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08 – Iterative Execution. Session Aims & Objectives. Aims 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:
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Session Aims & Objectives • Aims • 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)
Example: Hello v0 • 1 user click: 1 Hello (1 line of code) <html> <head><title>Hello</title></head> <body> <input id=btnHello type=button value=Hello /> <p id=lblHello></p> </body> </html> <script language=vbscript> Option Explicit Sub btnHello_OnClick() lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" End Sub </script>
Example: Hello v1 • 1 user click: 10 Hellos (10 lines of code) Lots of lines imagine 300 Hellos <script language=vbscript> Option Explicit Sub btnHello_OnClick() lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" End Sub </script>
Example: Hello v2 • 1 user click: 10 Hellos (4 lines of code) <script language=vbscript> Option Explicit Sub btnHello_OnClick() Dim h For h = 1 to 10 lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" Next End Sub </script>
For ... Next statement • repeat code known number of times • reduces length of code • easier to change • Syntax:Forvariable=startToendstatementblockNext
Advantages • Less code: • This makes program: • Easier to read • Easier to change (imagine 500 Hellos) Hello v1 Hello v2 Option Explicit Sub btnGo_OnClick() lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" End Sub Option Explicit Sub btnGo_OnClick() Dim h For h = 1 To 10 lblHello.innerHTML = lblHello.innerHTML & "Hello<br>" Next End Sub 10lines 4 lines
Example: Total • Real Power of loops • using counter variable • do something slightly different each time • Example: Dim num Dim tot tot = 0 For num = 1 To 5 tot = tot + num Next lblRes.InnerText = tot
Question: For … Next • What does the following code produce: Dim counter For counter = 1 To 10 lblNums.InnerText = lblNums.InnerText & counter Next • What does the following code produce: Dim i For i = 24 To 8 Step -2 lblNums.InnerText = lblNums.InnerText & i & i * 2 Next
Example: Letter Count <script language=vbscript> Option Explicit Sub btnCount_OnClick() Dim pos Dim count Dim char count = 0 For pos = 1 To Len(txtWords.value) char = Mid(txtWords.value, pos, 1) If char = "e" Then count = count + 1 End If Next lblCount.innerText = count End Sub </script>
Example: Tones <script language="vbscript"> Option Explicit Sub btnShow_OnClick() Const stTag = "<span style=""background: #" Dim h, p, msg, red msg = txtMsg.value h = "" red = 255 For p = 1 To Len(msg) h = h + stTag + Hex(red) + "0000"">" h = h + Mid(msg, p, 1) h = h + "</span>" red = red - 5 Next divTones.innerHTML = h End Sub </script>
Do ... Loop statement • repeat code unknown number of times • more flexible than For • slower than For • Syntax:Do [{While|Until} condition] [statementblock] Loop
Example: Do … Loop • Can do everything a For … Loop can:Dim I Dim i i = 1 Do While i <= 10 For i = 1 To 10lblN.InnerText = ipicN.InnerText = i i = i + 1 Loop Next • And more:Dim i i = 1 Do While i < 10 lblN.innertext = i If (i / 2) = Int(i / 2) then i = i + 1 Else i = i + 3 End If Loop
Question: Do … Loop • What does the following produce: Dim num num = 20 Do While num > -12 lblDo.InnerText = lblDo.InnerText & num num = num - 1.5 Loop • What does the following produce: Dim num num = 6 Do Until num > 4 num = num + 5 lblDo.InnerText = lblDo.InnerText & num Loop
Tutorial Exercise: Hello • Task 1: Get the Hello Example (from the lecture) working. • Task 2: Modify your page so that it uses a variable to temporarily build to html. • Task 3: Modify your page so that the user can control how many 'Hellos' appear.
Tutorial Exercise: Letter Count • Task 1: Get the Letter Count Example (from the lecture) working. • Task 2: Modify your Letter Count page, so that the user can control which letter is counted.Hint: You will need a text box for the user to type a letter into. • Task 3: Modify your Letter Count program, so that the user cannot type more than one letter in the letter text box.Hint: Use the text box’s change event, and the len function.
Tutorial Exercise: Vowel Count • Task 1: Create a new page that counts the number of vowels (a, e, i, o, u) in a piece of text.Hint: similar to the letter count example.
Tutorial Exercise: Tones • Task 1: Get the tones example from the lecture working. • Task 2: Modify the page so that it puts a space in between each letter. • Task 3: Change the program so that it uses shades of another colour instead. • Task 4: Create a new page that selects random shades of your selected colour. Hint: use the Rnd function.
Tutorial Exercise: Chess Board • Task 1: Use one for loop inside another to create a chess board.
