Programming Language 2 (PL2)

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1 Programming Language 2 (PL2)

2 Explain the concept of data storage in Random files Apply file manipulation functions for Random files

3 A file opened for random access is assumed to be composed of a set of identical-length records, or sets of fields containing information. You can employ user-defined types to create records made up of numerous fields, each of which can have different data types. Data is stored as binary information.

4 When dimensioning string variables to use as a record type for random access you must specify a length for the string since each record should be of equal length. In other words, use only fixed length strings for storing the records of a file opened for random access. Advantages of Random Access: Compared to Sequential access, you may save significant disk space required by the file. Files can be opened for both read and write at the same time.

5 Disadvantage of Random Access: You can waste space if fields in the records are left blank or if most of the record strings are significantly shorter than the length dimensioned for the string.

6 The bytes in random-access files form identical records where each record contains one or more fields. A record with one field corresponds to any standard type such as integer or fixed-length string. A record with more than one field corresponds to a user-defined type.

7 Figure 1: Sample Records in a File Named Random.xxx In a file opened for random access all records must correspond to one type. All of the records in Figure 1 could correspond to the following user-defined type: Type Person Lname as String*12 Fname as String*8 Age as Integer End Type

8 Before opening a file for random access, you must create a user-defined type that corresponds to the records the file contains. Because records in a file opened for random access must have the same length, user-defined type string elements must have a fixedlength. If the string copied into the file record contains fewer characters than the fixed length of the string variable, the field is padded with trailing spaces; this can create wasted space in the file. If the string copied into the file record contains more characters than the fixed length of the string variable, the string will be truncated.

9 To open a file for random access use the following syntax for the Open statement: Open filename For Random As Filenumber Len=RecordLength Len=RecordLength is used to specify the size of each record. If this value is not correct, access to the file will produce unexpected results. In your macro you can use the Len function to determine the size of the userdefined type or variable that represents a record in the file. To read the data in a file opened for random access use the Get statement. The Get statement has the following syntax: Get #filenumber,[recnumber],varname

10 When you use a file opened for random access, recnumber is the record number at which reading begins and varname is the variable into which data is read. The first record in such a file is at position 1, the second record is at position 2, and so on. If you omit recnumber the next record is read.

11 You can use the following macro to create the file shown in Figure 1.

12 Put is the companion statement to Get. When writing records to files opened for random access use the Put statement. The Put statement has the following syntax: Put [#]filenumber,[recnumber],varname Similar to Get, recnumber is the record number at which writing begins and varname is the variable data is written from. The first record in such a file is at position 1, the second record is at position 2, and so on. If you omit recnumber the next record is written.

13 To create a new file similar to Figure 1 use the following macro.

14 Using both Get and Put statements you can edit a single record by specifying the record number. Use Get to read the record into a variable make the necessary changes, and then use Put to write the record back to the file. For example, to change Richard Thompson's age from 4 to 5 in the file Random.xxx (note that Richard Thompson is the second record), use the following macro:

16 After you run the ChangeRecord macro, the file looks like Figure 2. Figure 2: Records in Random.xxx After Running the ChangeRecord Macro

17 To append records to the end of a file, determine the record number of the new record and write it to the file. To determine the new record number, calculate the number of records in the file and then add one.

18 The AppendRecord macro appends a new record to Random.xxx:

19 After you run the ChangeRecord macro, the file looks like Figure 3. Figure 3: Records in Random.xxx After Running the Macro AppendRecord

20 Before your application opens a file for random access, it should declare all variables required to handle data from the file. This includes userdefined types, which correspond to records in the file, as well as standard types for other variables that hold data related to processing a file opened for random access.

21 Before your application opens a file for random access, it should declare all variables required to handle data from the file. This includes userdefined types, which correspond to records in the file, as well as standard types for other variables that hold data related to processing a file opened for random access.

22 Type Person ID As Integer MonthlySalary As Currency LastReviewDate As Long FirstName As String * 15 LastName As String * 15 Title As String * 15 ReviewComments As String * 150 End Type

23 Because all records in a random-access file must have the same length, it is often useful for string elements in a user-defined type to have a fixed length, as shown in the Persontype declaration above, where, for instance, FirstName and LastName have a fixed length of 15 characters.

24 If the actual string contains fewer characters than the fixed length of the string element to which it is written, Visual Basic fills the trailing spaces in the record with blanks (character code 32). Also, if the string is longer than the field size, it is truncated. If you use variable-length strings, the total size of any record stored with Put or retrieved with Get must not exceed the record length specified in the Open statement s Len clause.

25 To replace records, use a Put statement, specifying the position of the record you want to replace; for example: Put #FileNum, Position, Employee This code will replace the record number specified by Position, with the data in the Employee variable.

26 To add new records to the end of a file opened for random access, use the Put statement shown in the preceding code fragment. Set the value of the Position variable equal to one more than the number of records in the file. For example, to add a record to a file that contains five records, set Position equal to 6. LastRecord = LastRecord + 1 Put #FileNum, LastRecord, Employee

27 You could delete a record by clearing its fields, but the record would still exist in the file. Usually you don t want empty records in your file, because they waste space and interfere with sequential operations. It is better to copy the remaining records to a new file, and then delete the old file.

28 To remove a deleted record in a random-access file 1. Create a new file. 2. Copy all the valid records from the original file into the new file. 3. Close the original file and use the Kill statement to delete it. 4. Use the Name statement to rename the new file with the name of the original file.

File Management. Logical Structure. Positioning Fields and Records. Primary and Secondary Keys. Sequential and Direct Access.

File Management. Logical Structure. Positioning Fields and Records. Primary and Secondary Keys. Sequential and Direct Access. File Management Logical Structure Positioning Fields and Records Primary and Secondary Keys Sequential and Direct Access Binary Search File Management File Indexing Chapter 2 Logical Structure File, on