CS 4 Intro to Programming using Visual Basic

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CS 4 Intro to Programming using Visual Basic

• For Loops
• Patchrawat UthaisombutUniversity of Pittsburgh

based on lecture notes by D. Schneider
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Chapter 6 Repetition

• 6.1 Do Loops
• 6.2 Processing Lists of Data with Do
• Loops
• 6.3 For...Next Loops
• 6.4 A Case Study Analyze a Loan

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6.3 ForNext Loops

• Declarations of Control Variables
• Nested For Next Loops

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ForNext Loops

• Used when we know how many times we want the loop
  to execute
• A counter controlled loop

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Sample

• Dim i As Integer
• For i 1 To 5
• lstTable.Items.Add(i " " i 2)
• Next
• The loop control variable, i, is
• Initialized to 1
• Tested against the stop value, 5
• Incremented by 1 at the Next statement

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Do While equivalent

• i 1
• Do While i lt 5
• lstTable.Items.Add(i " " i 2)
• i 1
• Loop

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ForNext Loop Syntax
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Set control variableto initial value
Is control variable gtterminating value?
Yes
No
ExecuteStatementsin loop body
Incrementcontrol variable
Executestatementsfollowing the loop
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Example 1 Output
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Example 1 Code

• Dim yr As Integer
• Dim pop as Double 300000
• Dim fmtStr As String _
• "0,41,12N0"
• For yr 2006 To 2010 lstPop.Items.Add(String.Form
  at( _
• fmtStr, yr, pop)
• pop 0.03 pop
• Next

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Example 2
Start value
Control variable
Stop value
Amount to add to index

• For index 0 To n Step s
• lstValues.Items.Add(index)
• Next

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Example with Negative Step

• Dim j As Integer
• For j 10 To 1 Step -1
• lstBox.Items.Add(j)
• Next
• lstBox.Items.Add("Blastoff")

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Declaration Inside For Statement

• Dim i As Integer
• For i 1 To 5
• lstTable.Items.Add(i " " i 2)
• Next
• For i As Integer 1 To 5
• lstTable.Items.Add(i " " i 2)
• Next

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Playing with For Loop
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Playing with For Loop

• Private Sub btnDisplay_Click() Handles
  btnDisplay.Click
• Dim n, s As Double
• Dim index As Double
•  
• n CDbl(txtN.Text)
• s CDbl(txtStep.Text)
•  
• lstDisplay.Items.Clear()
•  
• For index 0 To n Step s
• lstDisplay.Items.Add(index)
• Next
• End Sub

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Nested ForNext Loops
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Example Seats on an Airplane

• For i As Integer 65 To 70
• For j As Integer 1 To 25
• lstBox.Items.Add(Chr(i) j)
• Next
• Next
• OUTPUT A1
• A2
• A3

Outer loop
Inner loop
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Multiplication Table
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Multiplication Table

• Private Sub btnDisplay_Click() Handles
  btnDisplay.Click
• Dim row, entry As String
•  
• lstTable.Items.Clear()
• For j As Integer 1 To 5
• row ""
• For k As Integer 1 To 3
• entry j " x " k " " (j
  k)
• row entry " "
• Next
• lstTable.Items.Add(row)
• Next
• End Sub

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For and Next Pairs

• For and Next statements must be paired.
• If one is missing, the automatic syntax checker
  will complain with a wavy underline and a message
  such as
• A For must be paired with a Next.

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Start, Stop, and Step values

• Consider a loop beginning with
• For i As Integer 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,
• or if m is less than n and s is negative.

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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.

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Non-integer Step Values

• Can lead to round-off errors with the result that
  the loop is not executed the intended number of
  times.

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Non-integer Step Example

• A loop beginning with
• For i As Double 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 As Double _
• 1 To 2.01 Step 0.1