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12 – Enumerated Data-Types & Pass-by-reference

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Assignment• Object naming conventions:

txt – text box btn – command button pic – picture box

• Appropriate use of:– indentation– remarks– constants (no magic numbers)– variables (module level, and local)– procedures/functions– arrays– structures– enumerate data types

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Project Files• Ensure that you submit:

– project file (*.vbp)– all form files (*.frm, and *.frx)– all module files (*.bas)

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Session Aims & Objectives• Aims

– To introduce the idea of enumerated data types– To introduce the idea of passing by reference

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

– declare and use an enumerated data type– pass parameters by reference

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Enumerated Data Types• Often need to use numbers to represent

things (coding)

• For example, curry: mild, medium, or hot• Could store text: "mild", "medium", "hot"

– takes lots of space (1 byte per character)– easily becomes inconsistent, e.g. "hit"

• Alternatively, use numbers to represent text:1 "mild"2 "medium"3 "hot"

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Example 1: Curry v1Option Explicit

Private Sub Form_Load() lstCurry.AddItem "Mild", 0 lstCurry.AddItem "Medium", 1 lstCurry.AddItem "Hot", 2 picCurry.FillStyle = vbSolidEnd Sub

Private Sub lstCurry_Click() lblCurryCode.Caption = lstCurry.ListIndex lblCurryText.Caption = lstCurry.List(lstCurry.ListIndex) If lstCurry.ListIndex = 0 Then picCurry.FillColor = vbWhite ElseIf lstCurry.ListIndex = 1 Then picCurry.FillColor = vbYellow Else picCurry.FillColor = vbRed End If picCurry.Cls picCurry.Circle (1000, 750), 500End Sub

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Example 2: Curry v2Option Explicit

Private Sub Form_Load() lstCurry.AddItem "Mild", 0 lstCurry.AddItem "Medium", 1 lstCurry.AddItem "Hot", 2 picCurry.FillStyle = vbSolidEnd Sub

Private Sub lstCurry_Click()Dim CuCo As Long ' Curry code CuCo = lstCurry.ListIndex lblCurryCode.Caption = CuCo lblCurryText.Caption = lstCurry.List(CuCo) If CuCo = 0 Then picCurry.FillColor = vbWhite ElseIf CuCo = 1 Then picCurry.FillColor = vbYellow Else picCurry.FillColor = vbRed End If picCurry.Cls picCurry.Circle (1000, 750), 500End Sub

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Example 3: Curry v3Option Explicit

Const Mild = 0Const Medium = 1Const Hot = 2 Private Sub Form_Load()

lstCurry.AddItem "Mild", Mild lstCurry.AddItem "Medium", Medium lstCurry.AddItem "Hot", Hot picCurry.FillStyle = vbSolid End Sub

Private Sub lstCurry_Click() Dim CuCo As Long ' Curry code CuCo = lstCurry.ListIndex lblCurryCode.Caption = CuCo lblCurryText.Caption = lstCurry.List(CuCo) If CuCo = Mild Then picCurry.FillColor = vbWhite ElseIf CuCo = Medium Then picCurry.FillColor = vbYellow Else picCurry.FillColor = vbRed End If picCurry.Cls picCurry.Circle (1000, 750), 500 End Sub

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Example 4: Curry v4Option Explicit

Enum TSpice Mild = 0 Medium = 1 Hot = 2End Enum Private Sub Form_Load() lstCurry.AddItem "Mild", Mild lstCurry.AddItem "Medium", Medium lstCurry.AddItem "Hot", Hot picCurry.FillStyle = vbSolid End Sub

Private Sub lstCurry_Click() Dim CuCo As TSpice ' Curry code CuCo = lstCurry.ListIndex lblCurryCode.Caption = CuCo lblCurryText.Caption = lstCurry.List(CuCo) If CuCo = Mild Then picCurry.FillColor = vbWhite ElseIf CuCo = Medium Then picCurry.FillColor = vbYellow Else picCurry.FillColor = vbRed End If picCurry.Cls picCurry.Circle (1000, 750), 500 End Sub

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Exercise 1: EDTs• Create an EDT to store the following

classification of height: short, average, tall

• Create an EDT to store the following classification of publication: book, journal

Enum THeight Short = 0 Average = 1 Tall = 2End Enum

Enum TPublication Book = 0 Journal = 1End Enum

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Variables and Memory Addresses• The computer keeps track of where variables are

stored in memory by using memory addresses.• Every byte (position) in memory has a memory

address:

• In the above example the variable identified by the name x is stored at location 63542 (this is the address of the first byte of data allocated to the variable x)

23x IntegerIdentifier

Value TypeMemory

0

63542

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Parameter Passing Methods • There are 2 ways to pass parameters to

functions and procedures:– Passing by Value: a literal value is passed

from the call to the definition (you have already used this)Sub p1(x As integer) …End Sub

– Passing by Reference: a variable’s memory address (a reference to the variables position in memory) is passed from the call to the definitionSub p2(ByRef y As integer) …End Sub

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Why pass by reference?• It allows the value of the passed variable to

be changed– i.e. it allows functions and procedures to

change the value of things passed to them

• Normally parameters are for input data – only functions can output data via the return value

• Pass by reference allows data to be input and output via parameters

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Example: Change the ValueDim a As IntegerDim b As integer

Sub P1(x As integer) x = x * 2 End Sub

Sub P2(ByRef x AS integer) x = x * 2 End Sub

a = 11b = 12P1 a ' What is the value of a?P2 b ' What is the value of b?

Pass by Ref

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

passed (variables are substituted by their value)p1 y ' This is fine. p1 21 ' This is fine.

• Pass by reference – only variables can be passed (in fact the variable’s memory address is passed)literals cannot be passed – they have no memory addressp2 y ' This is fine. p2 21 ' This generates an error.

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Example: Pass by Ref vs. FunctionSub P2(x As Integer) x = x * 2End Sub

Function F2(x As integer) As Integer F2 = x * 2End Function

Dim b As integer b = 4 P2 b ' What is the value of b? b = 4 b = F2(b) ' What is the value of b?

P1var x: integer

F1x: integer integer

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Pass by Ref vs. Function• a procedure that changes the value of a single

parameter is equivalent to a function,– the procedure P2:

P2 b was equivalent to:

– the function F2: b = F2(b)

• However, – F2 is far more explicit,– P2 is a bit cryptic: not obvious that value of b changes

• this makes code difficult to read, which can lead to errors

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Example: TotalDim Nums(1 To 5) As IntegerDim tot As integer

Function Total() As Integer Dim tmpTot As integer Dim i As integer tmpTot = 0 For i = 1 to 5 tmpTot = tmpTot + Nums(i) Next Total = tmpTot End Function

Nums(1) = 23Nums(2) = 17Nums(3) = 28Nums(4) = 12Nums(5) = 25tot = Total() ' What is the value of tot?

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Example: AverageDim ave As Double

Function Average() As Double

Dim tmpTot As Integer

Dim i As Integer

tmpTot = 0

for i = 1 to 5

tmpTot = tmpTot + Nums(i)

Next

Average = tmpTot / 5

End Function

ave = Average() ' What is the value of ave?

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

code

• Useful to combine them

• Problem:– a function can only have 1 output– This:

is not possible (in VB, or Delphi anyway)

double (average)integer (total)TotAve

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Example: Total and Average Sub TotAve(ByRef T As Integer, _

ByRef A As Double)

Dim I As integer

T = 0

For i = 1 to 5

T = T + Nums(i)

Next

A = T / 5

End Sub

tot = 12

ave = 15

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

TotAvevar T: integervar A: double