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17 – Modular Design in ASP

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Questions: Session variables• Write a line of VB code to put 74 into a

session variable called score.

• Write VB code that adds 1 to a variable called g, when a session variable called i is over 25.

If Session("i") > 25 Then

g = g + 1

End If

Session("a") = 74

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Session Aims & Objectives• Aims

– Highlight modular design techniques, and demonstrate them in ASP

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

– Identify dependencies between lines of code– Determine whether a routine is self-contained– Use procedures, functions, parameters, and

modules (shared VB script files) in ASP

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Example: Apples (analysis)SPECIFICATION

• User Requirements – help young children learn to count from 1 to 10

• Software Requirements– Functional:

–computer selects number between 1 and 10–computer displays that number of apples–user types digits–computer compares digits to number of apples

– Non-functionalshould be easy to use and interesting

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Example: Apples v2 (design)• Functionality:

• computer selects number between 1 and 10• computer displays that number of apples• user types digits• computer compares digits to number of apples

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Example: Apples v2 (code)Dim n As Long

Sub Page_Load() Dim html As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next quest.InnerHtml = html Session("NumApples") = n msg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") If CInt(txtAns.Value) = CInt(n) Then msg.InnerHtml = "Correct, well done!" Else msg.InnerHtml = "Sorry, please try again." End If btnStart.Disabled = False btnCheck.Disabled = True End If End Sub

Apples.aspxDim n As Long

Sub Page_Load() Dim html As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next quest.InnerHtml = html Session("NumApples") = n msg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False

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Example: Apples v2 (code)Dim n As Long

Sub Page_Load() Dim html As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next quest.InnerHtml = html Session("NumApples") = n msg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") If CInt(txtAns.Value) = CInt(n) Then msg.InnerHtml = "Correct, well done!" Else msg.InnerHtml = "Sorry, please try again." End If btnStart.Disabled = False btnCheck.Disabled = True End If End Sub

Apples.aspx

ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") If CInt(txtAns.Value) = CInt(n) Then msg.InnerHtml = "Correct, well done!" Else msg.InnerHtml = "Sorry, please try again." End If btnStart.Disabled = False btnCheck.Disabled = True End If End Sub

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Problem Solving Strategies• bottom-up

– Create a detailed solution first– Then look for best solution– refactoring – process of:

• changing internal design of code,• without altering what it does

• top-down– plan overall design– fill in details

in practice mixed – novices favour bot-up, experts top-down

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• Useful to understand code:

Dim a As Long

Dim b As Long

Dim c As Double

a = 12

b = a + 5

c = b + 1.25

Dry Running

a b c

- - -

- - -

- - -

12 - -

12 17 -

12 17 18.25

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Dependencies: Numeric Variables

• consider the following code:1 Dim h As Long2 Dim q As Long3 h = 54 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

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Dependencies: String Variables• consider the following code:

1 Dim surname As String2 Dim forename As String3 Dim initials As String4 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

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Question: Variable Dependencies• What dependencies exist in the following code?

Dim q1 As String

Dim q2 As String

Dim u As Long

Dim o As Long

Dim g As Long

q1 = "It is not enough to have a good mind."

q2 = "The main thing is to use it well."

u = Len(q1)

o = Len(q2)

g = u + o

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Example: Apples (Dependencies)• Difficult to see

dependencies for lines far apart

Dim n As Long

Sub Page_Load() Dim html As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next quest.InnerHtml = html Session("NumApples") = n msg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False

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Example: Apples (Dependencies)• Put dependent

lines close together

Dim n As Long

Sub Page_Load() Dim html As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) Session("NumApples") = n html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next quest.InnerHtml = html msg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False

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Example: Apples v3 (design)• Functionality:

• computer selects number between 1 and 10• computer displays that number of apples• user types digits

• computer compares digits to number of apples

and displays number of apples typed by user

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Example: Apples v3 (code)Dim n As Long

Sub Page_Load() Dim html As String Dim msg As String Dim a As Long If Request.Form("btnStart") > "" Then n = 1 + Int(Rnd() * 9) Session("NumApples") = n html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next parQuest.InnerHtml = html parMsg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") msg = "" For a = 1 To txtAns.Value msg = msg & "<img src=Apple.gif>" Next If CInt(txtAns.Value) = CInt(n) Then msg = msg & "Correct, well done!" Else msg = msg & "Sorry, please try again." End If parMsg.InnerHtml = msg btnStart.Disabled = False btnCheck.Disabled = True End If End Sub

Apples.aspx

ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") msg = "" For a = 1 To txtAns.Value msg = msg & "<img src=Apple.gif>" Next If CInt(txtAns.Value) = CInt(n) Then msg = msg & "Correct, well done!" Else msg = msg & "Sorry, please try again." End If parMsg.InnerHtml = msg btnStart.Disabled = False btnCheck.Disabled = True End If End Sub

• copy + paste

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Modular Design• What do lines do (group summary)?

n = 1 + Int(Rnd() * 9) Session("NumApples") = n html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next parQuest.InnerHtml = html parMsg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False

Pick Num. of Apples

Display Question

Prepare for Response

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Modular Design (top level)• Top level reads like English algorithm:

Dim n As LongDim html As StringDim msg As StringDim a As Long

Sub Page_Load() If Request.Form("btnStart") > "" Then PickRandomNumberOfApples DisplayApplesQuest PrepareForResponse ElseIf Request.Form("btnCheck") > "" Then n = Session("NumApples") DisplayApplesUser DisplayFeedback PrepareForQuest End If End Sub

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Modular Design (detail)• Procedures contain (hide) detail:

Sub PickRandomNumberOfApples() n = 1 + Int(Rnd() * 9) Session("NumApples") = n End Sub Sub DisplayApplesQuest() Dim html As String html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next parQuest.InnerHtml = html End Sub Sub DisplayApplesUser() msg = "" For a = 1 To txtAns.Value msg = msg & "<img src=Apple.gif>" Next End Sub Sub DisplayFeedback() If CInt(txtAns.Value) = CInt(n) Then msg = msg & "Correct, well done!" Else msg = msg & "Sorry, please try again." End If parMsg.InnerHtml = msg End Sub Sub PrepareForResponse() parMsg.InnerHtml = "" txtAns.Value = "" btnStart.Disabled = True btnCheck.Disabled = False End Sub Sub PrepareForQuest() btnStart.Disabled = False btnCheck.Disabled = True End Sub

Sub DisplayApplesQuest() html = "How many apples are there?<br />" For a = 1 To n html = html & "<img src=Apple.gif>" Next parQuest.InnerHtml = html End Sub Sub DisplayApplesUser() msg = "" For a = 1 To txtAns.Value msg = msg & "<img src=Apple.gif>" Next End Sub

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Routines: Self-Contained• Good design principle:

– routines (functions and procedures)should be self-contained(easier to re-use in other programs)

Dim u As Long u = Twice(4)

Function Twice(a As Long) Return a * 2End Function

Dim u As LongDim a As Long a = 4 u = Twice()

Function Twice() Return a * 2End Function

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Question: Self-Contained Routines

• Are the following routines self contained?Dim num1

Dim num2

Dim diff

Sub Compare()

diff = num1 - num2

End Sub

Function Half(num As Double) As Double

Return num / 2

End Function

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Debugging• key skill:

– locate the cause of a bug• using testing methods

• first step– narrow it down as much as possible

• typical pattern in early tutorials:– student: it doesn't work– lecturer: what doesn't work– student: my code– lecturer: yes, but which bit exactly– student: ????– lecturer: run your program, take me through it, which bits work, and

where exactly does it go wrong– student: when I click this, nothing happens– lecturer: which bit of code is responsible for doing that?– student: this bit

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Problem Solving: 9 dots• Join all 9 dots

– with straight continuous lines

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Problem Solving Process (Name Split)

• Problem: a variable exists called n. This contains a person's full name (forename, then surname ). It needs to be split into two separate variables.

Dim n As String

n = "Ruth Jones"

• Solution Process:– What do I do to solve this manually (on paper)?– How do I know where the forename ends and the

surname begins?– The space is the key:

• Find the space• everything before the space is the forename• everything after the space is the surname

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Tutorial Exercise: Apples• LEARNING OBJECTIVE:

identify dependencies between lines of coderefactor code: dependent lines closerrefactor code: split into routines (procedures and functions)refactor code: make routines self-contained

• Task 1: Get the Apples v3 example (from the lecture) working• Task 2: Modify your page to keep a score.

HINT: Try to identify the routines first, then fill in the code.