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Once an appropriate data file has been selected, the averages will be calculated and displayed, as shown in Fig. 9.36. Thus, for the data contained in data file New Daily Temperatures.txt, we see that the average daily high temperature is 78.7 F, and the average daily low temperature is 64.7 F.
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Fig. 9.35
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Fig. 9.36
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9.5 RANDOM ACCESS (DIRECT) DATA FILES Random access data files consist of a collection of fixed-length records, each of which is assigned a unique record number. Once a random access file has been opened, an individual record can be accessed by referencing its record number. Information can then be read from the record or written to the record, as required by the individual application. With random access files, the Open statement takes on the form
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Open filename For Random As #n Len = record length
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Open filename As #n Len = record length
since Random is the default file type, in the absence of an explicit specification. The record length (expressed in bytes) must be large enough to accommodate each of the fixed-length data items that will be stored within the random access data file. The record length may be written as an integer constant or an expression. It is particularly common to use the library function Len to return the length, in bytes, of a previously defined data item. Records are usually defined as user-defined data types (see Sec. 2.4). The size of each string member must be included within the data type declaration.
EXAMPLE 9.8
A Visual Basic program that utilizes a random access data file. The program includes the following record declarations.
Private Type Customer Customername As String * 20 AcctNo As Integer Balance As Single End Type Dim OldCustomer As Customer, NewCustomer As Customer
Thus, OldCustomer and NewCustomer are user-defined variables that will be used to represent records. Note that we have specified the maximum length of the string member, since strings do not have a predefined standard size. Thus, each record will consist of 26 bytes (20 bytes for the string member, 2 bytes for the integer member, and 4 bytes for the single-precision member).
Data items are read from a random access data file, one record at a time, via the Get # statement. This statement is written in general terms as
Get #n, record number, data item
or simply
Get #n, , data item
If the record number is not stated explicitly, it is taken to be that of the record following the last Get # statement or the last Put # statement (see below). Data items are written to a random access data file, one record at a time, via the Put # statement. Syntactically, this statement resembles the Get # statement. It is written in general terms as
Put #n, record number, data item
or simply
Put #n, , data item
As with Get #, the record number is taken to be that of the record following the last Get # or Put # statement if it is not shown explicitly.
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EXAMPLE 9.9 CREATING A RANDOM-ACCESS DATA FILE: STATES AND THEIR CAPITALS
The following Visual Basic program creates a random access data file containing the names of the 50 states within the USA and their capitals. Each record will consist of the name of one state and the corresponding state capital. The records will be stored alphabetically, in the same order they are created, although the individual records need not be accessed in this order once the file has been created (see Example 9.10). Fig. 9.37 shows the preliminary control layout. The File menu entries are shown in Fig. 9.38.
Fig. 9.37
Here are the corresponding menu editor entries. Caption
File ....&New ........&Exit
Fig. 9.38
Name
mnuFile mnuNew mnuSep1 mnuExit
The required Visual Basic instructions are shown below. Note that the program defines a 30-byte, user-defined record type called StateRecord, consisting of two 15-byte strings, Name and Capital. Event procedure mnuNew_Click utilizes a variable of this type, called State. Hence, State will consist of two members, called State.Name and State.Capital. In all other respects the program logic should be straightforward, as it is similar to that utilized in earlier programs. Note, however, the use of the Put # statement, rather than the Input # statement, within event procedure mnuNew_Click.
Private Type StateRecord Name As String * 15 Capital As String * 15 End Type Private Sub Form_Load() Form1.Caption = "Creating a Random Access Data File" Label1.Caption = "States and their Capitals" Label1.FontSize = 12 Label1.Alignment = 2 'center text End Sub
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