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06 – Iteration

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Session Aims & Objectives• Aims

– Revise Variables and Conditional execution– To introduce the main concepts involved in

getting the machine to perform repetitive tasks.

• Objectives,by end of this week’s sessions, you should be able to:

– To be able to implement code that does repetitive tasks, using looping structures:

• known limits (for loop)• unknown limits (do loop)

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Revision• Variables

• Conditions

Tax

Variables

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Example 1: Hello v1Option Explicit

Private Sub btnGo_Click() picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello" picHello.Print "Hello"End Sub

Hello v1

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For ... Next statement• repeat code known number of times

– reduces length of code

– easier to change

• Syntax:For counter = start To end [Step increment] [statementblock]Next

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• Advantages:– Less code (4 v 10)– Easier to read– Easier to change

Example 2: Hello v2Option Explicit

Private Sub btnGo_Click()Dim i As Long For i = 1 To 10 picHello.Print "Hello" NextEnd Sub

Hello v2

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Example 3: Total

• Real Power of loops– using counter variable

– do something slightly different each time

• Example: Dim num As LongDim tot As Longtot = 0For num = 1 To 5 tot = tot + numNext Total

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Exercise 1: For … Next• What does the following code produce:

Dim counter As Integer For counter = 1 To 10 picNums.Print counter Next

• What does the following code produce:Dim i As Integer For i = 24 To 8 Step -2 picNums.Print i, i * 2 Next

Loops

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Example 4: Circles v1

Option Explicit

Private Sub btnCicles_Click() picCircles.Circle (2000, 2000), 200 picCircles.Circle (2000, 2000), 400 picCircles.Circle (2000, 2000), 600 picCircles.Circle (2000, 2000), 800 picCircles.Circle (2000, 2000), 1000 picCircles.Circle (2000, 2000), 1200 picCircles.Circle (2000, 2000), 1400 picCircles.Circle (2000, 2000), 1600End Sub

Circles v1

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Example 5: Circles v2

Option Explicit

Private Sub btnCicles_Click()Dim r As Long For r = 200 To 1600 Step 200 picCircles.Circle (2000, 2000), r NextEnd Sub

Circles v2

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Example 6: Circles v3 (Animation)Private Sub btnCicles_Click()Dim r As LongDim p As Long picCircles.DrawMode = vbInvert For r = 200 To 1600 Step 200 picCircles.Circle (2000, 2000), r

For p = 1 To 5000000 Next

picCircles.Circle (2000, 2000), r Next picCircles.DrawMode = vbCopyPenEnd Sub

Circles v3

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Example 7: Letter CountOption Explicit

Private Sub btnCount_Click()Dim pos As LongDim count As LongDim char As String * 1 count = 0 For pos = 1 To Len(txtWords.Text) char = Mid$(txtWords.Text, pos, 1) If char = "e" Then count = count + 1 End If Next lblCount.Caption = countEnd Sub

Letter Count

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Example 8: Face v3Private Sub btnDraw_Click()Dim x As SingleDim p As Long picFace.FillStyle = vbSolid For x = 1000 To 6000 Step 50 picFace.FillColor = vbYellow picFace.Circle (x, 2400), 1000 picFace.Line (x, 2300)-Step(0, 400) picFace.FillColor = vbWhite picFace.Circle (x - 400, 2000), 250 picFace.Circle (x + 400, 2000), 250 picFace.FillColor = vbBlack picFace.Circle (x - 300, 2000), 40 picFace.Circle (x + 300, 2000), 40 picFace.Circle (x, 2500), 600, , 3.4, 6

For p = 1 To 500000 Next

picFace.Cls NextEnd Sub

Face v3

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Do ... Loop statement

• repeat code unknown number of times– more flexible than For

– slower than For

• Syntax:Do [{While|Until} condition] [statementblock]Loop

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Example 9: Do … Loop• Can do everything a For … Loop can:

Dim i As Long Dim i As Longi = 1Do While i <= 10 For i = 1 To 10 picNums.Print i picNums.Print i i = i + 1Loop Next

• And more:Dim i As Longi = 1Do While i < 10 picNums.Print I If (i / 2) = Int(i / 2) then i = i + 1 Else i = i + 3 End If Loop

Loops

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Exercise 2: Do … Loop• What does the following produce:

Dim num As Single num = 20 Do While num > -12 picDo.Print num num = num - 1.5 Loop

• What does the following produce:Dim num As Single num = 6 Do Until num > 4 num = num + 5 picDo.Print num Loop

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Example 10: Face v4Private Sub btnDraw_Click()Dim times As LongDim x As SingleDim p As LongDim inc As Long x = 1000 inc = 50 times = 6 Do Until times = 0 picFace.FillColor = vbYellow picFace.Circle (x, 2400), 1000 … … picFace.Circle (x, 2500), 600, , 3.4, 6

For p = 1 To 500000 Next

x = x + inc If x <= 1000 Or x >= 6000 Then inc = -inc times = times - 1 lblTimes.Caption = times lblTimes.Refresh End If

picFace.Cls LoopEnd Sub

Face v4