a introduction to programming using visual basic 2010 （ eighth edition ） david i. schneider
TRANSCRIPT
A Introduction to Programming Using Visual Basic 2010
（ Eighth Edition）David I. Schneider
Chapter 6 – Arrays
6.1 Creating and Accessing Arrays
6.2 Using LINQ with Arrays
6.3 Arrays of Structures
6.4 Two-Dimensional Arrays
6.1 Creating and Accessing Arrays
• Declaring an Array Variable• The Load Event Procedure• Implicit Array Sizing• Calculating an Array Value with a Loop• The ReDim Statement• Flag Variables• For Each Loops
6.1 Creating and Accessing Arrays (continued)
• Passing an Array to a Procedure• User-Defined Array-Valued Functions• Searching for an Element in an Array• Copying an Array• Split Method and Join Function
Simple Variable vs Array
• A variable (or simple variable) is a name to which Visual Basic can assign a single value
• An array variable is a collection of simple variables of the same type with same name to which Visual Basic can efficiently assign a list of values.
Example advantage of arraySuppose you want to evaluate the exam grades
for 10 students and to display the scores which are above the average
Dim Score1, Score2, Score3, Score4, Score5 As IntegerDim Score6, Score7, Score8, Score9, Score10 As IntegerDim Sum As Integer, Average As DoubleScore1 = CInt(InputBox("Please input a new score"))Score2 = CInt(InputBox("Please input a new score"))Score3 = CInt(InputBox("Please input a new score"))Score4 = CInt(InputBox("Please input a new score"))Score5 = CInt(InputBox("Please input a new score"))Score6 = CInt(InputBox("Please input a new score"))Score7 = CInt(InputBox("Please input a new score"))Score8 = CInt(InputBox("Please input a new score"))Score9 = CInt(InputBox("Please input a new score"))Score10 = CInt(InputBox("Please input a new score"))Sum = Score1 + Score2 + Score3 + Score4 + Score5 + _ Score6 + Score7 + Score8 + Score9 + Score10
ExampleAverage = Sum / 10txtAve.Text = CStr(Average)
If Score1 > Average Then txtAbove.Text &= Score1 & " "End IfIf Score2 > Average Then txtAbove.Text &= Score2 & " "End IfIf Score3 > Average Then txtAbove.Text &= Score3 & " "End IfIf Score4 > Average Then txtAbove.Text &= Score4 & " "End IfIf Score5 > Average Then txtAbove.Text &= Score5 & " "End If
If Score6 > Average Then txtAbove.Text &= Score6 & " "End IfIf Score7 > Average Then txtAbove.Text &= Score7 & " "End IfIf Score8 > Average Then txtAbove.Text &= Score8 & " "End IfIf Score9 > Average Then txtAbove.Text &= Score9 & " "End IfIf Score10 > Average Then txtAbove.Text &= Score10 & " "End If .
ExampleRealize with Repetition-structure and simple variableDim Score As IntegerDim Sum As Integer, Average As DoubleFor i = 1 To 10 Score = CInt(InputBox("Please input the " & i & " th score")) Sum += ScoreNextAverage = Sum / 10txtAve.Text = CStr(Average) For i = 1 To 10 Score = CInt(InputBox("Please input the " & i & " th score")) If Score > Average Then txtAbove.Text &= Score & " " End IfNext
The 10 scores must be input twice, becausea simple variable only store a single value.
ExampleRealize with array
Dim Score(9) As IntegerDim Sum As Integer, Average As DoubleFor i = 0 To 9 Score(i) = CInt(InputBox("Please input the " & i & " th score ")) Sum += Score(i)NextAverage = Sum / 10txtAve.Text = CStr(Average) For i = 0 To 9 If Score(i) > Average Then txtAbove.Text &= Score(i) & " " End IfNext
No matter how many scores are there,the program structure is the same.
Array Terminology
Syntax ：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, n + 1, is the size of the array.
Putting Values into an Array
score(0) = 10
subscript
Read: "score sub zero equals ten"
Which means that the number 10 is being stored at the first location in the array called score because all arrays begin counting at 0
In other words, score(0) is the first element of array score().
Example 1Private Sub btnWho_Click(...) Handles btnWhoWon.Click Dim teamNames(3) As String Dim n As Integer teamNames(0) = "Packers" teamNames(1) = "Packers" teamNames(2) = "Jets" teamNames(3) = "Chiefs" n = CInt(txtNumber.Text) txtWinner.Text = teamName(n - 1)End Sub
Super Bowl 美国橄榄球超级杯大赛
Load Event Procedure
Occurs as the Form loads in memory
Private Sub frmName_Load(...) Handles MyBase.Load
The keyword MyBase refers to the form being loaded. This event procedure is a good place to assign values to an array.
Example 2: Form-Load procedure
Dim teamNames(3) As String
Private Sub frmBowl_Load(...) Handles MyBase.Load teamNames(0) = "Packers" teamNames(1) = "Packers" teamNames(2) = "Jets" teamNames(3) = "Chiefs"End Sub
Private Sub btnWhoWon_Click(...) Handles btnWhoWon.Click Dim n As Integer n = CInt(txtNumber.Text) txtWinner.Text = teamNames(n - 1)End Sub
The array can’t declared within any event procedure,It must be Class-Scope.
Initializing ArraysArrays may be initialized when created:
Dim arrayName() As DataType={value0,value1,value2,...,valueN}
declares an array having upper bound N and assigns value0 to arrayName(0), value1 to arrayName(1), ..., and valueN to arrayName(N).
Example:
Dim teamNames() As String={"Packers","Packers","Jets","Chiefs"}
Dim Score() As Integer={10, 20, 30, 40, 50, 60, 70, 80, 90, 100}
圆括号： parenthesis 大括号： curly bracket 逗号： comma 引号： quotation mark
Example 2: Simplified writing
Dim teamNames(3) As String
Private Sub frmBowl_Load(...) Handles MyBase.LoadDim teamNames() As String={"Packers","Packers","Jets","Chiefs"}
End Sub
Private Sub btnWhoWon_Click(...) Handles btnWhoWon.Click Dim n As Integer = CInt(txtNumber.Text) txtWinner.Text = teamNames(n - 1)End Sub
You can’t specify the subscript here, the size of the array is decided by the following list
Text Files Some times text file is used to offer the initial values of array. Such as Presidents.txt, States.txt etc.• Can be created, viewed, and managed by word processors
or by the Visual Basic IDE• Have the extension txt• Normally placed in the bin\Debug folder in the Solution
Explorer.
Using a Text File to Populate a String Array• Assume the text file named
theNamesOfTheFirstSixPresidents.txt
lies in the bin\Debug folder.• The text file can be used to fill string array with
the statement Dim PNames() As String
=IO.File.ReadAllLines("theNamesOfTheFirstSixPresidents.txt")
• The array PNames will have size 6 and upper bound 5.
Array Methods
Method Name Value Value in this case
arrayName.Count number of elements 6
arrayName.Max highest value(不区分大小写数字比字母小 ) Thomas Jefferson
arrayName.Min lowest value(不区分大小写数字比字母小 ) George Washington
arrayName.First first element George Washington
arrayName.Last last element John Q. Adams
arrayName.Average Average value(需为数值型数组 )
arrayName.Sum The sum of the elements(需为数值型数组 )
Dim PNames() As String =IO.File.ReadAllLines("theNamesOfTheFirstSixPresidents.txt")
Array Methods (continued)
• The upper bound of arrayName is arrayName.Count – 1
• arrayName.First is the same as arrayName(0)
Populating a Numeric Array with a Text File
If you want to evaluate the elements value numerically (such as average, sum etc.), you should change the array from stringto numeric type.
Dim FinalScores() As String=IO.File.ReadAllLines("Scores.txt")Dim Scores(FinalScores.Count-1) As IntegerFor i As Integer = 0 To FinalScores.Count-1 Scores(i) = CInt(FinalScores(i)) lstOutput.Items.Add(Scores(i))NextlstOutput.Items.Add("The highest value is " & Scores.Max)lstOutput.Items.Add("The lowest value is " & Scores.Min)lstOutput.Items.Add("The average is " & Scores.Average)lstOutput.Items.Add("The sum is " & Scores.Sum)
Using Loops Instead of Methods
Dim ages() As Integer = {55, 56, 61, 52, 69, 64, 46, 54, 47}
Dim max As Integer = ages(0)
For i = 1 To ages.Count - 1
If ages(i) > max Then
max = ages(i)
End If
Next
txtOutput.Text = "Greatest age: " & max
Output: Greatest age: 69
Using Loops Instead of Methods Cont.
Dim ages() As Integer = {55, 56, 61, 52, 69, 64, 46, 54, 47}
Dim max As Integer = 0
For i = 1 To ages.Count - 1
If ages(i) > ages(max) Then
max = i
End If
Next
txtOutpxt = "Greatest age: " & ages(max) & ". It’s the " & max+1 & "th element"
Output: Greatest age: 69. It’s the 5th element
Another way, you may find the max element with the subscript
ReDim Statement
• The size of an array may be changed after it has been created with the following statement ReDim arrayName(m)
• arrayName is the name of the already declared array• m is an Integer literal, variable, or expression• the upper bound of the array is changed to m.
You can’t change the type of an existing array,so there is no As DataType
Preserve Keyword
ReDim arrayName(m)
• resets all values to their default.This can be prevented with the keyword Preserve
ReDim Preserve arrayName(m)
• resizes the array and retains as many values as possible.
Flag VariablesFlag variables have type Boolean. They are used when looping through an array. It may provide information to be used after loop terminates. Or, allows for the early termination of the loop.
Dim Score() As StringPrivate Sub Button1_Click(…) Handles Button1.Click Score = IO.File.ReadAllLines("scores.txt") For i = 0 To Score.Count - 1 lstoutput.Items.Add(Score(i)) NextEnd Sub
Dim n As String = TextBox1.TextDim Found As Boolean = FalseFor i = 0 To Score.Count - 1 If Score(i) = n Then Found = True Exit For End IfNextIf Found = True Then MessageBox.Show(n & " is found.")Else MessageBox.Show("There is no " & n & " in the array.")End If
For Each LoopsFor i As Integer = 0 To ages.Count - 1 If ages(i) > max Then max = ages(i) End IfNext
can be replaced with
For Each x As Integer In ages If x > max Then max = x End IfNext
For i = 0 To Score.Count - 1 If Score(i) = n Then Found = True Exit For End IfNext
can be replaced with
For each Mark as string In Score If mark = n Then Found = True Exit For End IfNext
The Looping variable can be any name
Passing an Array Element
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
Function Triple(ByVal x As Integer) As Integer Return 3 * xEnd Function
Private Sub btnCalculate_Click(...) Handles btnCalculate.Click Dim ages() As Integer = {55, 56, 61, 52, 69, 64, 46, 54, 47} txtOutput.Text = “The range of the array is " & Range(ages)End Sub
Passing Arrays to Procedures cont.
Function Range(ByVal x() As Integer) As Integer
Dim Large, Small, N As Integer
Large = x.Max
Small = x.Min
N = Large - Small
Return N
End FunctionThe calling statement uses the name
of the array without parentheses.
The header of the Function procedure uses the name with an empty set of parentheses
User-Defined Array-Valued FunctionsHeaders have the formFunction FunctionName(ByVal var1 As Type1, ByVal var2 As Type2, ...) As DataType()
Function Prime(ByVal x() As Integer) As Boolean() Dim JG(x.Count - 1) As Boolean ‘ 记录结果 For i = 0 To x.Count - 1 Dim flag As Boolean = True For j = 2 To x(i) – 1 If x(i) Mod j = 0 Then flag = False Exit For End If Next j If flag Then JG(i) = True Else JG(i) = False End If Next i Return JGEnd Function
此函数的返回值为一个布尔型的数组，每个元素的值就是数组 x()对应元素的素数判断结果
Searching for an Element in an ArraynumVar = Array.IndexOf(arrayName, value)
• assigns to numVar the index of the first occurrence of value in arrayName.• Or assigns -1 if the value is not found.
Dim colors() As String = IO.File.ReadAllLines("Colors.txt")Private Sub btnDetermine_Click(…) Handles btnDetermine.Click Dim color As String = txtColor.Text If Existing(color, colors) Then txtOutcome.Text = color & " is a crayola color." Else txtOutcome.Text = color & " is not a crayola color." End If End Sub Function Existing(ByVal x As String, ByVal y() As String) As Boolean If Array.IndexOf(y, x) = -1 Then Return False Else Return True End If End Function
Copying an ArrayarrayOne = arrayTwo
makes arrayOne an exact duplicate of arrayTwo
If only arrayOne and arrayTwo have been declared with the same data type, no matter what is the size of arrayOne .
They share the same location in memory.
Copying an Array Cont.
Dim a() As Integer = {10, 20, 30, 40, 50}Dim b() As Integerb = aFor i = 0 To 4 ListBox1.Items.Add(b(i))NextListBox1.Items.Add("Ubound(b)=" & b.Count - 1)
Dim a() As Integer = {10, 20, 30, 40, 50}Dim b(4) As Integerb = aFor i = 0 To 4 ListBox1.Items.Add(b(i))NextListBox1.Items.Add("Ubound(b)=" & b.Count - 1)
Dim a() As Integer = {10, 20, 30, 40, 50}Dim b(2) As Integerb = aFor i = 0 To 4 ListBox1.Items.Add(b(i))NextListBox1.Items.Add("Ubound(b)=" & b.Count - 1)
Dim a() As Integer = {10, 20, 30, 40, 50}Dim b(8) As Integerb = aFor i = 0 To 4 ListBox1.Items.Add(b(i))NextListBox1.Items.Add("Ubound(b)=" & b.Count - 1) .
结论：只要数组 a 和 b 都是一维数组且类型相同，无论数组 b 原来的下标如何，都会被重新定义为和 a 一
样！
Copying an Array Cont.
Dim a() As Integer = {10, 20, 30, 40, 50}Dim b() As Integerb = aFor i = 0 To 4 b(i) = b(i) / 2Next
For i = 0 To 4 ListBox1.Items.Add(a(i))Next
结论：数组 a 和 b 经过直接复制操作后，它们在内存中指向同一位置（同一个数组的两个不同名称）
Dim a() As Integer = {10, 20, 30, 40, 50}
Dim b(4) As Integer
For i = 0 To 4
b(i) = a(i)
Next
For i = 0 To 4
b(i) = b(i) / 2
Next
For i = 0 To 4
ListBox1.Items.Add(a(i))
Next
结论：数组 a 和 b 经过逐元素赋值后，它们只有初始值相同，其它无任何联系。
Split Method
• Facilitates working with text files.• Split can convert a string containing comma-
separated data into a string array.• The 0th element of the array contains the text
preceding the first comma, the 1st element contains the text between the first and second commas, ..., and the last element contains the text following the last comma.
Split Example
For instance, suppose the string array employees has been declared without an upper bound, and the string variable line has the value “Bob,23.50,45”.
Dim employees() as String employees = line.Split(","c)
•sets the size of employees to 3•sets employees(0) = “Bob”•sets employees(1) = “23.50”•sets employees(2) = “45”
Split Comments
employees = line.Split(","c)• In this example, the character comma is called the
delimiter for the Split method, and the letter c specifies that the comma has data type Character instead of String
• Any character can be used as a delimiter. If no character is specified, the space character will be used as the delimiter by default.
ExamplePrivate Sub btnConvert_Click(...) Handles btnConvert.Click Dim stateData(), line As String line = "California,1850,Sacramento,Eureka" stateData = line.Split(","c) For Each entry As String In stateData lstOutput.Items.Add(entry) NextEnd Sub
California
1850
Sacramento
Eureka
Join Function
The reverse of the Split method is the Join function. Join concatenates the elements of a string array into a string containing the elements separated by a specified delimiter.Dim numbers() As String = {"One", "Two", "Three", "Four"}Dim s As Strings = Join(numbers, "-")txtOutput.Text = s
Output: One-Two-Three-Four
Out of Range Error
The following code references an array element that doesn't exist. This will cause an error.
Comments and ExercisesComments6.1 P206Exercises6.1 P207
6.2 Using LINQ with Arrays• LINQ Queries• The Distinct Operator• The ToArray Method• Use of Function Procedures in Queries• The Let Operator• The OrderBy Operator• The DataSource Property• Binary Search
What is LINQ?• LINQ stands for Language INtegrated Query• A query is a request for information
Note: Option Infer must be set to ON in order to use LINQ
LINQ QueryCode of the form range variable
Dim queryName = From var In arrayName source data
Where [condition on var] filter data 过滤数据
Select var Project data 投影数据
• declares the variable queryName and assigns to it a sequence of the values from arrayName that satisfy the stated condition
• The values in the sequence can be converted to an array, displayed in a list box, or written to a text file
query operators
Example 1'States.txt contains names of the 50 states
Dim states() As String = IO.File.ReadAllLines("States.txt")
Dim stateQuery = From s In states
Where s.Length = 5
Select s
For Each s As String In stateQuery
lstStates.Items.Add(s)
Next
lstStates.Items.Add(stateQuery.Count)
lstStates.Items.Add(stateQuery.Min)
lstStates.Items.Add(stateQuery(1))
Maine缅因州 , Texas德克萨斯州 , Idaho爱达荷州
Example 2Dim nums() As Integer = {7, 11, 12, 14, 15, 16, 18, 21, 30}
Dim numQuery = From n In nums
Where n > 10 And n < 20 And n Mod 2 = 0
Select n
For Each m As Integer In numQuery
lstBox.Items.Add(m)
Next
lstBox.Items.Add("")
lstBox.Items.Add("Largest number: " & numQuery.Max)
lstBox.Items.Add("Second number: " & numQuery(1))
lstBox.Items.Add("Sum of numbers: " & numQuery.Sum)
Another Variation of Example 2
Dim S() As Integer = {5, 12, 8, 7, 11, 5, 8}Dim numQuery = From num In S
Where num < 10
Select num * num changed
For Each X As Integer In numQuery
lstBox.Items.Add(X)
Next
Output: 25, 64, 49, 25, 64
Distinct Operator
Dim S() As Integer = {5, 8, 7, 5, 8}Dim numQuery = From num In S
Select num
Distinct
For Each X As Integer In numQuery
lstBox.Items.Add(X)
Next
Output: 5, 8, 7
There is no Where clause. It means select all the elements
The sequence created with a LINQ query might contain duplicate elements. Duplicates can be eliminated with key word Distinct.
ToArray Method Also the sequence returned by a LINQ query has many of the features of an array(it has Max, Min, Sum, Average, Count, First, Last methods and subscripts just like an array), but the query is not an array variable because it hasn’t datatype and its values can’t be altered with assignment statements.
The ToArray method can convert it to an array variable
Dim nums() As Integer = {8, 12, 5, 7, 12, 5}
Dim numQuery = From n In nums
Where n < 10
Select n
Distinct
Dim Small() As Integer = numQuery.ToArray
lstBox.Items.Clear()
For i = 0 To Small.Count - 1
lstBox.Items.Add(Small(i))
Next RESULT ： 8, 5, 7.
Function Procedures in QueriesFunction procedures may be used in Where and Select clauses
Dim nums() As Integer = {5, 7, 8, 11, 12}
Dim numQuery = From n In nums
Where IsPrime(n) = True
Select Triple(n)
For Each m As Integer In numQuery
lstBox.Items.Add(m)
Next
Let Operator A Let operator gives a name to an expression and makes queries easier to read
Function LargestDivisor(ByVal x As Integer) As Integer Dim Max As Integer = 2 For i = 3 To x \ 2 If x Mod i = 0 Then Max = i End If Next Return MaxEnd Function返回合数的最大质因子( 不含 1 和自身 )
Dim nums() As Integer = {15, 7, 39, 11, 21}Dim numQuery = From n In nums Where IsPrime(n) = False Let ZuiDaYinZi = LargestDivisor(n) Select ZuiDaYinZiFor Each m As Integer In numQuery lstBox.Items.Add(m)Next
Order By Operator Order By Operator may sort string values into alphabetical order or sort numbers into numeric order either ascending or descending(Ascending by default)
Dim nums() As Integer = {15, 17, 39, 5, 21, 13}Dim numQuery = From n In nums Where IsPrime(n) = True Order By n Descending Select nFor Each m As Integer In numQuery lstBox.Items.Add(m)Next
Example 5..
Dim stateQuery = From state In IO.File.ReadAllLines("States.txt")
Order By state.Length, state Descending
Select state
For Each state As String In stateQuery
lstOutput.Items.Add(state)
Next
如果想只显示长度大于 5 的州名称，如何改写？如果不需要过滤也不需要其它处理，只是为了对数组进行排序，大可不必动用 LINQ 来进行 Order By ， Array.Sort( 数据名 ) 就可以实现对数组的简单升序排序 。再进行一次 Array.Reverse( 数组名 )就是降序。
DataSource PropertyThe DataSource property fills a list box with the values returned by a query lstBox.DataSource = queryName.ToList
The first entry will be highlighed by default. The highlighting can be eliminated with lstBox.SelectedItem = Nothing
Dim stateQuery = From state In IO.File.ReadAllLines("States.txt")
Order By state.Length, state Descending
Select state
lstOutput.DataSource = stateQuery.ToList
'lstOutput.SelectedItem = Nothing
Binary Search
A large array in ascending order is most efficiently searched with a binary search. The Binary Search method looks for a value in the array by first determining in which half of the array it lies. The other half is then ignored, and the search is narrowed to the retained half.
A statement of the form
numVar = Array.BinarySearch(arrayName, value)
Assigns to numVar the index of an occurrence of the requested value in arrayName. If the value is not found, then a negative number is assigned to numVar.
注意：要想使用二分法查找，这个数组必须是升序的！
Comments and ExercisesComments6.2 P217Exercises6.2 P218
6.3 Arrays of Structures
• Structures• Arrays of Structures• The DataGridView Control• Searching an Array of Structures• Using General Procedures with Structures• Displaying and Comparing Structure Values• Complex Structures (optional)
Structures
A structure is a group of different types of data, Also called a UDT (User Defined Type)
Sample structure definition:
Structure Nation
Dim Name As String
Dim Continent As String
Dim Population As Double 'in millions
Dim Area As Double 'in square miles
End Structure
Structure Definition
Each subvariable in a structure is called a member
To declare a variable of a structure type:
Dim Country As Nation
Each member is accessed via
variableName.memberName
For example:
Country.Name = "China"
Country.continent = "Asia"
Example 1
Dim Country As Nation
Country.Name = "China"
Country.Continent = "Asia"
Country.Population = "1332.5"
Country.Area = "3696100"Dim Country As Nation
Dim line As String = "China,Asia,1332.5,3696100"
Dim data() As String = line.Split(","c)
Country.Name = data(0)
Country.Continent = data(1)
Country.Population = CDbl(data(2))
Country.Area = CDbl(data(3))
Example 1 (continued)
'Display data in text boxes
txtName.Text = country.name
txtContinent.Text = country.continent
txtPop.Text = FormatNumber(1000000*country.population,0)
txtArea.Text = FormatNumber(country.area, 0) & " square miles"
txtDensity.Text = FormatNumber(1000000*country.population / country.area) & " people per square mile"
Text File: UN.txt
The file UN.txt contains 192 countries each per line
There are 4 fields (name, continent, pop in millions, area in square miles) for each country
For Example:Canada,North America,32.9,3855000
France,Europe,63.5,211209
New Zealand,Australia/Oceania,4.18,103738
Australia,Australia/Oceania,21.9,2967909
China,Asia,1332.5,3696100
United States,North America,307.2,3794066
Example 3Dim nations(191) As Nation 'declare arrayDim line, data() As String 'fill with UN.txtDim countries() As String = IO.File.ReadAllLines("UN.txt")For i As Integer = 0 To 191 line = countries(i) 'Such as"China,Asia,1332.5,3696100" data = line.Split(","c) nations(i).name = data(0) nations(i).continent = data(1) nations(i).population = CDbl(data(2)) nations(i).area = CDbl(data(3))Next
Structure Nation
Dim Name As String
Dim Continent As String
Dim Population As Double
Dim Area As Double
End Structure
Example 3: More Partial CodeDim selectedContinent As String = lstContinents.Text
Dim query = From country In nations
Where country.continent = selectedContinent
Order By country.area Descending
Select country.name
For Each nation In query ' 此时 query 的结果都是国家的名称 lstCountries.Items.Add(nation)
Next
Structure College
Structure College
Dim name As String
Dim state As String 'state abbreviation
Dim yearFounded As Integer
End Structure
The file Colleges.txt contains the colleges founded before 1800 in US.
There are 3 fields (name, state, year founded) in each line
For Example:
Harvard U.,MA,1636 哈佛 , 马萨诸塞州 (Massachusetts)
Columbia U.,NY,1754 哥伦比亚大学 , 纽约州 (New York)
Example 4: Earliest CollegesDim colleges() As College
Private Sub frmColleges_Load(…) Handles MyBase.Load
Dim schools() = IO.File.ReadAllLines("Colleges.txt")
ReDim colleges(schools.Count - 1)
Dim line As String
Dim data() As String 'hold data for a single college
For i As Integer = 0 To schools.Count - 1
line = schools(i) 'such as "Columbia U.,NY,1754"
data = line.Split(","c)
colleges(i).name = data(0)
colleges(i).state = data(1)
colleges(i).yearFounded = CInt(data(2))
Next
End Sub
Structure College
Dim name As String
Dim state As String
Dim yearFounded As Integer
End Structure
Example 4: Earliest Colleges Cont.
Private Sub btnDisplay_Click(….) Handles btnDisplay.Click
Dim query = From C In colleges
Where C.state = mtbState.Text.ToUpper
Order By C.name Ascending
Select C.name, C.yearFounded '提取出了其中的两列 lstColleges.Items.Clear()
For Each institution In query
lstColleges.Items.Add(institution.name & " " & institution.yearFounded)
Next
End Sub
How to show information with several columns
• Method 1: Output with ListBox normally• Method 2: Formatting output with zones• Method 3: With DataGridView control
Formatting Output with Zones
Method 2:• Use a fixed-width font such as Courier New • Divide the characters into zones with a format stringDim fmtStr As String = "{0, 15}{1, 10}{2, 8}"
lstOutput.Items.Add(String.Format(fmtStr, data0, data1, data2))
Formatting Output with Zones (Display in tabular form)
DataGridView ControlMethod 3:• Useful when two or more pieces of information are to be displayed• Lies in the Data group or All Windows Forms group of the Toolbox• Displays a table with column headers• AutoSizeColumnsMode property is used to set the width of column.
DataSource Method
When the Select clause of a query contains two or more items, the pair of statements
DataGridView1.DataSource = queryName.ToList
DataGridView1.CurrentCell = Nothing
displays the items of data in a DataGridView control. (The second statement, which is optional, keeps all cells unhighlighted.)
DataGridView Headers
• By default the rows have blank headers and the column headers contain the names of the items in the Select clause
row headers
column headers
DataGridView Headers (cont.)
• Row headers can be removed by setting the RowHeadersVisible property of the DataGridView control to False.
• A column header can be customized with a statement such as DataGridView1.Columns("Name").HeaderText = "University" DataGridView1.Columns("YearlyTuition").HeaderText = "Tuition" DataGridView1.Columns("YearFounded").HeaderText = "Year Founded"
Searching an Array of Structures
The Where clause of a query can be used to locate specific items(定位出那些具有某个特定值的项，例如某个州、某个成立年份等 ). If the query returns a sequence, a method such as First, Last, Max, Min and so on may be used to display a certain item. Example: Dim query = From institution In University
Where institution.State = txtState.Text
Order By institution.YearFounded Ascending
Select institution
txtName.Text = query.First.Name
txtYear.Text = CStr(query.First.YearFounded)
Comments and ExercisesComments6.3 P233Exercises6.3 P234
6.4 Two-Dimensional Arrays
• Declaring a Two-Dimensional Array Variable• Implicit Array Sizing and Initialization• The ReDim Statement• Filling a Two-Dimensional Array with a Text File
Declaring a Two-Dimensional Array Variable
• One-dimensional arrays store a list of items of the same type
• Two-dimensional arrays store a table of items of the same type.
• Consider the rows of the table as numbered 0, 1, 2,…, m and the columns numbered 0, 1, 2, …, n. Then the array is declared with
Dim arrayName(m, n) As DataType• The item in ith row, jth column write as
arrayName(i,j)
Sized ( 指定大小的 ) two-dimensional array
Implicit Array Sizing and Initialization
A two dimensional array can be initialized when it is declared as Dim arrayName(,) As DataType={{row0},{row1},...,{rowN}}
• It declares a two-dimensional array where row0 consists of the entries in the top row of the corresponding table delimited by commas. For Example:Dim marks(,) As Integer={{50,20,80},{40,90,70},{60,10,30}}
50 20 80
40 90 70
60 10 30
Unsized ( 未指明大小的 ) two-dimensional array
Road-Mileage Array
Dim rm(,) As Double = {{0,2054,802,738},{2054,0,2786,2706},
{802,2786,0,100},{738,2706,100,0}}
declares and initializes an array of road-mileages table
Some elements of the array are rm(0,0)=0, rm(0,1)=2054, rm(1,2)=2786
Chicago LA NY Philly
Chicago 0 2054 802 738
LA 2054 0 2786 2706
NY 802 2786 0 100
Philly 738 2706 100 0
GetUpperBound Method
After execution of the statementDim arrayName(m, n) As varType
the value of arrayName.GetUpperBound(0) is m, and the value of arrayName.GetUpperBound(1) is n.
Dim seat(20, 15) As Integer
seat.GetUpperBound(0) is 20seat.GetUpperBound(1) is 15
第几维的上界，下界恒为 0
ReDim a Two-Dimensional Array
An already-created array can be resized with ReDim arrayName(m, n)
no matter it is a sized or unsized array But it will loses the current contents, or with
ReDim Preserve arrayName(m, n)• When Preserve is used, only the columns (the right
most dimension) can be resized• ReDim cannot change the number of dimensions in
the array.如果定义这个多维数组时，根本就不知道每一维的大小。
可以这样定义二维数组 Dim a(,) As DataType可以这样定义三维数组 Dim a(,,) As DataType
……
Filling a Two-Dimensional Array with a Text FileDim rm(3, 3) As Double 'road mileage
Dim rowOfNums() As String =IO.File.ReadAllLines("Distances.txt")Dim line, data() As StringFor i As Integer = 0 To 3 line = rowOfNums(i) 'The string consists 4 numbers data = line.Split(","c) 'The 4 distances For j As Integer = 0 To 3 rm(i, j) = CDbl(data(j)) NextNext Private Sub btnShow_Click(…) Handles btnShow.Click
Dim Row, Col As Integer Row = CInt(txtOrig.Text) Col = CInt(txtDest.Text) If (Row >= 1 And Row <= 4) And (Col >= 1 And Col <= 4) Then txtMiles.Text = CStr(rm(Row - 1, Col - 1)) Else MessageBox.Show(“the numbers must between 1 and 4", "Error") End If End Sub
Comments and ExercisesComments6.4 P245Exercises6.4 P245