File Handling

Common mistakes that lose marks (and lose data in real programs):

• Opening a file in WRITE mode when you meant to keep its existing data. WRITE mode overwrites the file, destroying all existing contents.
• Forgetting CLOSEFILE. Buffered data may not reach the disk; the file may be incomplete or corrupt.
• Reading past the end of the file without checking for it. There is no EOF function in CIE pseudocode, so always test the line you have just read against a sentinel value and stop the loop with a Boolean flag.
• Opening a file that does not exist in READ mode. Causes a runtime error; check it exists first if necessary.
• Mixing modes. A single OPENFILE opens for one specific mode. To switch between reading and writing, close and reopen the file.
• Forgetting to handle blank lines when reading. An empty line is often used as the end-of-file sentinel in the CIE pseudocode pattern, so be careful with files that genuinely contain blank lines in the middle.
• Always make a backup of important text files before writing to them. A program with a bug can wipe out hours of work in milliseconds.