To 4 For inner = 1 To 2 .. .. Next inner Next outer Chapter 6 - Visual Basic Schneider 46 Example: Display a 3x3 rectangle of stars Private Sub cmdDisplay_Click() Dim i As Integer, j As Integer For i = 1 To 3 For j = 1 To 3 picOutput.Print "*"; Next j picOutput.Print Next i End Sub Chapter 6 - Visual Basic Schneider *** *** *** 47 Review For i= -5 To -1 Step - 2 257 picOutput.Print i; Next i For i= 1 To 5 Step picOutput.Print i; 2 i=i+1 -5 picOutput.Print i; Next i Chapter 6 - Visual Basic Schneider 48 picOutput.Print i; Review *** ** * * ** *** Private Sub cmdDisplay_Click() Dim i As Integer, j As Integer For i = 1 To 3 For j = i To 3 picOutput.Print "*"; Next j picOutput.Print Next i End Sub Chapter 6 - Visual Basic Private Sub cmdDisplay_Click() Dim i As Integer, j As Integer For i = 1 To 3 For j = 1 To i picOutput.Print "*"; Next j picOutput.Print Next i End Sub Schneider 49 How many times the statement print "Good Luck" will be executed Dim i as integer, j as integer for i=0 To 3 for j=0 To 0 print "Good Luck" Next j Next i 4