vb.netexp的简单介绍
我是个VB初学者谁能给我解释一下VB中的 :Exp,log,是干什么的还有怎么用??
log:
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说明VB不支持ln函数，换句话说，VB的log函数就是数学中的ln函数，而常用对数lg只能使用换底公式lg(x)=log(x)/log(10)条件很简单x0。
Log
函数
返回一个
Double，指定参数的自然对数值。
语法
Log(number)
必要的
number
参数是
Double
或任何有效的大于
的数值表达式。
说明
自然对数是以
e
为底的对数。常数
e
的值大约是
2.718282。
如下所示，将
x
的自然对数值除以
n
的自然对数值，就可以对任意底
n
来计算数值
x
的对数值：
Logn(x)
=
Log(x)
/
Log(n)
下面的示例说明如何编写一个函数来求以
10
为底的对数值：
Static
Function
Log10(X)
Log10
=
Log(X)
/
Log(10#)End
Function
exp:
Exp
函数
返回
Double，指定
e（自然对数的底）的某次方。
语法
Exp(number)
必要的
number
参数
number
是
Double
或任何有效的数值表达式。
说明
如果
number
的值超过
709.782712893，则会导致错误发生。常数
e
的值大约是
2.718282。
注意
Exp
函数的作用和
Log
的作用互补，所以有时也称做反对数。
利用VB.NET实现导出DataTable数据到excel中，各位大侠帮帮忙，最好有源码，谢谢，江湖救急
刚好写了个Helper类，你试验一下DataTable2Exce(这个方法代码如下：
Imports System.IO
Imports System.Data
Imports System.Data.OleDb
Public MustInherit Class ExcelHelper
Private Shared Function buildConnStr(excelFilePath As String) As String
Dim excelFileInfo As New System.IO.FileInfo(excelFilePath)
Dim constr As String
If excelFileInfo.Extension = ".xlsx" Then
constr = String.Format("Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties='Excel 12.0 Xml;HDR=YES;IMEX=1'", excelFilePath)
Else
constr = String.Format("Provider=Microsoft.Jet.OLEDB.4.0;Data Source={0};Extended Properties='Excel 8.0;HDR=Yes;IMEX=1'", excelFilePath)
End If
Return constr
End Function
'将datatable导入到excel
Public Shared Function DataTable2Excel(dt As DataTable, excelFilePath As String) As Boolean
If File.Exists(excelFilePath) Then
Throw New Exception("该文件已经存在！")
End If
If dt.TableName.Trim.Length = 0 Or dt.TableName.ToLower = "table" Then
dt.TableName = "Sheet1"
End If
Dim colCount As Integer = dt.Columns.Count
Dim pa(colCount - 1) As OleDb.OleDbParameter
Dim tableStructStr As String = "Create Table " dt.TableName "("
Dim connString As String = buildConnStr(excelFilePath)
Dim objconn As New OleDbConnection(connString)
Dim objcmd As New OleDbCommand
objcmd.Connection = objconn
Dim dataTypeList As New ArrayList
dataTypeList.Add("System.Decimal")
dataTypeList.Add("System.Double")
dataTypeList.Add("System.Int16")
dataTypeList.Add("System.Int32")
dataTypeList.Add("System.Int64")
dataTypeList.Add("System.Single")
Dim i As Integer = 0
For Each col As DataColumn In dt.Columns
If dataTypeList.IndexOf(col.GetType.ToString) 0 Then
pa(i) = New OleDbParameter("@" col.ColumnName, OleDbType.Double)
objcmd.Parameters.Add(pa(i))
If i + 1 = colCount Then
tableStructStr += col.ColumnName + " double)"
Else
tableStructStr += col.ColumnName + " double,"
End If
Else
pa(i) = New OleDbParameter("@" col.ColumnName, OleDbType.VarChar)
objcmd.Parameters.Add(pa(i))
If i + 1 = colCount Then
tableStructStr += col.ColumnName + " VarChar)"
Else
tableStructStr += col.ColumnName + " VarChar,"
End If
End If
i += 1
Next
Try
objcmd.CommandText = tableStructStr
If objconn.State = ConnectionState.Closed Then objconn.Open()
objcmd.ExecuteNonQuery()
Catch ex As Exception
Throw ex
End Try
Dim InsertSql_1 As String = "Insert into " + dt.TableName + " ("
Dim InsertSql_2 As String = " Values ("
Dim InsertSql As String = ""
For colID As Integer = 0 To colCount - 1 Step 1
If colID + 1 = colCount Then
InsertSql_1 += dt.Columns(colID).ColumnName ")"
InsertSql_2 += "@" + dt.Columns(colID).ColumnName + ")"
Else
InsertSql_1 += dt.Columns(colID).ColumnName + ","
InsertSql_2 += "@" + dt.Columns(colID).ColumnName + ","
End If
Next
InsertSql = InsertSql_1 + InsertSql_2
For rowID As Integer = 0 To dt.Rows.Count - 1 Step 1
For colID = 0 To dt.Columns.Count - 1
If pa(colID).DbType = DbType.Double And dt.Rows(rowID)(colID).ToString.Trim = "" Then
pa(colID).Value = 0
Else
pa(colID).Value = dt.Rows(rowID)(colID).ToString.Trim
End If
Next
Try
objcmd.CommandText = InsertSql
objcmd.ExecuteNonQuery()
Catch ex As Exception
Throw ex
End Try
Next
Try
If objconn.State = ConnectionState.Open Then objconn.Close()
Catch exp As Exception
Throw exp
End Try
Return True
End Function
' 获取Excel文件数据表列表Sheets
Public Shared Function GetExcelTables(ExcelFileName As String) As ArrayList
'Dim sheets As New List(Of String)
'conn.Open()
'Dim dt As DataTable = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, Nothing)
'For Each r In dt.Rows
' sheets.Add(r("TABLE_NAME"))
'Next
'conn.Close()
'Return sheets
Dim dt As DataTable
If Not File.Exists(ExcelFileName) Then
Throw New Exception("指定的Excel文件不存在")
Return Nothing
End If
Dim tableList As New ArrayList
Using conn As OleDbConnection = New OleDbConnection(buildConnStr(ExcelFileName))
Try
conn.Open()
dt = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, New Object() {Nothing, Nothing, Nothing, "TABLE"})
Catch ex As Exception
Throw ex
End Try
For i As Integer = 0 To dt.Rows.Count - 1
Dim tableName As String = dt.Rows(i)(2).ToString.Trim.TrimEnd("$")
If tableList.IndexOf(tableName) 0 Then tableList.Add(tableName)
Next
End Using
Return tableList
End Function
'将Excel文件导出至DataTable(第一行作为表头)
Public Shared Function InputFromExcel(ExcelFileName As String, TableName As String) As DataTable
If Not File.Exists(ExcelFileName) Then
Throw New Exception("指定的Excel文件不存在")
End If
Dim tableList As ArrayList = GetExcelTables(ExcelFileName)
If tableList.IndexOf(TableName) 0 Then
TableName = tableList(0).ToString.Trim
End If
Dim dt As New DataTable
Dim conn As New OleDbConnection(buildConnStr(ExcelFileName))
Dim cmd As New OleDbCommand("select * from [" TableName "$]", conn) '调试是否需要$
Dim adapter As New OleDbDataAdapter(cmd)
Try
If conn.State = ConnectionState.Closed Then conn.Open()
adapter.Fill(dt)
Catch ex As Exception
Throw ex
Finally
If conn.State = ConnectionState.Open Then conn.Close()
End Try
Return dt
End Function
'查询excel文件中的一个数据
Public Shared Function ReadOneDataFromExcel(ExcelFileName As String, TableName As String, sql As String) As Object
If Not File.Exists(ExcelFileName) Then
Throw New Exception("指定的Excel文件不存在")
End If
Dim tableList As ArrayList = GetExcelTables(ExcelFileName)
If tableList.IndexOf(TableName) 0 Then
TableName = tableList(0).ToString.Trim
End If
Dim dt As New DataTable
Dim conn As New OleDbConnection(buildConnStr(ExcelFileName))
Dim cmd As New OleDbCommand(sql, conn) '调试是否需要$
Dim ret As Object
Try
If conn.State = ConnectionState.Closed Then conn.Open()
ret = cmd.ExecuteScalar()
Catch ex As Exception
Throw ex
Finally
If conn.State = ConnectionState.Open Then conn.Close()
End Try
Return ret
End Function
'获取Excel文件指定数据表的数据列表columnNames
Public Shared Function GetExcelTableColumns(ExcelFileName As String, TableName As String) As ArrayList
Dim dt As DataTable
If Not File.Exists(ExcelFileName) Then
Throw New Exception("指定的Excel文件不存在")
Return Nothing
End If
Dim ColList As New ArrayList
Using conn As OleDbConnection = New OleDbConnection(buildConnStr(ExcelFileName))
Try
conn.Open()
dt = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Columns, New Object() {Nothing, Nothing, TableName, Nothing})
Catch ex As Exception
Throw ex
End Try
For i As Integer = 0 To dt.Rows.Count - 1
Dim ColName = dt.Rows(i)("Column_Name").ToString().Trim()
ColList.Add(ColName)
Next
End Using
Return ColList
End Function
End Class
VB中的EXP函数
exp(arg) 表示自然对数 e的arg次方,e≈2.71828182845905因此exp(1)就是e的一次方,以此类推exp(0)=1exp(-1)=0.367879441171442exp(-2)=0.135335283236613
VB语言,EXP数字函数:返回以e底,以X为指数的值
exp(3)=e^3
e=2.718281828459045.....
e是个超越数，和圆周率pi类似，具体解说可以百度
exp(n)就是求这个数的n次方
例子
' 求e^13
Dim a as Double
a = exp(13)
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