Pascal - File Handling
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- Category: Chapter 4
- Published: Monday, 15 April 2013 14:00
- Written by Sternas Stefanos
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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;
1. 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.
2. 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
3. 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
4. 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
5. 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
6. 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 |