Using Files in Ada Programs

Files need to be:

1. declared
   • file variable set up
2. created/opened/reset
   • disk file linked to file variable
   • file opened for I/O
3. used for I/O
   • PUT, GET, etc
4. closed
   • after I/O finished

An example program shows how the contents of one file can be copied to another. The program includes all of the elements listed above and described below.

File operations

Declaring Text Files

        with TEXT_IO;

        ...

        filevar : TEXT_IO.FILE_TYPE;

Commonly two file variables

• input: file variable called something like filein
• output: file variable called something like fileout

Data type for file variable:

• A type is defined in TEXT_IO, for text  files
• TEXT_IO.FILE_TYPE
• should specify full type name

Preparing Files

Open an existing file for input:

        OPEN   (FILE => filein,
                MODE => IN_FILE,
                NAME => "oldstu.dat");

        OPEN   (filein,  IN_FILE, "oldstu.dat");

Create a new file for output:

        CREATE (FILE => fileout,
                MODE => OUT_FILE,
                NAME => "newstu.dat");

        CREATE (fileout, "newstu.dat");

Open an existing file for output (over-write its contents):

        OPEN   (FILE => fileout,
                MODE => OUT_FILE,
                NAME => "newstu.dat");

        OPEN   (fileout, OUT_FILE, "newstu.dat");

RESET

Sometimes a program needs to process a file twice

• it might write a file first, and then need to read from the file it has just written.
• it might need to read the data in the file twice

You could always close the file, and then reopen it, but Ada provides a shortcut for this situation.

RESET procedure:

• go back to beginning
• (optionally) change mode
• file must be open already

Example: read file twice

        OPEN (filevar, IN_FILE, filename);
        ... code to read from the file ...
        RESET (filevar);
        ... code to read the file all over again ...
        CLOSE (filevar);

Example: write, then read

        CREATE (filevar, OUT_FILE, filename);
        ... code to write to the file ...
        RESET (filevar, IN_FILE);
        ... read the information just written ...
        CLOSE (filevar);

Using files for I/O

PUT, GET etc may have a file variable as first argument

        PUT ("message");                 -- to screen

        PUT (fileout, "message");        -- to file

        SET_COL (fileout, 5);
        PUT     (fileout, "message");

Closing Files

        CLOSE (filevar);

Treating the terminal as a text file

Ada lets you treat the terminal as a text file (or files).

Input from keyboard:

• text file referred to by built-in file variable STANDARD_INPUT
• RETURN <end of line>
• Ctrl-L <end of page>
• Ctrl-D <end of file>

	GET (num);
	GET (STANDARD_INPUT, num);

Output to screen:

• text file referred to by built-in file variable STANDARD_OUTPUT
• NEW_LINE <end of line>
• NEW_PAGE <end of page>
• end main; <end of file>

	PUT ("message");
	PUT (STANDARD_OUTPUT, "message");