Lecture 21: Files

Output to File

Today we are going to learn how to read from files and write to file. As an example we will only learn how to read and write text files, files where the infomation is stored in ASCII format. Such files differ from binary format because they are also readable by humans. We already are using with text files, because all our programs written in the practical lessons are of this type.
These files can be placed on floppy disks, on the harddisk, or even on CD-ROMs (in which case they can - of course - only be read and not written).

Instructions

The following instruction are related to file access:

Var text
Assign
Rewrite
Reset
Close
Read, ReadLn
Write, WriteLn
Eol, Eof

Declaring a variable for file access:

Before we can open a file and have access to it (either reading or writing) we have to declare the existence of a file. In PASCAL we can declare a text file in the following way:

Var filehandle: text;

With filehandle the logical name of the file. This is not equal to the actual name of the file, as we will see in a moment. The place to declare this is together with the files.
Example:
Var f: text;

Assigning a name to the file

Inside the program we should assign an actual name to the file before we can open it:

Assign(filehandle,filename);

The filehandle is the one declared above and the filename is a string (constant or variable) containing the name of our file. For example:
Assign(f, 'MYFILE.TXT');

ReadLn(s);
Assign(f, s);

Input or output?

To open the file, we can use two forms, depending if we want to open the file for input (reading the file) or output (writing to the file):

Reset(filehandle);

Rewrite(filehandle);

For example:
Reset(f);
Rewrite(f);

Reading and Writing

The reading from file and writing to file are now the same as if we were reading from the keyboard or writing to the screen. We use the same instructions (Read, ReadLn, Write, and WriteLn). The only difference is that the first parameter has to be the file we have just opened:

Read(filehandle,...);

ReadLn(filehandle,...);

Write(filehandle,...);

WriteLn(filehandle,...);

Note that we can only use the Read and ReadLn instructions for files that have previously been opened for input and Write and WriteLn are only to be used for files opened for output. Examples:
WriteLn(f, r:0:2);
ReadLn(f, opcao);


Read a variable with the keyboard	Read a variable from file

Closing the file

When we are ready with the file, we must close it. This is especially the case for output files. If we forget to close the file before ending the program, probably not all the information will be written to the file. To close the file we use

Close(filehandle);

for example:
Close(f);

Summary

File input and output consist of the following steps:

Eol, Eof

Two instructions can be useful
Eol(filehandle): returns true if we are reading at the end of a line in the file
Eof(filehandle): returns true if we are reading at the end of the file.

Example:
While NOT Eof(f) do
ReadLn(s);
which will read from the file until the end of the file is encountered.

Examples

code screen file TEST.TXT
after running the program

PROGRAM WithFileOutPut;

Var f: text;
s: string;
i: integer;

begin
WriteLn('Name of File:');
ReadLn(s);
Assign(f, s);
Rewrite(f);
for i := 1 to 10 do
WriteLn(f, i, ' Hello');
Close(f);
end.

Name of File:
TEST.TXT

1 Hello
2 Hello
3 Hello
4 Hello
5 Hello
6 Hello
7 Hello
8 Hello
9 Hello
10 Hello

code screen file TEST.TXT before
running the program

PROGRAM WithFileInPut;

Var f: text;
    c: char;
    s: string;
    i: integer;

begin
WriteLn('Name of File:');
ReadLn(s);
Assign(f, s);
Reset(f);
While NOT Eof(f) do
    begin
       (* read a character
            from file: *)
      Read(f, c);
       (* show it on the screen: *)
      Write(c);
    end;
Close(f);
end.

Name of File:
TEST.TXT
1 Hello
2 Hello
3 Hello
4 Hello
5 Hello
6 Hello
7 Hello
8 Hello
9 Hello
10 Hello

1 Hello
2 Hello
3 Hello
4 Hello
5 Hello
6 Hello
7 Hello
8 Hello
9 Hello
10 Hello

Peter Stallinga. Universidade do Algarve, 30 Abril 2002