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- 7.1 Creating and Accessing Arrays
- 7.2 Using Arrays
- 7.3 Some Additional Types of Arrays
- 7.4 Sorting and Searching
- 7.5 Two-Dimensional Arrays
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- ReDim Statement
- Using an Array as a Frequency Table
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- A variable (or simple variable) is a name to which VB.NET can assign a
single value.
- An array variable is a collection of simple variables of the same type
to which VB.NET can efficiently assign a list of values.
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- Suppose that you want to evaluate the exam
grades for 30 students and to display the names of the students
whose scores are above average.
- Private Sub btnDisplay_Click(...)
- Handles btnDisplay.Click
Dim student1 As String, score1 As Double
Dim student2 As String, score2 As Double
Dim student3 As String, score3 As Double
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- Dim student(30) As String
- Dim score(30) As Double
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- Dim arrayName(n) As DataType
- 0 is the "lower bound" of the array
- n is the "upper bound" of the array – the last available
subscript in this array
- The number of elements is the size of the array
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- Private Sub btnWhoWon_Click(...) _
- Handles btnWhoWon.Click
- Dim teamName(4) As String
- Dim n As Integer
- 'Place Super Bowl Winners into
the array
- teamName(1) =
"Packers"
- teamName(2) =
"Packers"
- teamName(3) = "Jets"
- teamName(4) = "Chiefs"
- 'Access array
- n = CInt(txtNumber.Text)
- txtWinner.Text = teamName(n)
- End Sub
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- Dim teamName(3) As String
- Private Sub btnWhoWon_Click(...)Handles btnWhoWon.Click
- Dim n As Integer
- n = CInt(txtNumber.Text)
- txtWinner.Text = teamName(n)
- End Sub
- Private Sub Form1_Load(...) Handles MyBase.Load
- 'Place Super Bowl Winners into
the array
- teamName(1) =
"Packers"
- teamName(2) =
"Packers"
- teamName(3) = "Jets"
- End Sub
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- Arrays may be initialized when they are created:
- Dim arrayName() As varType = {value0, _
- value1, value2, ..., valueN}
- declares an array having upper bound N and assigns value0 to arrayName(0),
value1 to arrayName(1), value2 to arrayName(2), ..., and valueN to arrayName(N).
- Dim teamName() As String = {"", "Packers", _
-
"Packers", "Jets", "Chiefs"}
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- The size of an array may be changed after it is created:
- ReDim arrayName(m)
- where arrayName is the name of the already declared array and m is an
Integer literal,variable, or expression.
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- To keep any data that has already been stored in the array when resizing
it, use
- ReDim Preserve arrayName(m)
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- Private Sub btnAnalyze_Click(...) Handles btnAnalyze.Click
- 'Count occurrences of the various letters in a sentence
- Dim index, letterNum As Integer
- Dim sentence, letter As String
- Dim charCount(26) As Integer
- 'Examine and tally each letter
of the sentence
- sentence =
(txtSentence.Text).ToUpper
- For letterNum = 1 To sentence.Length
- letter =
sentence.Substring(letterNum - 1, 1)
- If (letter >=
"A") And (letter <= "Z") Then
- index = Asc(letter) - 64 'The
ANSI value of "A" is 65
- charCount(index) += 1
- End If
- Next
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- 'List the tally for each letter
of alphabet
- lstCount.Items.Clear()
- For index = 1 To 26
- letter = Chr(index + 64)
- If charCount(index) > 0 Then
- lstCount.Items.Add(letter
& " " & _
- charCount(index))
- End If
- Next
- End Sub
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- The following sets up an array, and then references an element that
doesn't exist. This will cause an
error.
- Dim trees() As String = {"", "Sequoia", _
-
"Redwood", "Spruce"}
- txtBox.Text = trees(5)
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- If arrayOne() and arrayTwo() have been declared with the same data type,
then the statement
- arrayOne = arrayTwo
- makes arrayOne() an exact duplicate of arrayTwo(). It will have the same
size and contain the same information.
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- Ordered Arrays
- Using Part of an Array
- Merging Two Ordered Arrays
- Passing Arrays to Procedures
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- An array is ordered if the elements are in ascending or descending
order.
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- Dim nom() As String = {"", "AL", "BOB",
"CARL", "DON","ERIC", _
- "FRED",
"GREG", "HERB", "IRA", "JACK"}
- Private Sub btnSearch_Click(...) Handles btnSearch.Click
- Dim name, name2Find As String
- Dim n As Integer 'Subscript of
the array
- name2Find = txtName.Text.ToUpper
- Do
- n += 1 'Add 1 to n
- Loop Until (nom(n) >=
name2Find) Or (n = 10)
- If nom(n) = name2Find Then
- txtResult.Text =
"Found."
- Else
- txtResult.Text = "Not
found."
- End If
- End Sub
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- 'Demonstrate using part of an array
- Dim stock(100) As String
- Dim counter As Integer
- Private Sub btnRecord_Click(...) Handles btnRecord.Click
- If (counter < 100) Then
- counter += 1 'Increment
counter by 1
- stock(counter) =
txtCompany.Text
- txtCompany.Clear()
- txtCompany.Focus()
- txtNumber.Text = CStr(counter)
- Else
- MsgBox("No space to
record additional companies.", , "")
- txtCompany.Clear()
- btnsummarize.Focus()
- End If
- End Sub
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- Private Sub btnsummarize_Click(...)
- Handles btnsummarize.Click
- 'List stock companies that have
been recorded
- Dim i As Integer
- lstStocks.Items.Clear()
- For i = 1 To counter
- lstStocks.Items.Add(stock(i))
- Next
- End Sub
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- To consolidate two lists into a single ordered list:
- Compare the two names at the top
of the first and second lists.
- If one name alphabetically precedes the other, copy it onto the third
list and cross it off its original list.
- If the names are the same, copy the name onto the third list and cross
out the name from the first and second lists.
- Repeat Step 1 with the current top names until you reach the end of
either list.
- Copy the names from the remaining list onto the third list.
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- An array declared in a procedure is local to that procedure
- An entire array can be passed to a Sub or Function procedure
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- Private Sub btnCompute_Click(...) Handles btnCompute.Click
- Dim score() As Integer = {0, 85,
92, 75, 68, 84, 86, _
- 94, 74,
79, 88}
- txtAverage.Text = CStr(Sum(score)
/ 10)
- End Sub
- Function Sum(ByVal s() As Integer) As Integer
- Dim total, index As Integer
- total = 0
- For index = 1 To s.GetUpperBound(0)
'The upper bound is 10
- total += s(index)
- Next
- Return total
- End Function
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- Searching successive elements of an ordered list beginning with the
first element is called a sequential search.
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- A single element of an array can be passed to a procedure just like any
ordinary numeric or string variable.
- Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
- Dim num(20) As Integer
- num(5) = 10
-
lstOutput.Items.Add(Triple(num(5)))
- End Sub
- Private Function Triple(ByVal x As Integer) As Integer
- Return 3 * x
- End Function
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- Control Arrays
- Array of Structures
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- Control arrays are arrays of controls, such as labels, text boxes, etc.
- They are created in much the same way as any other array:
- Dim arrayName(n) As ControlType
- or
- Dim arrayName() As ControlType
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- The following statements declare control arrays.
- Dim lblTitle(10) As Label
- Dim txtNumber(8) As TextBox
- Dim btnAmount() As Button
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- Dim lblDept(5) As Label
- Dim txtDept(5) As TextBox
- Private Sub Form1_Load(...) Handles MyBase.Load
- Dim depNum As Integer
- lblDept(1) = Label1
- lblDept(2) = Label2
- lblDept(3) = Label3
- lblDept(4) = Label4
- lblDept(5) = Label5
- txtDept(1) = TextBox1
- txtDept(2) = TextBox2
- txtDept(3) = TextBox3
- txtDept(4) = TextBox4
- txtDept(5) = TextBox5
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- For depNum = 1 To 5
- lblDept(depNum).Text =
"Department " & depNum
- txtDept(depNum).Clear()
- Next
- End Sub
- Private Sub btnCompute_Click(...)
- Handles btnCompute.Click
- Dim totalSales As Double = 0
- Dim depNum As Integer
- For depNum = 1 To 5
- totalSales += CDbl(txtDept(depNum).Text)
- Next
- txtTotal.Text =
FormatCurrency(totalSales)
- End Sub
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- A way of grouping heterogeneous data together
- Also called a UDT (User Defined Type)
- Sample structure definition:
- Structure College
- Dim name As String
- Dim state As String
- Dim yearFounded As Integer
- End Structure
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- Each subvariable in a structure is called a member or field
- To declare a variable of a structure type:
- Dim college1 As College
- Each member is accessed via the variable name dot member name
- college1.name = "Harvard"
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- Structure College
- Dim name As String
- Dim state As String
- Dim yearFounded As Integer
- End Structure
- Dim college1, college2, collegeOlder As College
- Private Sub btnFirst_Click(...) Handles btnFirst.Click
- Dim prompt As String
- college1.name =
InputBox("Enter name of college.", "Name")
- college1.state =
InputBox("Enter state.", "State")
- prompt = "Enter the year
the first college was founded."
- college1.yearFounded = CInt(InputBox(prompt,
"Year"))
- End Sub
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- Integer, String, Double, etc.
- Another User Defined Type
- Arrays
- Must not specify range
- Range must be set using ReDim
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- Structure FullName
- Dim firstName As String
- Dim lastName As String
- End Structure
- Structure Student
- Dim name As FullName
- Dim credits() As Integer
- End Structure
- Private Sub btnGet_Click(...) Handles btnGet.Click
- Dim numYears, i As Integer
- Dim person As Student
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- txtResult.Clear()
- person.name.firstName =
InputBox("First Name:")
- person.name.lastName =
InputBox("Second Name:")
- numYears = CInt(InputBox("Number
of years " & _
-
"completed:"))
- ReDim person.credits(numYears)
- For i = 1 To numYears
- person.credits(i) = CInt(InputBox("Credits
in yr "_
- &
i))
- Next
- DetermineStatus(person)
- End Sub
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- Sub DetermineStatus(ByVal person As Student)
- Dim i, total As Integer
- For i = 1 To person.credits.GetUpperBound(0)
- total += person.credits(i)
- Next
- If (total >= 120) Then
- txtResult.Text =
person.name.firstName & " " & _
- person.name.lastName &
" has enough credits" & _
- " to graduate."
- Else
- txtResult.Text =
person.name.firstName & " " & _
- person.name.lastName &
" needs " & _
- (120 - total) & "
more credits to graduate."
- End If
- End Sub
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- Bubble Sort
- Shell Sort
- Searching
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- Sorting is an algorithm for ordering an array.
- We discuss two sorting algorithms:
- Both use the swap algorithm:
- temp = varl
- varl = var2
- var2 = temp
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- Private Sub btnAlphabetize_Click(...) _
- Handles btnAlphabetize.Click
- Dim firstWord, secondWord, temp As
String
- firstWord = txtFirstWord.Text
- secondWord = txtSecondWord.Text
- If (firstWord > secondWord) Then
- temp = firstWord
- firstWord = secondWord
- secondWord = temp
- End If
- txtResult.Text = firstWord &
" before " & _
- secondWord
- End Sub
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- Compare the first and second items. If they are out of order, swap them.
- Compare the second and third items. If they are out of order, swap them.
- Repeat this pattern for all remaining pairs. The final comparison and
possible swap are between the next-to-last and last elements.
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- Begin with a gap of g = Int(n/2)
- Compare items 1 and 1 + g, 2 and 2 + g, . . ., n and n - g. Swap any
pairs that are out of order.
- Repeat Step 2 until no swaps are made for gap g.
- Halve the value of g.
- Repeat Steps 2, 3, and 4 until the value of g is 0.
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- Sequential search starts at the beginning of a list and keeps looking
one by one until the item is found or the end of the list is reached.
- For a sequential search, the list need not be sorted.
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- Usually more efficient than sequential search
- List must be sorted
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- Denote the subscript of the first item in the list by first and the subscript of the
last item by last. Initially, the value of first is 1, the value of last
is the number of items in the list, and the value of flag is False.
- Look at the middle item of the current list, the item having the
subscript middle = Int((first +
last ) / 2)
- If the middle item is quarry (what you are looking for), then flag is
set to True and the search is over.
- If the middle item is greater than quarry, then quarry should be in the
first half of the list. So the subscript of quarry must lie between
first and middle - 1. That is,the new value of last is middle - 1.
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- If the middle item is less than quarry, then quarry should be in the
second half of the list of possible items. So the subscript of quarry
must lie between middle +1 and last. That is, the new value of first is middle
+1.
- Repeat Steps 2 through 5 until quarry is found or until the halving
process uses up the entire list. (When the entire list has been used up,
first > last.) In the second case, quarry was not in the original
list.
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- One-dimensional arrays require one subscript to access each element in
the array.
- Two-dimensional arrays require two subscripts to access each element.
- Dim rm(4, 4) As Double
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- Dim rm(4, 4) As Double
- Private Sub Form1_Load(...) Handles MyBase.Load
- 'Fill two-dimensional array with intercity mileages
- Dim sr As IO.StreamReader =
-
IO.File.OpenText("DISTANCE.TXT")
- Dim row, col As Integer
- For row = 1 To 4
- For col = 1 To 4
- rm(row, col) = CDbl(sr.ReadLine)
- Next
- Next
- sr.Close()
- End Sub
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- An unsized two-dimensional array can be declared with a statement of the
form
- Dim arrayName(,) As varType
- and a two-dimensional array can be declared and initialized at the same
time with a statement of the form
- Dim arrayName(,) As varType = {{ROW0}, {ROW1},... {ROWm}}
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- An already-created array can be resized
- ReDim arrayName(r, s)
- which loses the current contents, or with
- ReDim Preserve arrayName(m, s)
- When Preserve is used, only the column can be resized.
- ReDim cannot change the number of dimensions in an array.
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- 7.1 Creating and Accessing Arrays
- 7.2 Using Arrays
- Passing arrays to functions and sub procedures
- 7.3 Some Additional Types of Arrays
- Structures and arrays of structures
- Merging two ordered arrays
- 7.4 Sorting and Searching
- Linear and binary searches
- Exchange sorts
- 7.5 Two-Dimensional Arrays
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