Simply put a file is a
binary sequence of some type. In
Delphi, there are three classes of
file:
typed, text, and untyped. Typed files are files that contain data of a particular type, such as Double, Integer or previously defined custom Record type. Text files contain readable ASCII characters. Untyped files are used when we want to impose the least possible structure on a file.
Typed Files
While text files consist of lines terminated with a CR/LF (
#13#10) combination,
typed files consist of data taken from a particular type of data structure.
For example, the following declaration creates a record type called TMember and an array of TMember record variables.
type
TMember = record
Name : string[50];
eMail : string[30];
Posts : LongInt;
end;
var Members : array[1..50] of TMember;
Before we can write the information to the disk we have to declare a variable of a file type. The following line of code declares an F file variable.
Note: To create a typed file in Delphi, we use the following
syntax:
var SomeTypedFile : file of SomeType
The base type (SomeType) for a file can be a scalar type (like Double), an array type or record type. It should not be long string, dynamic array, class, object or a pointer.
In order to start working with files from Delphi, we have to link a file on a disk to a file variable in our program. To create this link we must use AssignFile procedure in order to associate a file on a disk with a file variable.
AssignFile(F, 'Members.dat')
Once the association with an external file is established, the file variable F must be 'opened' to prepare it for reading and/or writing. We call Reset procedure to open an existing file or Rewrite to create a new file. When a program completes processing a file, the file must be closed using the CloseFile procedure. After a file is closed, its associated external file is updated. The file variable can then be associated with another external file.
In general, we should always use
exception handling; many errors may arise when working with files. For example: if we call CloseFile for a file that is already closed Delphi reports an I/O error. On the other hand, if we try to close a file but have not yet called AssignFile, the results are unpredictable.
Write to a File
Suppose we have filled an array of Delphi members with their names, e-mails, and number of posts and we want to store this information in a file on the disk. The following piece of code will do the work:
var
F : file of TMember;
i : integer;
begin
AssignFile(F,'members.dat') ;
Rewrite(F) ;
try
for j:= 1 to 50 do
Write (F, Members[j]) ;
finally
CloseFile(F) ;
end;
end;
Read from a File
In order to retrieve all the information from the 'members.dat' file we would use the following
code:
var
Member: TMember
F : file of TMember;
begin
AssignFile(F,'members.dat') ;
Reset(F) ;
try
while not Eof(F) do begin
Read (F, Member) ;
{DoSomethingWithMember;}
end;
finally
CloseFile(F) ;
end;
end;
Note: Eof is the EndOfFile checking function. We use this function to make sure that we are not trying to read beyond the end of the file (beyond the last stored record).
Seeking and Positioning
Files are normally accessed sequentially. When a file is read using the standard procedure Read or written using the standard procedure Write, the current file position moves to the next numerically ordered file component (next record). Typed files can also be accessed randomly through the standard procedure Seek, which moves the current file position to a specified component. The FilePos and FileSizefunctions can be used to determine the current file position and the current file size.
{go back to the beginning - the first record}
Seek(F, 0) ;
{go to the 5-th record}
Seek(F, 5) ;
{Jump to the end - "after" the last record}
Seek(F, FileSize(F)) ;
Change and Update
You've just learned how to write and read the entire array of members, but what if all you want to do is to seek to the 10th member and change the e-mail? The next procedure does exactly that:
procedure ChangeEMail(const RecN : integer; const NewEMail : string) ;
var DummyMember : TMember;
begin
{assign, open, exception handling block}
Seek(F, RecN) ;
Read(F, DummyMember) ;
DummyMember.Email := NewEMail;
{read moves to the next record, we have to
go back to the original record, then write}
Seek(F, RecN) ;
Write(F, DummyMember) ;
{close file}
end;
Completing the Task
