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)