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

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

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

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  1. 12 – Enumerated Data-Types & Pass-by-reference

  2. 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

  3. Project Files • Ensure that you submit: • project file (*.vbp) • all form files (*.frm, and *.frx) • all module files (*.bas)

  4. 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

  5. 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"

  6. Example 1: Curry v1 Option Explicit Private Sub Form_Load() lstCurry.AddItem "Mild", 0 lstCurry.AddItem "Medium", 1 lstCurry.AddItem "Hot", 2 picCurry.FillStyle = vbSolid End 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), 500 End Sub

  7. Example 2: Curry v2 Option Explicit Private Sub Form_Load() lstCurry.AddItem "Mild", 0 lstCurry.AddItem "Medium", 1 lstCurry.AddItem "Hot", 2 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 = 0 Then picCurry.FillColor = vbWhite ElseIf CuCo = 1 Then picCurry.FillColor = vbYellow Else picCurry.FillColor = vbRed End If picCurry.Cls picCurry.Circle (1000, 750), 500 End Sub

  8. Example 3: Curry v3 Option Explicit Const Mild = 0 Const Medium = 1 Const 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

  9. Example 4: Curry v4 Option Explicit Enum TSpice Mild = 0 Medium = 1 Hot = 2 End 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

  10. 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 = 2 End Enum Enum TPublication Book = 0 Journal = 1 End Enum

  11. 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) 0 63542 x 23 Integer Identifier Type Value Memory

  12. Parameter Passing Methods • There are 2 ways to pass parameters to functions and procedures: • Passing by Value: a literalvalue 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

  13. 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

  14. Example: Change the Value Dim a As Integer Dim 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 = 11 b = 12 P1a ' What is the value of a? P2b ' What is the value of b? Pass by Ref

  15. 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. 

  16. Example: Pass by Ref vs. Function P1 var x: integer F1 x: integer integer Sub P2(x AsInteger) x = x * 2 End Sub Function F2(x As integer) AsInteger F2 = x * 2 End Function Dim b As integer b = 4 P2b ' What is the value of b? b = 4 b = F2(b) ' What is the value of b?

  17. Pass by Ref vs. Function • a procedure that changes the value of a single parameter is equivalent to a function, • the procedure P2:P2bwas 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

  18. Example: Total Dim Nums(1 To 5)AsInteger Dim tot As integer Function Total() AsInteger Dim tmpTot As integer Dimi As integer tmpTot = 0 For i = 1 to 5 tmpTot = tmpTot + Nums(i) Next Total = tmpTot End Function Nums(1) = 23 Nums(2) = 17 Nums(3) = 28 Nums(4) = 12 Nums(5) = 25 tot = Total() ' What is the value of tot?

  19. Example: Average Dim ave AsDouble Function Average() AsDouble Dim tmpTot AsInteger Dim i AsInteger 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?

  20. Two results? integer (total) TotAve double (average) • 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)

  21. Example: Total and Average SubTotAve(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? var T: integer TotAve var A: double

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