计算机网络英文文献和vb数据库外文文献 第5页
push-to-talk systems ,were installed in several cities beginning in the late 1950s.CB-radio,taxis,and police cars on television programs often use this technology.
In the 1960s,IMTS(Improved Mobile Telephone System) was installed .It ,too ,used a high-powered (200-watt) transmitter ,on top of a hill , but now had two frequencies ,one for sending and one for receiving , so the push-to-talk button was no longer needed . Since all communication from the mobile telephones went inbound on a different channel than the outbound signals ,the mobile users could not hear each other (unlike the push-to-talk system used in taxis).
IMTS supported 23 channels spread out from 150 MHz to 450 MHz .Due to the small number of channels ,users often had to wait a long time before getting a dial tone .Also ,due to the large power of the hilltop transmitter ,adjacent systems had to be several hundred kilometers apart to avoid interference . All in all ,the limited capacity made the system impractical.
In VB6.0 MSChart controls is one function formidable high-level graph tool, has the rich graph plan function, may demonstrate two-dimensional and the three dimensional good chart, the linear chart, the cake chart and so on the many kinds of commonly used graph. Recently I in order to in double struck the graph when some region will the database content which corresponded with this region controlled on in DataGrid demonstrates, encountered many difficulties, finally used one to solve the problem from definition variable SelectSeries, concrete method as follows:
Supposes the database name is " the student information ", has one Access table " the student result ", its content is one class student's test result, includes the student number, the name, the result 3 fields, the result field form is the character, the value is " is superior ", " is good ", " center ", " the difference " center one.
Window Form1 includes one MSChart controls MC$score, the type for the two-dimensional cake chart, uses in to demonstrate each kind of result the student counts; One ADO controls AdScore uses in to connect the database; One DataGrid controls DgScore uses in by the form form demonstration database content.
The work process is: Double strikes the cake chart some region, then DgScore demonstrates the corresponding result the student name list.
Code as follows:
Option Explicit
Dim SelectedSeries as Integer ' from definition variable
Dim Rs () as String ' withdraws the character string array which the record compendium uses
Private Sub Form_Load ()
' Establishes DataGrid controls the data pool
DgScore.DataSource= " AdScore "
' Establishes ADO controls the connection character string and the initial record source, namely demonstration content
AdScore.ConnectString= " Provider=Microsoftc.oledb.3.51; Persist_Security Info=False; Data Source= student 毕业论文
http://www.751com.cn/student result order by result "
AdScore.Refresh
' Supposes in advance withdraws the SQL sentence which the recording uses
Rs (1) = " select * from the student result where result = " + Chr (34) + " is superior " + _Chr (34) + " order the by result "
Rs (2) = " select * from the student result where result = " + Chr (34) + " is good " + _Chr (34) + " order the by result "
Rs (3) = " select * from student result where result = " + Chr (34) + " center " + _Chr (34) + " order by result "
Rs (4) = " select * from student result where result = " + Chr (34) + " difference " _Chr (34) + " order by result "
End Sub
Private Sub MC$score_SeriesSelected (Series as Integer, MouseFlags as _Integer, Cancel as Integer)
SelectedSeries=Series
End Sub
Private Sub MC$score_PointSelected (Series as Integer, DataPoint as_Integer, MouseFlags as
Integer, Cancel as Integer)
SelectedSeries=Series
End Sub
Private Sub MC$score_Db1Click ()
' Changes ADO controls the record source and renovates
Adoc1.RecordSource=Rs (SeletedSeries)
Adoc1.Refresh
End Sub
---- VB does for the rapid application development (RAD) the tool more and more obtains the development personnel's approval and accepts. It to many API (for example ODBC API, SOCKET API and so on) the seal causes the programming to change is simple. At the same time, it supports under the integrated development environment visible, the event actuates, to be object-oriented and so on the programming characteristic. Under, we chat in VB transfer the memory process the realization method and its the matters needing attention.
---- We know, the VB database programming has permits the many kinds of method, for instance straight takes over the use of ODBC the API programming, this method is flexible, is highly effective, the programmer may realize to the database complex control; Also may use in VB the data object, like RDO (long-distance data object), DAO (data visit object), ADO (ActiveX data object), this method realizes conveniently, quickly, but flexible worse somewhat. As a result of memory process in realization data
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