157212 Week 3 lecture 2 Lists, Loops

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157212Week 3 lecture 2Lists, Loops Arrays
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Tutorials start next week

• See web site for handout
• www.massey.ac.nz/tsrichar

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ListBoxes ComboBoxes

• Provide a list for the user to select from
• Various styles, choose based on
• Amount of data to be displayed
• Space available
• User's ability to add to the list

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Types of List Controls

• ListBox tool
• 1st prefix
• Simple List Box with/without scroll bars
• ComboBox tool
• cbo prefix
• List may allow for user to add new items
• List may "drop down" to display items in list

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List Controls Visually
Dropdown Combo Box
Simple Combo Box
List Boxes
Dropdown List Box
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Choosing List Type

• Many items, little space
• Dropdown Combo Box
• Dropdown List
• Few items
• List Box
• User allowed to add to the list
• Simple Combo Box
• Dropdown Combo Box

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Name and Text Properties

• ListBoxes
• Name displayed at Design Time
• Text accessible only at Run Time
• ComboBoxes
• Text displayed and accessible at Design Time
• Text also accessible at Run Time
• If user enters new data in ComboBox then VB
  stores it temporarily in the Text property

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DropDownStyle Property

• Used to indicate the type of ComboBox
• Dropdown Combo DropDown
• Simple Combo Simple
• Dropdown List DropDownList

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Items Collection

• List of items in a ListBox or ComboBox is a
  collection
• Collections are objects that have properties and
  methods that allow you to
• Add items
• Remove items
• Refer to individual items
• Count items
• Clear the collection

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Index Property

• Zero based value used to reference individual
  items in the collection
• Position of an item in the list
• 1st item Index 0 (1-10)
• 2nd item Index 1 (2-11)
• 3rd item Index 2 (3-12)
• So we can say that . . .
• nth item Index n-1

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SelectedIndex Property

• Use to identify currently selected item in the
  list
• If no item is selected ListIndex -1
• Use to select an item in the list

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Items.Count Property

• Use to determine number of items in the list

Remember Items.Count is the actual number of
items in the list BUT the SelectedIndex property
and Index will be 1 less. For example, it there
are 20 items in the list Items.Count20 BUT
Index 19 for the last item in list
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Using the Items Collection

• Use the index of the item to reference a specific
  item in the collection
• Remember that the index is zero based so the 1st
  item in the list is index position zero

lstUni.Items(5) Massey" ' Next line
references the currently selected
item strSelectedFlavor lstFlavor.Items(lstFlavor
.SelectedIndex)
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Filling a List Hint on test

• Design time in properties window
• Items property
• Click on ellipses to open String Collection
  Editor
• Separate items with ENTER key
• Run time methods
• Items.Add
• OR
• Items.Insert

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Filling List - Design Time
Click ellipses button to open
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Items.Add Method

• Use to add new items to the list at run time
• General Form
• Examples

Object.Items.Add(ItemValue)
lstNames.Items.Add("Brandon") lstDeptNums.Items.Ad
d(100) cboMajors.Items.Add(txtMajors.Text) 'Next
line adds new item user typed in to
combo cboGrade.Items.Add(cboGrade.Text)
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Items.Insert Method

• Use to add new items to the list at run time at a
  specific location(index) in the collection
• General Form
• Examples

Object.Items.Insert(IndexPosition, ItemValue)
lstDeptNums.Items.Insert(1, 100) cboGrade.Items.in
sert(0, "Freshman") cboMajors.Items.Insert(11,
txtMajors.Text)
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Items.RemoveAt Method

• Use to specifically remove items from the list at
  run time by specifying the index of the item
• General Form
• Examples

Object.Items.RemoveAt(IndexPosition)
lstDeptNums.Items.RemoveAt(1) ' Next line
removes the currently selected item cboGrade.Items
.RemoveAt(cboGrade.SelectedIndex)
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Items.Remove Method

• Use to specifically remove items from the list at
  run time by specifying the Text of the item
• General Form
• Examples

Object.Items.Remove(TextString)
lstDeptNums.Items.Remove("USC") ' Next line
removes the currently selected item cboGrade.Items
.Remove(cboGrade.Text)
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Clear Method

• Empty the entire list from list box or combo box,
  remove all the items
• General Form
• Examples

Object.Items.Clear( )
lstDeptNums.Items.Clear( ) cboGrade.Items.Clear( )
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ListBox and ComboBox Events

• TextChanged Event
• Occurs when user types text into Combo
• ListBox does not have TextChanged Event
• SelectedIndexChanged Event
• Enter Event (receive focus)
• Leave Event (lose focus)

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Loops

• Repeating a series of instructions
• Each repetition is called an iteration
• Types of Loops
• Do
• Use when the number of iterations is unknown
• For Next
• Use when the number of iterations known

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Do Loop

• Ends based on a condition you specify, either
• Loop While a condition is True
• Loop Until a condition becomes True
• Condition can be located at
• Top of Loop, Pretest
• Bottom of Loop, Posttest

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Do Loop General Form

• Do While Until condition
• Statements to execute
• Loop
• OR
• Do
• Statements to execute
• Loop While Until condition

Top of Loop Condition, Pretest
Bottom of Loop Condition, Posttest
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Do's - When Evaluated

• Top Evaluation, not guaranteed to run once since
  evaluated BEFORE running
• Do While Loop
• Do Until Loop
• Bottom Evaluation, will always run at least one
  time
• Do Loop While
• Do Loop Until

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For Next Loops

• Use when you know the number of iterations
• Uses a numeric counter variable, called Loop
  Index, to control number of iterations
• Loop Index is incremented at the bottom of the
  loop on each iteration
• Step value can be included to specify the
  incrementing amount to increment Loop Index, step
  can be a negative number

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For Next Loop General Form

• For LoopIndex InitialValue To TestValue Step
  Increment
• Statements to execute
• Next LoopIndex

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Do Loops / For Next Loops Deciding Which To Use
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Manually Exiting For Next Loops

• In some situations you may need to exit the loop
  prematurely
• Use the Exit For statement inside the loop
  structure
• Generally the Exit For is part of an If statement

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..and then there was CASE

• Case Arrays

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Case Structure

• Best alternative for testing a single variable or
  expression for multiple values
• Any decisions coded with nested If statements can
  also be coded using Case structure
• Case Structure is simpler, cleaner, more
  efficient than the nested If

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Select Case General Form(also notice indenting)

• Select Case expression
• Case ConstantList
• code to run
• Case ConstantList
• code to run
• Case Else
• code to run
• End Select

If expressionvalue in constant list
If expressionvalue in constant list
If expressionlt gtvalues in any of the preceding
constant lists
Case Else is an optional clause
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Select Case - Numeric Value Example 1

• Select Case intScore
• Case Is gt 100
• lblMsg.Text"Excellent Score"
• Case 80 to 99
• lblMsg.Text"Very Good"
• Case 60 to 79
• lblMsg.Text"Excellent Score"
• Case Else
• lblMsg.Text"Improvement Needed"
• End Select

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Select Case - Numeric Value Example 2

• Select Case intListIndex
• Case 0
• HandleItemZero( )
• Case 1, 2, 3
• HandleItems( )
• Case Else
• HandleNoSelection( )
• End Select

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Select Case - String Value Example

• Select Case txtTeam.Text.ToUpper( )
• Case "TIGERS"
• (Code for Tigers)
• Case "LEOPARDS"
• (Code for Leopards)
• Case "COUGARS", "PANTHERS"
• (Code for Cougars and Panthers)
• End Select

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Arrays

• List or series of values all referenced by the
  same name
• Similar to list of values for list boxes and
  combo boxes - without the box
• Use an array to keep a series of variables for
  later processing such as
• Reordering
• Calculating
• Printing

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Array Terms

• Element
• Individual item in the array
• Index (or subscript)
• Zero based number used to reference the specific
  elements in the array
• Must be an integer
• Boundaries
• Lower Subscript, 0 by default
• Upper Subscript

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Simple Array Example
strName Array
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Defining Arrays

• Use Dim statement to declare
• Specify the number of elements in the array as
  the UpperSubscript
• Each element of the array will be assigned a
  default value
• Numeric gt 0
• String gt empty string, 0 characters

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Defining Arrays - Alternate Form

• Optionally, the elements in the array may be
  assigned values in the Dim statement
• However, if values are assigned, you cannot
  declare the Upper Subscript
• Advice Dont do it

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General Form Dim Statement for Arrays
Dim ArrayName(UpperSubscript) as Datatype Dim
ArrayName( ) as Datatype InitialValueList
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Dim Statement for Arrays Examples - Default Values

• Dim strName(3) as String
• Results in an array of 4 elements strName(0),
  strName(1),
• strName(2), strName(3)
• Dim decBalance(99) as Decimal
• Results in an array of 100 elements decBalance(
  0), . . . , decBalance(99)

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Dim Statement for Arrays Examples - Assigned
Values

• Dim strDept( ) as String "ACT", "MKT", "HR"
• Dim intActCode( ) as Integer 10, 20, 30, 40

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What does VB do with the array?

• When the DIM statement for the array is processed
  VB sets aside room for it in memory.
• Ex Dim strName(3) as String
• VB sets aside a memory location for 4 strings

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Referencing Array Elements

• Use the Index(s) of the Element

strName(0) "Sam Smith" strName(1) "Jill
Creech" strName(2) "Paul Fry" strName(3)
"Rich Wells"
Sam Smith
Jill Creech
Paul Fry
Rich Wells
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Working with Arrays

• Use Loops to reference each element in the array
• For / Next
• For Each / Next

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For Each / Next

• VB references EACH element of the array
• VB assigns its value to ElementName
• The variable used for ElementName must be same
  datatype as the array elements or an Object
  datatype
• Makes one pass through the loop per element
• Use Exit For statement within loop to exit early

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For Each Loop General Form

• For Each ElementName In ArrayName
• Statements to execute
• Next ElementName

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For Each / Next Examples
' Assumes array strName previously
dimensioned Dim strOneName As String For Each
strOneName In strName Debug.WriteLine(strOneName)
' Write one array element Next strOneName '
Assumes array intTotal previously dimensioned
Dim intOneTotal As Integer For Each intOneTotal
In intTotal intOneTotal0 ' reinitialize the
array Next intOneTotal
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The following slides are for reference
structures will not be used in programming
testStructures

• Combine multiple fields of data into a single
  unit
• Declaration (by default a Structure is Public)
• Cannot be declared inside a procedure
• Generally declared in General Declarations
• Define using Structure, End Structure
• Once created, declare variable of the Structure
  as if it were another datatype make up a
  meaningful prefix

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Structure/End Structure General Form

• PublicPrivate Structure NameOfStructure
• Dim FirstField As Datatype
• Dim SecondField As Datatype
• . . .
• End Structure

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Structure Example 1

• Public Structure Employee
• Dim intEmpID As Integer
• Dim strLName As String
• Dim strFName As String
• Dim datHireDate As Date
• End Structure
• ' Declaring a variable based on the Structure
• Dim empOffice As Employee

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Structure Example 2

• Public Structure Product
• Dim strDesc As String
• Dim strProdID As String
• Dim intQuan As Integer
• Dim decPrice As Decimal
• End Structure
• ' Declaring a variable array based on the
  Structure
• Dim prdInven(100) As Product

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Accessing the Elements in a Structure Variable

• Each field of data in Structure is an Element
• To access specify Variable.Element
• Examples

empOffice.intEmpID empOffice.strFName empOffice.s
trLName empOffice.datHireDate
prdInven(intIndex).strDesc prdInven(intIndex).strI
D prdInven(intIndex).intQuan prdInven(intIndex).de
cPrice
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Including An Array In A Structure

• Arrays can be included as elements within a
  Structure
• VB does not, however, allow you to declare the
  number of elements in the array within the
  Structure declaration
• Use the ReDim statement inside a procedure to
  define the size of the array

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ReDim Statement

• Used to redeclare the UpperSubscript/Size of an
  array
• Can preserve original data in the array
• General Form (basic)
• ReDim Preserve ArrayName(UpperSubscript)
• Examples
• ReDim Preserve strDept(20)
• ReDim sdtHousewares.decSale(6)

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Using Array Elements for Accumulators
intGroupNumCInt(txtGroup.Text)-1
mintTotal array
(0) (1) (2) (3) (4) (5) (6) (7)
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Table Lookup

• Problem Often values used to identify a series
  of elements are not
• Numerically sequential
• Numerically separated by a constant value
• Numeric at all, may be strings
• Solution Create another array to hold the
  identifying values and use a table lookup process
  to find the correct element in the array

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Table Lookup (cont.)
intIndex2
mgiGroup array
(0) (1) (2) (3) (4) (5) (6) (7)
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Using List Boxes With Arrays

• Use List Boxes or Combo Boxes rather than using
  text boxes for the user to enter data used for
  looking up information in the array
• Use the list's SelectedIndex property as the
  subscript of the corresponding array

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List Boxes With Arrays (cont.)
SelectedIndex 2 of List Box
mgiGroup array
(0) (1) (2) (3) (4) (5) (6) (7)
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Multidimensional Arrays

• Arrays can have more than one dimension
• Like a table of values
• You must specify the boundaries for each
  dimension using subscripts
• Example Two Dimensional Array
• Dim intScoreBoard (1, 8) as Integer

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Referencing Elements in Multidimensional Array
intScoreBoard(row,column)
0 1 2 3 4 5 6 7 8
0 1
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
intScoreBoard(1,0) intScoreBoard(1,1) intScoreBoar
d(1,2) intScoreBoard(1,3) intScoreBoard(1,4) intSc
oreBoard(1,5)
intScoreBoard(0,0) intScoreBoard(0,1) intScoreBoar
d(0,2) intScoreBoard(0,3) intScoreBoard(0,4) intSc
oreBoard(0,5)
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General Form Dim Statement for Two-Dimensional
Arrays
Dim ArrayName(HighestSubscript, Highest
Subscript) as Datatype Dim ArrayName( , ) as
Datatype ListOfValues
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Dim Statement for Two Dimensional-Arrays Examples

• Dim strName(2, 3) as String
• Results in an array of 12 elements 3 rows
  0, 1, 2, 3
• 4 columns 0, 1, 2, 3, 4
• Dim strName( , ) as String "Jim", "Mary",
  "Sam", "Sean", "Tom", "Sue", "Fred", "Paul",
  "Tim", "Al", "Bob", "Pete", "Joy", "Wes",
  "Kim", "Beth"
• Results in same array as above with default
  values

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Working WithTwo-Dimensional Arrays

• Initializing/Reinitializing
• Use Nested For/Next Loops
• Use For Each/Next Loop
• Printing
• Use For Each/Next Loop
• Summing
• Include a total field for each row and each
  column
• Use For/Next Loop to calculate sums

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Initializing For/Next Example
Dim intRow As Integer Dim intColumn As
Integer For intRow 0 to 2 For intColumn 0 to
3 strName(intRow, intColumn) " " Next
intColumn Next intRow
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Initializing For Each/Next Example
Dim strElement As String For Each strElement In
strName strElement " " Next strElement
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Summing Example
decAmount(3,5)
decRowTotal(3)
0 1 2 3 4 5
0 1 2 3
0 1 2 3
ROWTOTALS
COLUMNTOTALS
decColTotal(5)
0 1 2 3 4 5
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Summing Code Example
'Crossfoot Total a 2D table Dim decAmount(3,5) As
Decimal Dim decRowTotal(3) As Decimal Dim
decColTotal(5) As Decimal Dim intRowIndex As
Integer Dim intColIndex As Integer For
intRowIndex 0 to 3 For intColIndex 0 to
5 decRowTotal(intRowIndex)
decAmount(intRowIndex, intColIndex) decColTotal(
intColIndex) decAmount(intRowIndex,
intColIndex) Next intColIndex Next intRowIndex
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Lookup Two-Dimensional Tables

• Use same techniques as for single dimension
  arrays
• Direct Reference (if meaningful row and column
  subscripts are available)
• Table Lookup
• Many 2D tables used for lookup will require
  additional one-dimensional arrays or list boxes
  to aid in the lookup process

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Lookup Example Using List Box
Weight Subscript of Array uses lstWeight.SelectedI
ndex
Zone Subscript of Array uses lstZone.SelectedIndex
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Debugging Array Programs

• View the array elements in Break Time in the
  Autos Window or the Locals Window

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End of fundamental programming constructs

• You must know all about
• Variables, Constants
• If..Then..Else, Case
• Loops
• Lists
• Arrays
• Full Stop (.) method of accessing Properties
  Methods
• Property and method names of common controls
• Questions?