Repetition

1
Chapter 6

• Repetition

2
Outline Objectives

• Loop Structure
• Elements of a Loop Structure
• Processing Lists of Data with Do Loops

3
Types of LOOP Structures

• Do While . Loop
• Do Until Loop
• For Next loop

4
Basic Definition

• Looping the process of repeating a series of
  statements as many times as needed.
• Looping also called iteration.

5
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

6
The Do While . Loop

• Do While condition is true
• statement(s)
• Loop

7
Flowchart for a Do While Loop
Is the condition true
No
Yes
Execute statements within the loop
Execute statements that follow the loop
8
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 lt 10
• picNumbers.Print num
• num num 1
• Loop
• End Sub

9
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.

10
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.

11
Example of event-controlled loops

• passWord ""
• Do While passWord ltgt "SHAZAM"
• passWord UCase(InputBox("What is the
  password?"))
• Loop

12
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.

13
Example

• num 1
• Do While num lt 10
• picOutput.Print num
• num num 1
• Loop

14
Do Until . Loop

• Is executed until the condition becomes True
• Any Do While. Loop can be rewritten as a Do
  Until .. Loop

15
Example (requires the user to give a password
before opening a file)

• Private Sub cmdDisplay_Click()
• Dim passWord As String, info As String
• If UCase(txtName.Text) "SECRET.TXT" Then
• Do
• passWord UCase(InputBox("What is the
  password?"))
• Loop Until passWord "SHAZAM"
• End If
• Open txtName.Text For Input As 1
• Input 1, info
• picItem.Cls
• picItem.Print info
• Close 1
• End Sub

16
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 lt 0
• picResult.Print "It takes" yrs "years to
  deplete the account."
• End Sub

17
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

18
Processing List of Data with Loops

• EOF Function
• The EOF function tells us if we have read to the
  end of a file.
• Checks for the end-of-file marker.

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EOF Function

• EOF(n) where n is the reference number for the
  file.
• If there are more records to be read, EOF is
  False.
• When the end-of-file marker is reached EOF
  becomes True.

20
Example of EOF

• Open Phone.txt for Input As 1
• Do While Not EOF(1)
• Input 1, nom, phoneNum
• picOutput.Print nom, phoneNum
• Loop
• Close 1

21
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.

22
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

23
Example of a Data File

• 1, 1, 5, 10, 10, 25

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Output for the Previous Example

• The value of the 6 coins is 52 cents.

25
Boolean Variables and Flags

• A Boolean variable can hold one of two values
  True or False.
• Initial (default) value is False.
• Flags are Boolean variables
• - Used within a loop to provide information that
  will be used after the loop terminates
• - Provide an alternative for terminating the loop
• Dim myFlag as Boolean

26
Programming Assignment 6

• Page 281, Number 27

27
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.

28
Syntax of For Next Loop

• For loop-control-variable initial To terminal
• statement(s)
• Next loop-control-variable

29
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
• For i 1 To 5
• picTable.Print i i 2
• Next i
• End Sub

Loop Control variable
Terminating value
Initial Value
30
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.

32
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.

34
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.

35
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.

36
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

37
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

38
Example (the step value is negative)

• For Counter 8 To 1 Step -2
• picOutput.Print Counter
• Next Counter

39
Nested Loops

• For Outer 1 To 4
• For Inner 1 To 2
• ..
• ..
• Next Inner
• Next Outer

40
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

41
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.