09 Iterative Execution Mark Dixon Page 1 Questions
- Slides: 38
09 – Iterative Execution Mark Dixon Page 1
Questions: Dry Running • Dry run the following code: Dim a Dim b Dim c a = 12 b=a+5 c = b + 1. 25 Mark Dixon a - b - c - - - - 12 17 18. 25 Page 2
Questions: Variables • Write a line of code to declare a variable called h Dim h • Write a line of code that: 1) reads the value of the variable called h 2) adds 1, and 3) puts the result back into h h = h + 1 Mark Dixon Page 3
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: – identify and correct errors in For loops – create a For loop to repeat code a known number of times – identify and correct errors in Do loops – create a Do loop to repeat code an unknown number of times Mark Dixon Page 4
Dependencies: Numeric Variables • consider the following code: 1 Dim h 2 Dim q 3 h=5 4 q=h+2 • line 3 is dependent on line 1 (it involves h, it needs line 1) • line 4 is dependent on line 3 and line 2 Mark Dixon Page 5
Dependencies: Data Flow (Pipes) • think of connecting pipes (like plumbing in a house): Dim h Dim q h=5 q=h+2 Mark Dixon Page 6
Dependencies: String Variables • consider the following code: 1 Dim surname 2 Dim forename 3 Dim initials 4 surname = "Jones" 5 forename = "Alice" 6 initials = Left(surname, 1) & Left(forename, 1) • line 6 is dependent on lines 4 and 5 (it uses the values in the surname and forename variables) • line 5 is dependent on line 2 • line 4 is dependent on line 1 Mark Dixon Page 7
Question: Variable Dependencies • What dependencies exist in the following code? Dim q 1 Dim q 2 Dim u Dim o Dim g q 1 = "It is not enough to have a good mind. " q 2 = "The main thing is to use it well. " u = Len(q 1) o = Len(q 2) g=o+u Mark Dixon Page 8
Example: Hello v 0 • 1 user click: 1 Hello (1 line of code) <html> <head><title>Hello</title></head> <body> <input id="btn. Hello" type="button" value="Hello" /> <p id="par. Hello"></p> </body> </html> <script language="vbscript"> Option Explicit Sub btn. Hello_On. Click() par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " End Sub </script> Mark Dixon Page 9
Example: Hello v 1 • 1 user click: 10 Hellos (10 lines of code) Option Explicit Lots of lines imagine 300 Hellos Sub btn. Hello_On. Click() par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " End Sub Mark Dixon Page 10
Example: Hello v 2 • 1 user click: 10 Hellos (6 lines of code) Option Explicit Sub btn. Hello_On. Click() Dim h h=1 Do While h <= 10 par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " h=h+1 Loop End Sub Mark Dixon Page 11
Hello v 2: Do Loop • variable h – used as counter Mark Dixon Page 12
Example: Hello v 3 • 1 user click: 10 Hellos (4 lines of code) Option Explicit Sub btn. Hello_On. Click() Dim h For h = 1 To 10 par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " Next End Sub Mark Dixon Page 13
Hello v 3: For Loop • variable h – set and incremented automatically Mark Dixon Page 14
Advantages • Less code: Hello v 1 Option Explicit 10 lines Hello v 3 Option Explicit Sub btn. Go_On. Click() par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " Dim h par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " For h = 1 To 10 par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " Next par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " End Sub par. Hello. inner. HTML = par. Hello. inner. HTML & "Hello " End Sub 4 lines • This makes program: – Easier to read – Easier to change (imagine 500 Hellos) Mark Dixon Page 15
Do. . . Loop statement • repeat code unknown number of times – more flexible than For – slower than For • Syntax (Do While): Do While condition statementblock Loop • Syntax (Do Until): Do Until condition statementblock Loop Mark Dixon Page 16
For. . . Next statement • repeat code known number of times – reduces length of code – easier to change • Syntax: For variable = start To end statementblock Next Mark Dixon Page 17
Example: Do … Loop • Can do everything a For … Loop can: Dim i i = 1 Do While i <= 10 lbl. N. inner. Text = i i = i + 1 Loop Dim i For i = 1 To 10 lbl. N. inner. Text = i Next • And more: Dim i i = 1 Do While i < 10 lbl. N. inner. Text = i If (i / 2) = Int(i / 2) then i = i + 1 Else i = i + 3 End If Loop Mark Dixon Page 18
Example: Total • Real Power of loops – using counter variable – do something slightly different each time • Example: Mark Dixon Option Explicit Dim num Dim tot = 0 For num = 1 To 4 tot = tot + num Next lbl. Res. inner. Text = tot Page 19
Example: Total Mark Dixon Page 20
Example: Letter Count <script language="vbscript"> Option Explicit Sub btn. Count_On. Click() Dim pos Dim last Dim char Dim count last = Len(txt. Words. value) count = 0 For pos = 1 To last char = Mid(txt. Words. value, pos, 1) If char = "e" Then count = count + 1 End If Next lbl. Count. inner. Text = count End Sub </script> Mark Dixon Page 21
1) 2) 3) 4) 5) Mark Dixon Letter Count Start at first letter If no more letters then go to 5 If letter is an e then add 1 to count Go to 2 Display count Page 22
Question: For … Next • What does the following code display in par. Nums: Dim s Dim counter s = "" For counter = 1 To 10 s = s & " " & counter Next par. Nums. inner. Text = s 1 2 3 4 5 6 7 8 9 10 Mark Dixon Page 23
Example: Pendulum v 1 <html> <head><title>Pendulum</title></head> <body style="margin: 0; "> <img id="img. Mid" src="Dot. gif" style="position: absolute; " /> <img id="img. Pend" src="Pend. gif" style="position: absolute; " /> </body> </html> <script language="vbscript"> Option Explicit Dim ang Dim speed Sub window_onload() img. Mid. style. pixel. Left = document. body. client. Width / 2 img. Mid. style. pixel. Top = document. body. client. Height / 3 window. set. Interval "Swing()", 25 ang = 0 speed = 0. 04 End Sub Swing() ang = ang + speed If ang > 0. 5 Or ang < -0. 5 Then speed = -speed End If img. Pend. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 150 img. Pend. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 150 End Sub </script> Mark Dixon Page 24
Example: Pendulum v 2 <body style="margin: 0; "> <img id="img. Mid" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 1" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 2" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 3" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 4" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 5" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 6" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 7" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 8" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 9" src="Dot. gif" style="position: absolute; " /> <img id="img. Pend" src="Pend. gif" style="position: absolute; " /> </body> 56 lines of code … Mark Dixon Sub Swing() ang = ang + speed If ang > 0. 5 Or ang < -0. 5 Then speed = -speed End If img. Pend. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 150 img. Pend. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 150 img. Arm 1. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 15 img. Arm 1. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 15 img. Arm 2. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 30 img. Arm 2. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 30 img. Arm 3. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 45 img. Arm 3. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 45 img. Arm 4. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 60 img. Arm 4. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 60 img. Arm 5. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 75 img. Arm 5. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 75 img. Arm 6. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 90 img. Arm 6. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 90 img. Arm 7. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 105 img. Arm 7. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 105 img. Arm 8. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 120 img. Arm 8. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 120 img. Arm 9. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 135 img. Arm 9. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 135 End Sub </script> Page 25
Example: Pendulum v 3 <body style="margin: 0; "> <img id="img. Mid" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 1" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 2" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 3" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 4" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 5" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 6" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 7" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 8" src="Dot. gif" style="position: absolute; " /> <img id="img. Arm 9" src="Dot. gif" style="position: absolute; " /> <img id="img. Pend" src="Pend. gif" style="position: absolute; " /> </body> 45 lines of code … Sub Swing() Dim arm ang = ang + speed If ang > 0. 5 Or ang < -0. 5 Then speed = -speed End If img. Pend. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * 150 img. Pend. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * 150 For a = 1 To 9 Set arm = document. get. Element. By. Id("img. Arm" & a) arm. style. pixel. Left = img. Mid. style. pixel. Left + sin(ang) * (a * 15) arm. style. pixel. Top = img. Mid. style. pixel. Top + cos(ang) * (a * 15) Next End Sub </script> Mark Dixon Page 26
Example: Graph Option Explicit Sub window_onload() Const x. S = 10 ' Horizontal scaling factor. Const y. S = 10 ' Vertical scaling factor. Dim x. O ' Horizontal offset. Dim y. O ' Vertical offset. Dim x ' Horizontal position. Dim y ' Vertical position. Dim h ' html data for plotted points. x. O = 50 y. O = document. body. client. Height - 50 h = "" For x = 0 To 30 y=x h = h & "<img src='dot. gif' style='position: absolute; " h = h & "left: " & (x. O + (x * x. S)) & "; " h = h & "top: " & (y. O - (y * y. S)) & "; " h = h & "' />" Next document. body. inner. HTML = h End Sub Mark Dixon Page 27
Question: Do … Loop • What does the following display in par. Nums: Dim s Dim num s = "" num = 10 Do While num > -6 s = s & " " & num = num - 1. 5 Loop par. Nums. inner. Text = s 10 8. 5 7 5. 5 4 2. 5 1 -0. 5 -2 -3. 5 -5 Mark Dixon Page 28
Question: Do … Loop • What does the following display in par. Nums: Dim num = 6 Do Until num > 4 num = num + 5 par. Nums. inner. Text = par. Nums. inner. Text & num Loop nothing, 6 is already greater than 4 Mark Dixon Page 29
Loops: Errors <script language="vbscript"> Option Explicit Sub window_on. Load() For x = 1 To 10 Next End Sub </script> Mark Dixon Page 30
Loops: Errors <script language="vbscript"> Option Explicit Sub window_on. Load() Dim x For x = 1 To 10 End Sub Statement Expected </script> (missing Next) Mark Dixon Page 31
Loops: Errors <script language="vbscript"> Option Explicit Sub window_on. Load() Dim x Next End Sub </script> Mark Dixon Page 32
Loops: Errors <script language="vbscript"> Option Explicit Sub window_on. Load() Dim x For x = 1 To 10 Next End Sub </script> Mark Dixon Page 33
Tutorial Exercise: Hello • LEARNING OBJECTIVE: use variables to make a for loop more flexible • Task 1: Get the Hello Examples (0 to 2) 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. Mark Dixon Page 34
Tutorial Exercise: Letter Count • LEARNING OBJECTIVE: use a loop to search through a string (text) • 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 into. • Task 3: Modify your code so that it responds immediately. Hint: Remove the button, and link your code to the Key. Up event of the text box. • 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 len function. Mark Dixon Page 35
Tutorial Exercise: Vowel Count • LEARNING OBJECTIVE: build your own page from scratch, using a loop to search a string (piece of text) • 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. Mark Dixon Page 36
Tutorial Exercise: Pendulum • LEARNING OBJECTIVE: use a loop to shorten code responsible for visual display • Task 1: Get the Pendulum examples (1 to 3) working. • Task 2: Increase the number of dots for the arm. • Task 3: Modify your code so that the arm and pendulum are centred correctly. hint: deduct half the width of the image. Mark Dixon Page 37
Tutorial Exercise: Graphs • LEARNING OBJECTIVE: use functions and operators to change the behaviour of code that uses a loop • Task 1: Get the Graph example from the lecture working. • Task 2: Modify the page so that it plots the graph: y=x/2 • Task 3: Create a new page that plots the graph: y = Sin(x) Hint: change the loop to use values of x from 0 to 6. 2 in steps of 0. 1 (VBScript pocket reference p. 85) Mark Dixon Page 38
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