Pascal - File Handling

Pascal treats a file as a sequence of components which must be of uniform type. A file's type is determined by the type of the components. File data type is defined as:

type
file-name = file of base-type;
 

Where, the base-type indicates the type of the components of the file. The base type could be anything like, integer, real, Boolean, enumerated, subrange, record, arrays and sets except another file type. Variables of a file type are created using the var declaration:

var
f1, f2,...: file-name;
 

Following are some examples of defining some file types and file variables:

type
   rfile = file of real;
   ifile = file of integer;
   bfile = file of boolean;
   datafile = file of record
   arrfile = file of array[1..4] of integer;
var
   marks: arrfile;
   studentdata: datafile;
   rainfalldata: rfile;
   tempdata: ifile;
   choices: bfile;
 

 

Creating and Writing to a File

Let us write a program that would create a data file for students' records. It would create a file named students.dat and write a student's data into it:

program DataFiles;
type
   StudentRecord=Record
      s_name:AnsiString;
      s_addr:AnsiString;
      s_batchcode:AnsiString;
   end;
var
   Student:StudentRecord;
   f: file;
begin
   AssignFile(f,'students.dat');
   Rewrite(f);
   Student.s_name :='John Smith';
   Student.s_addr :='United States of America';
   Student.s_batchcode :='Computer Science';
   BlockWrite(f,Student,sizeof(StudentRecord));
   CloseFile(f);
end.
 

When compiled and run, the program would create a file named students.dat into the working directory. You can open the file using a text editor, like notepad, to look at John Smith's data.

 

Reading from a File

We have just created and written into a file named students.dat. Now let us write a program that would read the student's data from the file:

program DataFiles;
type
   StudentRecord=Record
      s_name:AnsiString;
      s_addr:AnsiString;
      s_batchcode:AnsiString;
   end;
var
   Student:StudentRecord;
   f: file;
begin
   AssignFile(f,'students.dat');
   reset(f);
   while not eof(f) do
   begin
      Blockread(f,Student,sizeof(StudentRecord));
      writeln('Name: ',Student.s_name);
      writeln('Address: ',Student.s_addr);
      writeln('Batch Code: ',Student.s_batchcode);
   end;
   readln;       // to see the output
   closeFile(f);
end.
 

When the above code is compiled and executed, it produces following result:

Name: John Smith
Address: United States of America
Batch Code: Computer Science

 

Files as Subprogram Parameter

Pascal allows file variables to be used as parameters in standard and user defined subprograms. The following example illustrates this concept. The program creates a file named rainfall.txt, and stores some rainfall data. Next, it opens the file, reads the data and computes the average rainfall.

Please note that, if you use a file parameter with subprograms, it must be declared as a var parameter.

program addFiledata;
const
   MAX =4;
type
   raindata = file of real;
var
   rainfile: raindata;
   filename:ShortString;    
procedure writedata(var f: raindata);
var
   data: real;
   i: integer;
begin
   rewrite(f,sizeof(data));
   for i:=1 to MAX do
   begin
      writeln('Enter rainfall data: ');
      readln(data);
      write(f, data);
   end;
   closeFile(f);
end;
procedure computeAverage(var x: raindata);
var
   d, sum: real;
   average: real;
begin
   reset(x);
   sum:=0.0;
   while not eof(x) do
   begin
      read(x, d);
      sum := sum + d;
   end;
   average := sum/MAX;
   CloseFile(x);
   writeln('Average Rainfall: ', average:7:2);
   readln;  // to see the output              
end;
 
begin
   writeln('Enter the File Name: ');
   readln(filename);
   AssignFile(rainfile, filename);
   writedata(rainfile);
   computeAverage(rainfile);
end.
 

When the above code is compiled and executed, it produces following result:

Enter the File Name:
rainfall.txt
Enter rainfall data:
34
Enter rainfall data:
45
Enter rainfall data:
56
Enter rainfall data:
78
Average Rainfall: 53.25

 

Text Files

A text file, in Pascal, consists of lines of characters where each line is terminated with an end-of-line marker. You can declare and define such files as:

type
file-name = text;
 

Difference between a normal file of characters and a text file is that a text file is divided into lines, each terminated by a special end-of-line marker, automatically inserted by the system. The following example creates and writes into a text file named contact.txt:

program exText;
var
   filename, data:ShortString;
   myfile: text;
begin
   writeln('Enter the file name: ');
   readln(filename);
   AssignFile(myfile, filename);
   rewrite(myfile);
   writeln(myfile,'Note to Students: ');
   writeln(myfile,'For details information on Pascal Programming');
   writeln(myfile,'Contact: Tutorials Point');
   writeln('Completed writing');
   closeFile(myfile);
end.
 

When the above code is compiled and executed, it produces following result:

Enter the file name:
contact.txt 
Completed writing

 

Appending to a File

Appending to a file means writing to an existing file that already has some data without overwriting the file. The following program illustrates this:

program exAppendfile;
var
   myfile: text;
   info:ShortString;
begin
   assignFile(myfile,'contact.txt');
   append(myfile);
   writeln('Contact Details');
   writeln('dddd street');
   closeFile(myfile);
   //let us read fromthis file
   AssignFile(myfile,'contact.txt');
   reset(myfile);
   while not eof(myfile) do
   begin
      readln(myfile, info);
      writeln(info);
   end;
   CloseFile(myfile);
   readln; // to see the output
end.
 

When the above code is compiled and executed, it produces following result:

Contact Details
dddd street
Note to Students:
For details information on Pascal Programming
Contact: Tutorials Point

 

File Handling Functions

Free Pascal provides the following functions/procedures for file handling:

S.N	Function Name & Description
1	procedure Append( var t: Text ); Open a file in append mode
2	procedure AssignFile( out f: file; const Name: ); Assign a name to a file
3	procedure AssignFile( out f: file; p: PChar ); Assign a name to a file
4	procedure AssignFile( out f: file; c: Char ); Assign a name to a file
5	procedure AssignFile( out f: TypedFile; const Name: ); Assign a name to a file
6	procedure AssignFile( out f: TypedFile; p: PChar ); Assign a name to a file
7	procedure AssignFile( out f: TypedFile; c: Char ); Assign a name to a file
8	procedure AssignFile( out t: Text; const s: ); Assign a name to a file
9	procedure AssignFile( out t: Text; p: PChar ); Assign a name to a file
10	procedure AssignFile( out t: Text; c: Char ); Assign a name to a file
11	procedure BlockRead( var f: file; var Buf; count: Int64; var Result: Int64 ); Read data from a file into memory
12	procedure BlockRead( var f: file; var Buf; count: LongInt; var Result: LongInt ); Read data from a file into memory
13	procedure BlockRead( var f: file; var Buf; count: Cardinal; var Result: Cardinal ); Read data from a file into memory
14	procedure BlockRead( var f: file; var Buf; count: Word; var Result: Word ); Read data from a file into memory
15	procedure BlockRead( var f: file; var Buf; count: Word; var Result: Integer ); Read data from a file into memory
16	procedure BlockRead( var f: file; var Buf; count: Int64 ); Read data from a file into memory
17	procedure BlockWrite( var f: file; const Buf; Count: Int64; var Result: Int64 ); Write data from memory to a file
18	procedure BlockWrite( var f: file; const Buf; Count: LongInt; var Result: LongInt ); Write data from memory to a file
19	procedure BlockWrite( var f: file; const Buf; Count: Cardinal; var Result: Cardinal ); Write data from memory to a file
20	procedure BlockWrite( var f: file; const Buf; Count: Word; var Result: Word ); Write data from memory to a file
21	procedure BlockWrite( var f: file; const Buf; Count: Word; var Result: Integer ); Write data from memory to a file
22	procedure BlockWrite( var f: file; const Buf; Count: LongInt ); Write data from memory to a file
23	procedure CloseFile( var f: file ); Close a file
24	procedure CloseFile( var t: Text ); Close a file
25	function EOF( var f: file ):Boolean; Check for end of file
26	function EOF( var t: Text ):Boolean; Check for end of file
27	function EOF: Boolean; Check for end of file
28	function EOLn( var t: Text ):Boolean; Check for end of line
29	function EOLn: Boolean; Check for end of line
30	procedure Erase( var f: file ); Delete file from disk
31	procedure Erase( var t: Text ); Delete file from disk
32	function FilePos( var f: file ):Int64; Position in file
33	function FileSize( var f: file ):Int64; Size of file
34	procedure Flush( var t: Text ); Write file buffers to disk
35	function IOResult: Word; Return result of last file IO operation
36	procedure Read( var F: Text; Args: Arguments ); Read from file into variable
37	procedure Read( Args: Arguments ); Read from file into variable
38	procedure ReadLn( var F: Text; Args: Arguments ); Read from file into variable and goto next line
39	procedure ReadLn( Args: Arguments ); Read from file into variable and goto next line
40	procedure Rename( var f: file; const s: ); Rename file on disk
41	procedure Rename( var f: file; p: PChar ); Rename file on disk
42	procedure Rename( var f: file; c: Char ); Rename file on disk
43	procedure Rename( var t: Text; const s: ); Rename file on disk
44	procedure Rename( var t: Text; p: PChar ); Rename file on disk
45	procedure Rename( var t: Text; c: Char ); Rename file on disk
46	procedure Reset( var f: file; l: LongInt ); Open file for reading
47	procedure Reset( var f: file ); Open file for reading
48	procedure Reset( var f: TypedFile ); Open file for reading
49	procedure Reset( var t: Text ); Open file for reading
50	procedure Rewrite( var f: file; l: LongInt ); Open file for writing
51	procedure Rewrite( var f: file ); Open file for writing
52	procedure Rewrite( var f: TypedFile ); Open file for writing
53	procedure Rewrite( var t: Text ); Open file for writing
54	procedure Seek( var f: file; Pos: Int64 ); Set file position
55	function SeekEOF( var t: Text ):Boolean; Set file position to end of file
56	function SeekEOF: Boolean; Set file position to end of file
57	function SeekEOLn( var t: Text ):Boolean; Set file position to end of line
58	function SeekEOLn: Boolean; Set file position to end of line
59	procedure SetTextBuf( var f: Text; var Buf ); Set size of file buffer
60	procedure SetTextBuf( var f: Text; var Buf; Size: SizeInt ); Set size of file buffer
61	procedure Truncate( var F: file ); Truncate the file at position
62	procedure Write( Args: Arguments ); Write variable to file
63	procedure Write( var F: Text; Args: Arguments ); Write variable to file
64	procedure Writeln( Args: Arguments ); Write variable to file and append newline
65	procedure WriteLn( var F: Text; Args: Arguments ); Write variable to file and append newline