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Private Sub mnuNew_Click() Dim State As StateRecord, RecordNumber As Integer Dim NewFile As String CommonDialog1.CancelError = True On Error GoTo CancelButton CommonDialog1.Filter = "Data file (*.dat) |*.dat" CommonDialog1.ShowSave NewFile = CommonDialog1.FileName Open NewFile For Random As #1 Len = Len(State) RecordNumber = 1 State.Name = InputBox("State (type END when finished):", "State") Do While UCase(Mid(State.Name, 1, 3)) <> "END" State.Capital = InputBox("Capital:", "Capital") Put #1, RecordNumber, State RecordNumber = RecordNumber + 1 State.Name = InputBox("State (type END when finished):", "State") Loop Close #1 MsgBox (NewFile & " has been created") CancelButton: Exit Sub End Sub Private Sub mnuExit_Click() End End Sub
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When the program is executed, we first see the form shown in Fig. 9.39. If we then select New from the File menu, the Save As dialog box shown in Fig. 9.40 appears. This dialog box allows us to specify a name for the new file. Note that we are currently selecting the name States.dat.
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Fig. 9.39
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Once the new file name has been specified, a pair of input boxes is generated, prompting for the name of the state and the state capital, respectively, as shown in Figs. 9.41 and 9.42. This process continues until the user enters the word End for the name of the state, as shown in Fig. 9.43. The message box shown in Fig. 9.44 then appears, indicating that the new file has been created.
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Fig. 9.40
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Fig. 9.41
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Fig. 9.42
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Fig. 9.43
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Fig. 9.44
Upon completion of the program execution, the random access data file shown in Fig. 9.45 will have been created. This particular data file consists entirely of strings; hence, it can be viewed within a text editor, as in Fig. 9.45. However, this is not true of all random access files. If some of the data items had been numeric rather than strings, we would not have been able to view the file within the text editor.
Fig. 9.45 Although the records within a random access data file are not arranged in any special order, the locations of the records are numbered consecutively from the start of the file, beginning with record number 1 and increasing by one unit for each consecutive data item. A pointer is used to to indicate the location of any particular record. The pointer must be positioned properly before a record can be read from or written to a direct data file. We have already encountered some use of pointer control in the previous example, where the pointer location is determined by assigning values to the variable RecordNumber. Visual Basic uses the LOF function to determine the file size and the Loc function to determine the pointer location. In addition, Visual Basic uses the Seek function to set the pointer for the next file I/O operation.
EXAMPLE 9.10 READING A RANDOM-ACCESS DATA FILE: LOCATING STATE CAPITALS VIA BINARY SEARCH
In this example we present a Visual Basic program that will allow the user to specify the name of a state and have the computer return the name of the state capital. In fact, the user will not have to enter the entire name of the state, but only those first few letters that uniquely identify each state (e.g., Alas for Alaska, Ca for California, Tex for Texas). The search procedure will make use of the random access data file States.dat, created in the last example.
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The basic idea will be to search the data file until the record containing the desired state has been found and then display the corresponding state capital. We will make use of an efficient search technique known as binary search to locate the desired record as quickly as possible. The method is based upon the assumption that the the records are stored in alphabetical order with respect to the names of the states. Thus, the method is intended for a random access data file in which the records are arranged consecutively with respect to the search parameter. To see how the binary search technique works, consider a search interval consisting of several consecutive records within the file. Our overall strategy will be to compare the desired state with the state within the middle record. One of three possible conditions will be obtained. 1. 2. 3. The desired record will be the middle record, in which case we display the corresponding state capital and end the search. The desired record will be in the first half of the search interval. Hence we eliminate the second half of the search interval and compare the desired state with the state within the middle record of the remaining subinterval. The desired record will be in the second half of the search interval. In this case we eliminate the first half of the search interval and compare the desired state with the state within the middle record of the remaining subinterval.
This process is repeated until either the desired record has been found or it has been determined that the desired state cannot be found. The relationship between a given search interval and its two possible subintervals is illustrated in Fig. 9.46.
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