1. Brent M. Dingle Texas A&M University Chapter 12 – section 1
CPSC Pascal
Brent M. Dingle Texas A&M University
Chapter 12 – section 1

2. Chapter 12
For this chapter we will really only be concerned with the first section – though we may touch on some of the other sections.

3. Chapter 12.1 – stuff to know assign close read/readln, write/writeln
reset
rewrite
append
close
read/readln, write/writeln
eof
eoln

4. Before the new stuff
Before getting into the new stuff be certain you understand how the commands write, writeln, read and readln work.
We are about to add some functionality to them.
Effectively they will work as you know them to, but they will be writing or reading from a file instead of to the screen or from the keyboard.

5. Files for output
Sooner or later your TA’s are going to get tired of just reading your program’s output on the screen and will want it put in a file.
Fortunately, with very little modification, Pascal will allow you to output stuff to a file instead of to the screen.

6. New TYPE of variable
The secret to outputting text to a file is the variable type TEXT.
This works just like other variables in declaring it: VAR file_ref : text;

7. Associating a variable with a file on the disk.
So once you have a text variable you need to associate with a filename on your disk drive:.
Use assign to do this.
Say the filename was: data.txt
To associate the variable file_ref with data.txt you would type: assign(file_ref, ‘data.txt’);
Can you include a path with the file name?
Try it and see!

8. 3 Options
Once a text variable is associated with a file, you need to say what you plan to do with the file.
You have 3 options:
Read the file.
Write (overwrite) the file.
Append stuff to the file (i.e. add stuff at the end).

9. Preparing to write (overwrite) a file
To PREPARE to write to the file named data.txt you would need a variable of type text declared and TWO lines of code: VAR file_ref : text : : BEGIN assign(file_ref, ‘data.txt’); rewrite(file_ref); : :

10. Actually writing stuff
To actually write to a file is easy, you just use write or writeln – but with an additional parameter.
So if you wanted to write a line of text to the file referenced by file_ref you would type: writeln(file_ref, ‘This line is written to a file’);

11. Ending writing
Before your program ends you MUST close the file you were writing to.
If you do not close the file, some data may be lost – it doesn’t get written to the file.
To close the file add this line before the ’END.’ of your program: close(file_ref);

12. Writing to a File Program Example
PROGRAM WriteStuff;
CONST
BLANK = ‘ ‘;
VAR
file_ref : text;
n : integer;
BEGIN
writeln(‘Program running…’);
assign(file_ref, ‘data.txt’);
rewrite(file_ref);
writeln(file_ref, ‘Howdy’);
writeln(file_ref, 3, BLANK, 4);
n := 5;
writeln(file_ref, n);
close(file_ref);
writeln(‘… end program’);
END.

13. Program Result
The above program WriteStuff would create or overwrite the filenamed data.txt.
Usually you will specify a path with the file name.
For example if you wanted data.txt created on the root directory of the a: floppy disk you would change assign(file_ref, ‘data.txt’); to: assign(file_ref, ‘a:\data.txt’);

14. Appending to a file
Appending is similar to writing – except anything you write or writeln will be tacked onto the end of the file, rather than overwriting what was previously in the file.
There is one difference and that is in the preparation to append to a file .

15. Preparing to Append to a file
To PREPARE to append to the file named data.txt you would need a variable of type text declared and TWO lines of code: VAR file_ref : text : : BEGIN assign(file_ref, ‘data.txt’); append(file_ref); : :

16. Appending – Close the file
When you are done appending to the file…
As with writing to a file, you must remember to close the file: close(file_ref);

17. Preparing to Read a file
To PREPARE to read the file named data.txt you would need a variable of type text declared and TWO lines of code: VAR file_ref : text : : BEGIN assign(file_ref, ‘data.txt’); reset(file_ref); : :

18. Actually reading stuff
To read something from a file is pretty easy – just use read or readln.
Of course you need to make sure your type of variables are set correctly and your file is in the correct format.

19. Before our example
Before you can read a file you need to know at least:
if it exists
what format its contents are in
Assume we have a file named stuff.txt that contains the following: Si 7 3
Now we want a program to read that file and display its contents on the screen…

20. Reading to a File Program Example
PROGRAM ReadStuff;
VAR
in_file : text;
first_letter, second_letter : char;
n1, n2 : integer;
BEGIN
assign(in_file, ‘stuff.txt’);
reset(in_file);
writeln(‘Program begins…’);
readln(in_file, first_letter, second_letter);
writeln(first_letter, second_letter);
readln(in_file, n1);
readln(in_file, n2);
writeln(n1);
writeln(n2);
close(in_file);
writeln(‘… end program’);
END.

21. Read Program Result
Assuming all went well, the ReadStuff program above would output: Si 7 3
Notice we had to be certain to read our variables in the correct order so their type matched that of what was being read.
Could we have made everything of type char?
Try it and find out !

22. Reading – Close the file
When you are done reading from the file…
As with writing and appending to a file, you must remember to close the file. Notice near the end of the above program was the statement: close(in_file);

23. Other stuff eof(file_ref) eoln(file_ref)
Is used to check if you have reached the End Of the File.
eoln(file_ref)
Is used to check if you have reached the End Of the Line.
eof tends to be used more than eoln.
A program example using eof should be found in the sample program list – the reading a file example.

24. Near the end
Well, that is all I’m going to explicitly talk about in Chapter 12.
You may wish to read more about assign, eof, eoln, reset, rewrite and append as they may be useful in some of your assignments.
I would encourage you to try doing Exercises 3, 4, 5 and 6 on page 471. Who knows… maybe one of them might be an assignment later… =)

25. End of Chapter 12