Lesson 19 · 19 of 20

Text files · Pascal

Write and read text files in Pascal with AssignFile, Rewrite, Append, Reset, ReadLn and CloseFile: a shopping list saved to disk.

Pascal · Text files in DMarlon Studio: the example's code and its output in the terminal

A text file is handled through a variable of type `TextFile`. `AssignFile(f, 'shopping.txt')` links it to the file name; the file is created in the current folder, next to the program.

`Rewrite` creates the file for writing (emptying it if it already existed), and WriteLn(f, ...) writes just as it does to the console, but with the file as the first argument. `CloseFile` closes it and makes sure everything is saved.

`Append` opens the file again and writes at the end, keeping what was there: that is how coffee is added to the list.

`Reset` opens it for reading from the first line. The while not Eof(f) loop reads one line at a time with ReadLn(f, line) until it reaches the end (`Eof`, end of file), numbering each item with count.

Opening a file that does not exist for reading causes a run-time error. You can check first with `FileExists`, from SysUtils: notes.txt does not exist, so it gives FALSE.

Code · main.pas

program main;uses  SysUtils;var  f: TextFile;  line: String;  count: Integer;begin  // Rewrite creates the file (or empties it) to write in it.  AssignFile(f, 'shopping.txt');  Rewrite(f);  WriteLn(f, 'bread');  WriteLn(f, 'milk');  CloseFile(f);  // Append opens it again and writes at the end.  Append(f);  WriteLn(f, 'coffee');  CloseFile(f);  // Reset opens it to read, from the first line.  Reset(f);  count := 0;  while not Eof(f) do  begin    ReadLn(f, line);    Inc(count);    WriteLn(count, '. ', line);  end;  CloseFile(f);  WriteLn('Items: ', count, ' | Is there a notes.txt? ', FileExists('notes.txt'));end.

Output · fpc main.pas && ./main

1. bread
2. milk
3. coffee
Items: 3 | Is there a notes.txt? FALSE

Key points

AssignFile links the variable to the file name.
Rewrite creates, Append adds and Reset opens for reading.
WriteLn(f, ...) and ReadLn(f, ...) write and read lines.
Eof(f) tells you when the end is reached; CloseFile closes.

Exercise

Save another file, prices.txt, with one price per line (2.50, 1.10 and 3.75); read it back with ReadLn(f, price) into a Double variable and print the total.