File Processing and Exception Handling

Computer Science A2 · Further Programming. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A file keeps data after the program ends. A file must be opened in a mode before use and closed afterwards. Read mode reads existing data. Write mode makes a new file, and deletes the old contents if the file already exists. Append mode adds data to the end.

A serial file stores records in the order they arrive. A sequential file keeps them in key order. Both are read from the start. A random (direct access) file lets the program go straight to a record at a given position.

An exception is an unexpected event at run time, such as a missing file or division by zero. Exception handling catches it so the program does not crash.

Rules to remember

OPENFILE "F.txt" FOR READ (or WRITE, APPEND, RANDOM); CLOSEFILE "F.txt" when finished.
READFILE "F.txt", Line reads the next line; WRITEFILE "F.txt", Line writes one line; EOF("F.txt") is TRUE at the end of the file.
Read every line with: WHILE NOT EOF("F.txt") ... READFILE ... ENDWHILE.
Random files: SEEK "F.dat", Address moves to a record; GETRECORD reads the record there; PUTRECORD writes a record there.
To insert into a sequential file or delete from a serial or sequential file: copy the records to a new file, adding or leaving out the one record, then replace the old file.
Python: code that may fail goes in try:, the handler goes in except:, and the program then carries on after the handler.
Use exception handling for events the program cannot prevent: file not found, wrong data type entered, division by zero, disk full.

Common mistake

Students open an existing file in write mode to add a record, which wipes everything already in it. Use append mode to add to the end, and remember that read mode on a missing file raises an exception.

Worked example

Temps.txt holds 15, 22, 9, 30 and 18, one on each line. Count ← 0, then the file is opened FOR READ and a WHILE NOT EOF loop does READFILE "Temps.txt", Line and adds 1 to Count IF STR_TO_NUM(Line) > 16. What is Count at the end?

15 is not greater than 16, so Count stays 0. 22 is greater, so Count = 1.
9 is not greater, so Count stays 1. 30 is greater, so Count = 2.
18 is greater, so Count = 3. EOF is now TRUE, so the loop ends and the file is closed.

Answer: Count = 3.

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