11 – Parameters

Mark Dixon Page 1

11 – Parameters

Mark Dixon Page 2

Question: Arrays• How many array variables are in the

following code:

Dim xDim yDim f(4) x = 12 y = 6 f(2) = 21

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Question: Arrays• How many errors are in the following code:

Dim xDim yDim f(4) f = 3 y(3) = 6 f(1) = 19 f(5) = 21

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Session Aims & Objectives• Aims

– To introduce passing parameters/arguments to procedures to make them more general purpose

• Objectives, after this week’s sessions, you should be able to:

– use parameters in your programs to make procedures more flexible

– distinguish between formal and actual parameters

– pass parameters by reference and by value– identify and correct parameter errors

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Example: Clock (analysis)SPECIFICATION

• User Requirements – to know what the time is

• Software Requirements– Functional:

–display current time in analogue (traditional clock face) format

– Non-functionalshould be easy to read

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Example: Clock (user interface)• 14 objects:

– 12 numbers– mid point– second point

(moving)

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Example: Clock v0 (HTML)<html>

<body>

</body>

</html>

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Example: Clock v1 (HTML)• Remove 'hard coded' tags

<html>

<body>

</body>

</html>

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Example: Clock v1 (code)Option Explicit

Sub window_OnLoad() Dim ang Dim s Dim i s = "" For i = 1 To 12 s = s & "<div id=div" & i & " style=""position: absolute;"">" & i & "</div>" Next document.body.innerHTML = s & document.body.innerHTML imgMid.style.pixelleft = document.body.clientwidth / 2 imgMid.style.pixeltop = document.body.clientheight / 2 ang = 6.2 / 12 div1.style.pixelleft = imgMid.style.pixelleft + Sin(ang) * 200 div1.style.pixeltop = imgMid.style.pixeltop - Cos(ang) * 200 div2.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 2) * 200 div2.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 2) * 200 div3.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 3) * 200 div3.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 3) * 200 div4.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 4) * 200 div4.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 4) * 200 div5.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 5) * 200 div5.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 5) * 200 div6.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 6) * 200 div6.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 6) * 200 div7.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 7) * 200 div7.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 7) * 200 div8.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 8) * 200 div8.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 8) * 200 div9.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 9) * 200 div9.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 9) * 200 div10.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 10) * 200 div10.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 10) * 200 div11.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 11) * 200 div11.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 11) * 200 div12.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 12) * 200 div12.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 12) * 200 window.setinterval "ShowHands", 500 End Sub

Sub ShowHands() Dim ang Dim s ang = 6.2 / 60 s = Second(Now()) imgSec.style.pixelleft = imgMid.style.pixelleft + Sin(ang * s) * 200 imgSec.style.pixeltop = imgMid.style.pixeltop - Cos(ang * s) * 200 End Sub

Sub window_OnLoad() Dim ang Dim s Dim i s = "" For i = 1 To 12 s = s & "<div id=div" & i & " style='position: absolute;'>" & i & "</div>" Next document.body.innerHTML = s & document.body.innerHTML imgMid.style.pixelleft = document.body.clientwidth / 2 imgMid.style.pixeltop = document.body.clientheight / 2 ang = 6.2 / 12 div1.style.pixelleft = imgMid.style.pixelleft + Sin(ang) * 200 div1.style.pixeltop = imgMid.style.pixeltop - Cos(ang) * 200 … … … div12.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 12) * 200 div12.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 12) * 200 window.setInterval "ShowHands", 500 End Sub

• use loop to put in tags

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Example: Clock v1 (code)

current time and date

Option Explicit

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Sub window_OnLoad() Dim ang Dim s Dim i s = "" For i = 1 To 12 s = s & "<div id=div" & i & " style=""position: absolute;"">" & i & "</div>" Next document.body.innerHTML = s & document.body.innerHTML imgMid.style.pixelleft = document.body.clientwidth / 2 imgMid.style.pixeltop = document.body.clientheight / 2 ang = 6.2 / 12 div1.style.pixelleft = imgMid.style.pixelleft + Sin(ang) * 200 div1.style.pixeltop = imgMid.style.pixeltop - Cos(ang) * 200 div2.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 2) * 200 div2.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 2) * 200 … … … div10.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 10) * 200 div10.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 10) * 200 div11.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 11) * 200 div11.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 11) * 200 div12.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 12) * 200 div12.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 12) * 200 window.setinterval "ShowHands", 500 End Sub

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Example: Clock v1 (code)

Option Explicit

• same 2 lines duplicated– not identical

• different object• different angle

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Problem: Inflexible Procedure• Procedure does exactly the same each time

Sub ShowHands() Dim ang Dim s ang = 6.2 / 60 s = Second(Now()) Position End Sub

Sub Position() imgSec.style.pixelleft = imgMid.style.pixelleft + Sin(ang * s) * 200 imgSec.style.pixeltop = imgMid.style.pixeltop - Cos(ang * s) * 200End Sub

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Parameters/Arguments: What• Sometimes procedures need information

– Making a cup of tea:• milk, and number of sugars

• Makes them more flexible

Make cupof tea

Sugars

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Parameters: How• Procedure Declaration

– formal parameters define name

• Procedure Call– actual parameters list values in order

Option ExplicitDim res

Sub Calc(Num1, Num2) res = Num1 * Num2 End Sub

Sub btnCalc_Click() Calc 5, 2 Calc 11, 15 End Sub

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Parameters: How

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Error: Wrong number of arguments

Option Explicit

Sub Twice(num) parResult.innerText = num * 2 End Sub Twice 3 Twice 4, 3 Twice

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Questions: Parameters• Name the procedures in the following code

and state how many parameters they take:

Sub hello(s, x)Dim i s = "" For i = 1 To x s = s + "Hello" NextEnd Sub

Sub DisplayScore() parScore.innerText = scoreEnd Sub

hello takes 2 parameters

DisplayScore takes 0 parameters

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Questions: ParametersGiven the following declaration:

• What will the following put in parRes?

Sub thing(Num1, Num2, Num3)Dim tmpOutput tmpOutput = (Num1 + Num2) * Num3 parRes.innerText = tmpOutputEnd Sub

thing 2, 3, 6thing 6, 3, 2thing 20, 5, 2

301850

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Sub window_OnLoad()

Dim ang

s = ""

For i = 1 To 12

s = s & "<div id=div" & i & " style=""position: absolute;"">" & i & "</div>"

document.body.innerHTML = s & document.body.innerHTML

imgMid.style.pixelleft = document.body.clientwidth / 2

imgMid.style.pixeltop = document.body.clientheight / 2

ang = 6.2 / 12

Position div1, ang

Position div2, ang * 2

window.setinterval "ShowHands", 500

End Sub

Sub ShowHands()

Dim ang

ang = 6.2 / 60

s = Second(Now())

Position imgSec, ang * s

End Sub

Sub Position(objO, a)

objO.style.pixelleft = imgMid.style.pixelleft + Sin(a) * 200

objO.style.pixeltop = imgMid.style.pixeltop - Cos(a) * 200

End Sub

Sub Position(objO, a) objO.style.pixelleft = imgMid.style.pixelleft + Sin(a) * 200 objO.style.pixeltop = imgMid.style.pixeltop - Cos(a) * 200End Sub

Position imgSec, ang * s

Position div1, angPosition div2, ang * 2Position div3, ang * 3Position div4, ang * 4… … …Position div9, ang * 9Position div10, ang * 10Position div11, ang * 11Position div12, ang * 12

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Comparison of Clock Code

Sub window_OnLoad() Dim ang Dim s Dim i s = "" For i = 1 To 12 s = s & "<div id=div" & i & " style=""position: absolute;"">" & i & "</div>" Next document.body.innerHTML = s & document.body.innerHTML imgMid.style.pixelleft = document.body.clientwidth / 2 imgMid.style.pixeltop = document.body.clientheight / 2 ang = 6.2 / 12 Position div1, ang Position div2, ang * 2 Position div3, ang * 3 Position div4, ang * 4 Position div5, ang * 5 Position div6, ang * 6 Position div7, ang * 7 Position div8, ang * 8 Position div9, ang * 9 Position div10, ang * 10 Position div11, ang * 11 Position div12, ang * 12 window.setinterval "ShowHands", 500 End Sub

Sub ShowHands() Dim ang Dim s ang = 6.2 / 60 s = Second(Now()) Position imgSec, ang * s End Sub

Sub Position(objO, a) objO.style.pixelleft = imgMid.style.pixelleft + Sin(a) * 200 objO.style.pixeltop = imgMid.style.pixeltop - Cos(a) * 200 End Sub</script>

Sub ShowHands() Dim ang Dim s ang = 6.2 / 60 s = Second(Now()) imgSec.style.pixelleft = imgMid.style.pixelleft + Sin(ang * s) * 200 imgSec.style.pixeltop = imgMid.style.pixeltop - Cos(ang * s) * 200 End Sub</script>

Sub window_OnLoad() Dim ang imgMid.style.pixelleft = document.body.clientwidth / 2 imgMid.style.pixeltop = document.body.clientheight / 2 ang = 6.2 / 12 div1.style.pixelleft = imgMid.style.pixelleft + Sin(ang) * 200 div1.style.pixeltop = imgMid.style.pixeltop - Cos(ang) * 200 div2.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 2) * 200 div2.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 2) * 200 div3.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 3) * 200 div3.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 3) * 200 div4.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 4) * 200 div4.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 4) * 200 div5.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 5) * 200 div5.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 5) * 200 div6.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 6) * 200 div6.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 6) * 200 div7.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 7) * 200 div7.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 7) * 200 div8.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 8) * 200 div8.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 8) * 200 div9.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 9) * 200 div9.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 9) * 200 div10.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 10) * 200 div10.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 10) * 200 div11.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 11) * 200 div11.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 11) * 200 div12.style.pixelleft = imgMid.style.pixelleft + Sin(ang * 12) * 200 div12.style.pixeltop = imgMid.style.pixeltop - Cos(ang * 12) * 200 window.setinterval "ShowHands", 500 End Sub

Sub ShowHands() Dim ang Dim s ang = 6.2 / 60 s = Second(Now()) imgSec.style.pixelleft = imgMid.style.pixelleft + Sin(ang * s) * 200 imgSec.style.pixeltop = imgMid.style.pixeltop - Cos(ang * s) * 200 End Sub</script>

v0 (64 lines) v1 (59 lines) v2 (51 lines)

(2474 bytes) (2064 bytes) (941 bytes)

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Example: Clock v3• Use for loop instead of repeated calls

Position div1, angPosition div2, ang * 2Position div3, ang * 3Position div4, ang * 4Position div5, ang * 5Position div6, ang * 6Position div7, ang * 7Position div8, ang * 8Position div9, ang * 9Position div10, ang * 10Position div11, ang * 11Position div12, ang * 12

For i = 1 To 12 Position document.getElementById("div" & i), ang * iNext

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Example: Clock v4

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Memory Addresses• Memory addresses - keep track of where

variables are stored in memory• Every byte (position) in memory has a memory

address:

• e.g. variable identified by name xstored at location 63542(address of the first byte of data for variable x)

23xIdentifier

Computer Memory

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Example: Change the Value

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Parameter Passing Methods • 2 ways to pass parameters to procedures:

– Passing by Value: literal value is passed from the call to the definitionSub Change1(ByVal x) …End Sub

– Passing by Reference: variable’s memory address (a reference to the variables position in memory) is passed from the call to the definitionSub Change2(ByRef y) …End Sub

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Why pass by reference?• Previously, used parameters to pass data

into a procedure (input)

• Pass by reference, allows:– the procedure to change the value of the

passed variable– data to be input and output via parameters

ChangeByVal x

SetByRef x

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What can be passed• Pass by value – both literals and variables

can be passed (variables are substituted by their value)

Change1 y Change1 21

• Pass by reference – only variables can be passed (in fact the variable’s memory address is passed)

Change2 y literals cannot be passed – they have no memory address

Change2 21 ERROR

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Two results?• Total and Average algorithms share a lot of

• Useful to combine them

• TotAve procedure:

double (average)integer (total)TotAve

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Example: Total and Average Sub TotAve(ByRef T, ByRef A)

For i = 1 to 5

T = T + Nums(i)

A = T / 5

End Sub

tot = 0

ave = 0

TotAve tot, ave ' What is the value of ave and tot?

TotAvevar Tvar A

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Questions: Parameter Passing• Consider the following code: Dim a Sub Add(ByRef item, ByVal extra) item = item + extra End Sub a = 5

• What is the value of a, after each of these: Add a, 3 Add a, a Add 4, 4

Error – cannot pass 4 by reference

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Tutorial Exercise: Clock• LEARNING OBJECTIVE:

– use parameters provided by event procedures

• Task 1: Get the Clock (v2) example (from the lecture) working.

• Task 2: Modify your code – display lots of dots for the second hand (as per v3).

• Task 3: Modify your code – add an hour and minute hand

11 – Parameters

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