Chapter 8

FILES

Until now, all data in our programs has been stored in the computer's memory. Data can also be stored in files. This chapter will show how to use text files to store program data, and how to retrieve this data.

Data in a text file can be thought of as a sequence of characters stored in a sequence of lines. Each line has an end-of-line (EOLN) marker after it. Each line has an end-of-file (EOF) marker after the last line in the file.

To access a text file, the file's name must be associated with a variable of type TEXT. This requires two steps. First, a variable of type TEXT must be declared. Second, the filename must be associated to the variable using the ASSIGN statement.

The file must also be opened to access it. When the program finishes accessing the file it must be closed. The RESET, REWRITE, and CLOSE keywords are used for this purpose.

Program Intro_to_Files (input, output);

{This program opens a file }
 {and writes some text to it}

var
  My_File : text;

Begin
 
  {associate data.txt to the}
  { variable My_File}
  Assign(My_File, 'c:\data.txt');   

  Rewrite(My_File);       {open the file}

  {write to My_File}
  Writeln(My_File, 'First line');  
  Writeln(My_File, 'second line');

  Close(My_File);

End.

The first statement in the main program associates the filename with the file variable. The second statement opens the file for output. The third line writes to the file. Notice that the name of the file is the first argument that is passed to the Writeln statement. After writing to the file the close statement closes the file.

The following program will read the output from the previous program. It will use a loop to read the file character by character

Program Intro_to_reading (input, output);

var
  My_File : text;
  ch : char;

Begin
  Assign(My_File, 'c:\data.txt');

  Reset(My_File);  {open the file for input}

   
   Repeat
    
     Repeat

  {read a single character from the file}
        read(My_File,ch);            
        write(ch);         {display it}

  {keep looping until it reaches the EOLN}
     Until EOLN(My_File);       
  

     {move to the next line}
     Readln(My_File,ch); 
     {write to the next line}
     Writeln;                  

 {keep looping until it reaches the EOF} 
   Until EOF(My_File);    


End.