Transcript Document 7898397

Chapter 6
Repetition
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Types of LOOP Structures
• Do While ……. Loop
• Do Until …… Loop
• For …… Next loop
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Basic Definition
• Looping : the process of repeating a series
of statements as many times as needed.
• Looping also called iteration.
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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
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The Do While ……. Loop
Do While condition is true
statement(s)
Loop
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Flowchart for a Do While Loop
No
Is the condition true
Yes
Execute statements
within
the loop
Execute statements
that follow the loop
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Example (Displays the numbers from 1 through 10)
Private Sub cmdDisplay_Click()
Dim intNum As Integer
' Display the numbers from 1 to 10
intNum = 1
Do While intNum <= 10
picNumbers.Print intNum;
intNum = intNum + 1
Loop
End Sub
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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.
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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.
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Example of event-controlled loops
strPassWord = ""
Do While strPassWord <> "SHAZAM"
strPassWord = UCase(InputBox("What is the password?"))
Loop
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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.
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Example
num = 1
Do While num <= 10
picOutput.Print num;
num = num + 1
Loop
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Do Until ……. Loop
• Is executed until the condition becomes True
• Any Do While…. Loop can be rewritten as a Do
Until ….. Loop
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Example (requires the user to give a password
before opening a file)
Private Sub cmdDisplay_Click()
Dim strPassWord As String, info As String
If UCase(txtName.Text) = "SECRET.TXT" Then
Do
strPassWord = UCase(InputBox("What is the password?"))
Loop Until strPassWord = "SHAZAM"
End If
Open txtName.Text For Input As #1
Input #1, info
picItem.Cls
picItem.Print info
Close #1
End Sub
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Example (years to deplete a saving account)
Private Sub cmdEstimate_Click()
Dim amt As Single, yrs As Integer
' Years to deplete savings account
picResult.Cls
amt = 15000
yrs = 0
Do
amt = amt * 1.05 - 1000
yrs = yrs + 1
Loop Until amt <= 0
picResult.Print "It takes"; yrs; "years to deplete the account."
End Sub
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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
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Counters and Accumulators
• A Counter is a numeric variable that keeps track
of the number of items that have been processed
in the loop.
• An Accumulator is a numeric variable that totals
numbers.
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Example: Counter & Accumulator
Private Sub cmdAnalyze_Click()
Dim numCoins As Integer, sum As Single, value As Single
Open "COINS.TXT" For Input As #1
numCoins = 0
sum = 0
Do While Not EOF(1)
Input #1, value
numCoins = numCoins + 1
sum = sum + value
Loop
picValue.Print "The value of the"; numCoins; "coins is"; sum; "cents."
Close #1
End Sub
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For ……... Next Loop
• Is used to create a counting loop.
• Loop control variable has an initial value.
• Loop control variable has a terminating
value.
• Loop control variable has a Step value.
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Syntax of For…… Next Loop
For loop-control-variable = initial To terminal
statement(s)
Next loop-control-variable
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Example ( display a table of the first 5 numbers and their
square)
Private Sub cmdDisplayTable_Click()
Dim i As Integer
‘Display a table of the first 5 numbers and their sqares
picTable.Cls
Loop Control variable
For i = 1 To 5
Terminating value
Initial Value
picTable.Print i; i ^ 2
Next i
End Sub
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Example ( step value of 2)
For counter = 1 To 5 Step 2
picOutput.Print counter
Next counter
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When the For statement is first
encountered
This explanation assumes a positive step value
• The initial, terminal, and (if given ) step
value expression are evaluated.
• The loop control variable is assigned to the
initial value.
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This explanation assumes a positive step
value
• The value of the loop control variable is tested
against the terminal value.
• If the loop control variable is less than or equal to
the terminal value, the loop body is executed.
• If the loop control variable is greater than the
terminal value, the loop body is skipped, and
control passes to the first statement following the
Next.
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When the Next statement is encountered
• The step value is added to the loop control
variable. If there is no step value, +1 is added.
• A check is performed to determine if the value of
the loop control variable exceeds the terminal
value.
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Continued
• If the loop control variable is less than or equal to
the terminal value, control transfers back to the
statement after the For statement and the loop is
repeated.
• Otherwise, the loop is exited, and execution
continues with the statement following the Next.
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Rules for Using For ... Next loop
• The initial, terminal, and step values
cannot be modified in the loop body.
• You should never modify the value of the
loop control variable in the loop body.
• Each For statement must end with a Next
statement.
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Example (display 10 stars)
Private Sub cmdDisplay_Click()
Dim i As Integer
' Display a row of ten stars
picOutput.Cls
For i = 1 To 10
picOutput.Print "*";
Next i
End Sub
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Example
(request a number and display a row of that many stars)
Private Sub cmdDisplay_Click()
Dim i As Integer, stars As Integer
' Display a row of stars
picOutput.Cls
stars = Val(InputBox("Row length (1-20) : "))
For i = 1 To stars
picOutput.Print "*";
Next i
End Sub
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Example (the step value is negative)
For Counter 8 To 1 Step -2
picOutput.Print Counter
Next Counter
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Nested Loops
For Outer = 1 To 4
For Inner = 1 To 2
..
..
Next Inner
Next Outer
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Example (display a 10 by 10 array of stars)
Private Sub cmdDisplay_Click()
Dim i As Integer, j As Integer
' Display 10 x 10 square of stars
For i = 1 To 10
For j = 1 To 10
picOutput.Print "*";
Next j
picOutput.Print
Next i
End Sub
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Guidelines for Choosing a Loop:
• If counting loop, use a For…… Next Loop.
• If trailer-values and body is executed at least
once, use Do Until….. Loop.
• If trailer-value and nothing is known about the
first execution, use Do While…. Loop.
• If you are not sure use Do While….. Loop.
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