Working with Typed Files


Typed files are binary files that contain items of identical size. Typed files are usually files that contain records. To create a typed file, you first have to create a new file data type using the following syntax:

type   NewFileType = file of DataType;

The following code illustrates how to create a new file type that can be used to read and write records to a typed file:

type   TPerson = record     FirstName: string[20];     LastName: string[30];   Age: Integer; end; TPersonFile = file of TPerson; 

Notice that string fields in the record declaration have an explicitly defined length. The string length must be explicitly defined because the size of the entire record must be constant. If you want to store the record to a disk file, you cannot use normal strings since their length can change at any time and the compiler cannot determine their length at compile time.

There are several differences between text and typed files:

  • When you reset a typed file you can read and write to the file (you can only read from a text file when you reset it).

  • When you read or write from a typed file, you have to use the Read and Write procedures, not ReadLn and WriteLn.

The following example shows how to work with typed files.

Listing 8-6: Working with typed files

image from book
program Project1; {$APPTYPE CONSOLE} uses   SysUtils; type   TPerson = record     FirstName: string[20];     LastName: string[30];     Age: Integer;   end;   TPersonFile = file of TPerson; procedure ReadRecord(const AFileName: string; var Rec: TPerson); var   F: TPersonFile; begin   AssignFile(F, AFileName);   {$I-}   Reset(F);   {$I+}   if IOResult = 0 then   begin     Read(F, Rec);     CloseFile(F);   end;      // if IOResult end; procedure WriteRecord(const AFileName: string; var Rec: TPerson); var   F: TPersonFile; begin   AssignFile(F, AFileName);   Rewrite(F);   Write(F, Rec);   CloseFile(F); end; procedure DisplayRecord(var Rec: TPerson); begin   WriteLn('First Name: ', Rec.FirstName);   WriteLn('Last Name: ', Rec.LastName);   WriteLn('Age: ', Rec.Age);   WriteLn; end; var   TestRec: TPerson;   ReadRec: TPerson; begin   TestRec.FirstName := 'Stephen';   TestRec.LastName := 'King';   TestRec.Age := 58;   WriteRecord('c:\info.dat', TestRec);   ReadRecord('c:\info.dat', ReadRec);   DisplayRecord(ReadRec);   ReadLn; end.
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Another major difference between text and typed files is that while text files only allow sequential access, typed files allow you to randomly access records in the file. Random access is possible because every record in the file is the same size, and to read a specific record, procedures only have to skip a certain, easily determinable, amount of bytes.

To determine the number of records in a file, you can use the FileSize function. To move to a specific record in the file, you can use the Seek procedure. The Seek procedure accepts two parameters: the file variable and a zero-based integer that specifies the number of the record to move to.

var   F: TPersonFile;   RecCount: Integer; begin   RecCount := FileSize(F);   if RecCount = 0 then     WriteLn('File is empty')   else     Seek(F, FileSize(F)); end.



Inside Delphi 2006
Inside Delphi 2006 (Wordware Delphi Developers Library)
ISBN: 1598220039
EAN: 2147483647
Year: 2004
Pages: 212
Authors: Ivan Hladni

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