Header in Untyped Files

and this about typed files:

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Typed files or data files used to store homogeneous data such as bytes, integers, records, user-defiened types, and other Object Pascal types.

Typed files in Object Pascal

Typed files in Object Pascal declaration depends on record type but the main structure of declaration is:

var
  F: file of <Type>;

Type can be any object Pascal type but not Object such as:

var
  IntF: file of integer;
  CharF: file of char;
  RecF: file of TRec;

The most important procedures and functions used with typed files are below:

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Operation                            proc/func
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Declaration                           var  F: file of <Type>;
Assigning                             AssignFile(F);
Opening for reading               Reset(F); or Rewrite(F);
Opening for writing                Rewrite(F) or Reset(F) with FileMode = 2
Opening for read/write           Reset(F) with FileMode = 2
Appending                           FileMode:= 2; Reset(F); Seek(F, FileSize(F));
Reading                               Read(F, X); { X is a variable of same file type }
Writing                                 Writ(F, X);
Goto random position            Seek(F, Position); { Position is a number of desired record }
Closing file                           CloseFile(F);
Getting records count            FileSize(F);
Getting current position         FilePos(F);
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Typed file is organized as random access file so that you can open it in reading/writing mode and you can goto any record directly using Seek procedure.

Reset in typed file opens file for reading, writing, or reading and writing mode according to FileMode variable:

If FileMode = 0  that means read only
If FileMode = 1  that means write only
If FileMode = 2  that means read and write access

Befor openning file with reset make sure that the file is exists.

Rewrite in typed file create a new file and it overwrite the file if it is already exists. When you create file with Rewrite you can read and write from and to file.

Append does not work with typed files, insead you must open file with reset and goto beyond last record such as:

FileMode:= 2;  // Read and write mode
Reset(F);
Seek(F, FileSize(F));

If file size if 5 that means there is 5 records in file from 0 to 4 so that going to record number 5 will ensures writing on an empty record. This works only if file exists but it will rais an exception if file does not exists, so that you must write:

AssignFile(F, FileName);
FileMode:= 2;  // Read and write mode
if FileExists(FileName) then
  Reset(F)
else
  Rewrite(F);  // Create new file
Seek(F, FileSize(F));





Example:

(*********  Save data  **********)

procedure SaveData;
var
  F: file of integer;
  Data: integer;
begin
  AssignFile(F, 'Test.Dat');
  Rewrite(F);
  Repeat
    Data:= StrToInt(InputBox('Data entry',
      'Enter any numbers, enter 0 to stop data entry', ''));
    Write(F, Data);
  until Data = 0;
  CloseFile(F);

end;

(*********  Read data  **********)

procedure ReadData;
var
  F: file of integer;
  Data: integer;
begin
  AssignFile(F, 'Test.Dat');
  FileMode:= 0;  // Read only
  Reset(F);
  while not Eof(F) do
  begin
    Read(F, Data);
    ShowMessage(IntToStr(Data));
  end; // while
  CloseFile(F);

end;


Notes:

- If you want to open typed file for reading you must set FileMode to  0 because if you do not assign 0 to FileMode the default value of it will be 2 which means read and write access. If you open a read-only file or a file in CD-ROM and you want to read it, openning it with reset means read and write if FileMode = 2 which will generates access denied error and prevent you from reading file.

- Records in typed files have a fixed length, so that it does not require any seperator, example if you write three records in a typed file and record size is 10 bytes, actual file size will be 30.

- You can not  write long string in typed file, instead you have to determine it's size such as:

type
  TLine = String[30];
var
  F: file of TLine;
  Line: TLine;


---------------

Motaz

hi wimhhf;

Your header:

Type
  THeader = Record
     ID      : String[4];
     Version : Word;
     Author  : String[30];
  End;

VAR
  myHeader : tHeader;
  f        : File;
Begin
 // Init. your Header
 AssignFile(f,myHeader );
 Rewrite(f,1);
 BlockWrite ( f,Header,SizeOf(Header));
 // Read other Blocks
 CloseFile(f);
End.


The difference between typed and untyped files:

Typed Files:
  Example :

    TAdr = Record
      Name : String[30];
      Tele : String[30];
    End;

VAR
  F : File OF TADR;
  Rec : TAdr;

To Read an Addres form the File Read ( f, Rec);
To Write an Addres to the file ( f, Rec);

Best Regards

Cesario

u can use a typed file and use the first record for the header. This is how I do. I think its the easyest way

Thanks for your detailed answer, but I have still a question.

I have 2 records:

TPerson = record
  ID: LongInt;
  Name: string[15];
  Phone: string[10];
end;

TOther = record
  Owner = string[20];
  Date = string[20];
  Count = LongInt;
end;

Is it possible to make a typed file where I store records of TPerson.
But in the beginning of the typed file I put a record of TOther (like a header).

or in a different way:

Is it possible to make a untyped file where I store records of TPerson and other records/integer/strings.
But I don't understand exactly how to read/write the records and the strings like I do in a typed file...

In short: I'd like to store records in a file, but I'd to store also other variables in the same file. I know it is possible, but how to do is a bit fuzzy for me.

Greetings,

Wim

You can do this:

var
  Person: arra [1 .. 20] of TPerson;
  Other: TOther;
  F: file;
  i: Integer;
begin
  Assign..
  Rewrite(F, 1);
  // Write other as a header:
  BlockWrite(F, Other, SizeOf(Other));
 
  // Write persons:
  for i:= 1 to 20 do
    BlockWrite(F, Person[i], SizeOf(TPerson));
  CloseFile(F);
end;

to read:

  BlockRead(F, Other, SizeOf(Other), NumRead);
  i:= 0;
  while not eof (F) do
  begin
    Inc(i);
    BlockRead(F, Person[i], SizeOf(TPerson), NumRead);
    // Display the records
  end;
  CloseFile(F);


I hope you mean this

Motaz

Thanks!