Visual Basic VB: Decisions and Iteration

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Visual Basic (VB)Decisions and Iteration
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Decision Making

• Remember we talked about an objects state.
• Depending on state, which may well be determined
  by a variables value inside an object, we often
  need to take different courses of action.
• Remember the hungry/not hungry scenario?
• To make such a decision, we need to carry out a
  test to see if some condition is true or false,
  e.g.
• To withdraw money from the bank, you need to
  check the account balance.
• If the balance is greater than the amount that
  you want to withdraw, then you withdraw it
  otherwise you can arrange an overdraft.

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Decision Making in VB

• Two-way selection
• If test Then
• ElseIf test Then
• ElseIf test Then
• Else
• End If
• Multi-way selection
• Select Case testExpression
• Case expressionList
• Case expressionList
• Case Else
• End Select

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The If Statement

• Test is carried out and is either true or false.
• If true, then the statements associated with the
  Then are executed
• Otherwise, if there is an Else statement, then
  the statements associated with the Else are
  executed.
• If we wanted to set a student's grade depending
  on the exam mark if mark is greater than or
  equal to fifty, the grade will be pass otherwise
  the grade will be fail.
• If mark gt 50 Then
• grade "pass
• Else
• grade "fail
• End If

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The Select Case Statement

• The testExpression is evaluated once and compared
  with each expression in the expressionList
  associated with each Case.
• If there is a match, then the statements
  associated with that Case are executed.
• If a customer wants to buy a product and supply
  the product number and quantity, the computer
  calculates the total price
• Select Case productNo
• Case Is 21
• unitPrice 24.5
• Cost unitPrice quantity
• Case 54, Is gt 99, 78 To 83, ProductOfTheWeek
• unitPrice 14.3
• Cost unitPrice quantity
• Case Else
• unitPrice 10.0
• Cost unitPrice quantity
• End Select

Is there a better way to write this?
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Iteration

• Enables us to repeat a set of instructions many
  times.
• In VB there are various types of iteration
  statement
• Do While test
• statement
• ...
• Loop
• ---------------------------
• Do
• statement
• ...
• Loop While test
• ---------------------------
• For counterstartValue To endValue Step increment
• statement
• ...
• Next

Here we can replace While with Until and reverse
to logic
Theres also a For Each...Next, which you can
explore yourself
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Iteration

• The statements are known as the loop body.
• These are executed repeatedly while the condition
  is true.
• When the test for staying in the loop becomes
  false, the program moves to the statement after
  the loop.
• The Do While and the For loops perform the test
  before the loop body is executed.
• If the test evaluates to false the very first
  time it is tested then the loop body is never
  executed.
• In contrast, the Do...Loop statement tests the
  condition after the loop body has been executed
• the loop body is executed at least once.

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Which one to use?

• Well weve just heard about the difference of the
  Do loop over the other two
• The loop body is executed once in a Do loop.
• If were waiting for a state to change then Do
  While and Do loops are useful.
• If were moving though an array then the For loop
  is neater than a Do While
• But whats an array?

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Arrays

• Every item in an array contains some data.
• You could also think of arrays as tables in rows
  and columns
• The individual entries, or elements, are accessed
  by the array name, followed by the position
  (index), of the entry, eg myData(4).
• Like a street name and house number provides an
  address.
• However, array indexes start at 0, so
• Dim myArray(6) As Integer
• reserves space for 7 elements, indexed thus 0,
  1, 2, 3, 4, 5, 6.
• This is a bit different to most other languages.

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Declaring Arrays

• As with other variables, arrays must be declared
  before they are used
• Why?
• Examples
• Dim marks(6) As Integer 'creates an array which
  will contain ___ integers
• Dim sizes(4) As Double 'creates an array which
  will contain ___ doubles
• Dim names(3) As String 'creates an array which
  will contain ___ strings

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Setting/Getting Data in Arrays

• marks(3) 76 'sets the 4th element of the array
  marks to 76
• sizes(4) 2.5 'sets the 5th element of the
  array, sizes, to 2.5
• names(0) "Bridget" 'sets the 1st element of
  the array
• Strudents(0).lastName lastName(1) One array
  sets another
• Label2.Text names(3) get the 4th element of
  an array
• TextBox2.Text myControls(i).text gets the
  ith element of array object

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Returning to Iterating over Arrays

• Private Sub blahblah()
• Dim numbers(Size) As Integer
• Dim i As Integer
• numbers(0) 0
• For i 1 To Size Step 1
• numbers(i) i numbers(i - 1)
• Debug.WriteLine(Str(numbers(i)))
• Next
• End Sub

• Private Sub moreblah()
• Dim numbers(Size) As Integer
• Dim i As Integer
• numbers(0) 0
• i 1
• Do While i lt Size
• numbers(i) i numbers(i - 1)
• Debug.WriteLine(Str(numbers(i)))
• i i 1
• Loop
• End Sub

Which is better and why?
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Anatomy of a Loop

• Looking at the previous slide you can see three
  components common to all loops
• Initialisation
• Test
• Increment

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Multi-Dimensional Arrays

• Weve already said that arrays could be thought
  of as tables, but those we already seen would
  only have one row, ie one dimension.
• Imagine we could have more than one dimension

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Multi-Dimensional Arrays

• Declaring
• Dim printedTable(1000,5) As String
• One thousand one rows and six columns
• Space for 6006 strings.
• Setting
• printedTable(0,0) First Name
• printedTable(0,1) Last Name
• Getting
• headerRowLabel(0).Text printedTable(0,1)
• headerRowLabel(1).Text printedTable(0,2)

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Dynamic Arrays

• Sometimes you dont know how big the array will
  be when you initially declare it
• When you read in a file for instance.
• Dont want to declare a huge array initially
• Might run out of memory
• We need to re-size arrays when necessary using
  ReDim
• Dim charactersInString() declare with empty
  dimension list
• stringLength stringOfInterest.Length
• ReDim charactersInString(stringLength-1)
• ReDim destroys the contents of the array unless
  you use
• ReDim Preserve .

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Array Lists

• A special kind of dynamic array built into some
  of the standard controls.
• Has really handy methods to add, delete, count,
  etc. items from the array
• Dim listOfMates As ArrayList
• Dim numberOfMates As Integer
• listOfMates.Add("bob")
• listOfMates.Add(tam")
• numberOfMates listOfMates.Count
• listOfMates.RemoveAt(0)

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Array Lists in List Boxes
Imagine we had
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Inside the List Box

• we have a special secret array list called
  Items
• We can get at it by saying

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We need some subs

• After an enter name button click
• If TextBox1.Text.Length gt 0 Then
• ListBox1.Items.Add(TextBox1.Text)
• End If
• For a count,
• MessageBox.Show(CStr(ListBox1.Items.Count))
• For a remove,
• If ListBox1.SelectedIndex gt 0 Then
• ListBox1.Items.RemoveAt(ListBox1.Selec
  tedIndex)
• End If
• For a search, you need all sorts that you might
  like to figure out for yourself

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Next Time

• well discuss procedures.
• In the meantime, you should be near finishing all
  the lab exercises.
• You should be on the verge of starting your
  coursework by now?!
• Remember, you should be working outside of
  timetabled classes.
• Also remember, developing away from the labs can
  be problematic when we assess your work.
• You should try the exercises given on the first
  tutorial sheet available from the module Web
  pages.