File processing
Introduced| English | Meaning |
|---|---|
| overwrite/ˌəʊvəˈraɪt/ | overwrite |
| APPEND/əˈpend/ | A file-opening mode or operation that writes new data after a file's existing contents, preserving the earlier contents. |
| temporary file/ˈtemprəri faɪl/ | temporary file |
You cannot change one line of a text file
- A program must correct a spelling in the middle of a customer file. It seems obvious: find the line, change it, save.
- Almost no language will let you. A text file is a stream of characters, and the corrected name is a different length from the old one, so every byte after it would have to move.
- So the standard answer is not to edit at all: copy the whole file, changing that one line as it passes, and put the copy in place of the original.
- This lesson is the file operations, that copy-through pattern for updating, and the pitfalls that cause real bugs.
The operations
OPENFILE name FOR READ | WRITE | APPENDopens the file. READ opens an existing file at the start; WRITE creates it, or overwrites 覆盖 an existing one; APPEND 追加 opens it and adds at the end.READFILE name, variablereads the next line.WRITEFILE name, valuewrites one.CLOSEFILE namecloses it.EOF(name)is TRUE when the end has been reached.- Choose the file layout and access method first: serial, sequential and random files serve different operations. The copy-through examples here use sequential access to text records.
- The pattern for reading a whole file is always the same:
OPENFILE "names.txt" FOR READ
WHILE NOT EOF("names.txt") DO
READFILE "names.txt", thisName
OUTPUT thisName
ENDWHILE
CLOSEFILE "names.txt"
Open, use, close, every time
File access route
Follow a file from storage to program and back safely.
What does opening a file FOR WRITE do to existing contents?
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WRITE creates or overwrites the file. To keep existing contents and add more, use APPEND.
Match each file operation to what it does.
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WRITE overwrites, APPEND adds; EOF marks the end; the search loop stops early once the value is found.
Searching a file
- To search, read line by line and stop when the target is found. Two conditions are needed:
WHILE NOT EOF(…) AND NOT found. - The
foundflag matters. Without it the loop reads the whole file even after a match, which is wasteful, and there is nothing to test afterwards to report "not found".
found ← FALSE
OPENFILE "names.txt" FOR READ
WHILE NOT EOF("names.txt") AND NOT found DO
READFILE "names.txt", thisName
IF thisName = target THEN
found ← TRUE
ENDIF
ENDWHILE
CLOSEFILE "names.txt"
When searching a file for a value, the loop condition usually includes:
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Stopping when found avoids reading the rest of the file unnecessarily.
Why does a file search loop use WHILE NOT EOF AND NOT found? Select all that apply.
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Every file must still be closed. The two conditions handle early exit and the end of the file respectively.
Updating: the temporary-file pattern
- Because a text file cannot be edited in place, updating uses a temporary file 临时文件.
- Open the original FOR READ and a temporary file FOR WRITE. Read each line in turn; write it to the temporary file either changed or unchanged.
- Close both files, then replace the original with the temporary file.
- The same pattern does everything: to delete a record, skip writing that line; to insert one, write the new line at the right point as you copy.
To update a text file when in-place editing is not possible, you:
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Copy each line (changed or not) to a temp file, then swap it in for the original.
Put the steps of deleting one record from a text file in order.
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The last step is the one candidates leave out, and without it nothing has changed. The same pattern inserts or edits a record.
Worked example: delete a record
- Describe how a program deletes one record from a text file. [4]
- Open the original file for reading and a temporary file for writing.
- Read each line in turn. If the line is not the one to delete, write it to the temporary file; if it is, do not write it.
- When the end of the original is reached, close both files.
- Replace the original file with the temporary file. All four steps score, and the last one is the one candidates forget.
A program writes 500 records then ends without closing the file, and the last few are missing. Why?
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Writes are buffered for speed. Closing the file flushes the buffer, and it may also release a lock other programs are waiting on.
The pitfalls
- Forgetting to close a file: buffered data may never be written, so the last records vanish, and the file may stay locked against other programs.
- Opening FOR WRITE when APPEND was meant: the entire existing contents are destroyed the instant the file is opened, before a single line is written.
- Reading past EOF: test
EOFbefore every read, not after. - Hard-coded paths: put the filename in a constant so it appears once and the program can be moved.
Forgetting to close a file can lose buffered writes and lock other programs out — so you should always close a file when finished.
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Closing flushes buffered data to disk and releases the lock — a FINALLY block is a good place to guarantee it.
To add today's entries to a log that must keep its history, open the file FOR ____.
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WRITE would destroy every previous entry the instant the file was opened, before a single line was written.
Worked example: which mode
- A program adds today's transactions to a running log that must keep its history. APPEND: WRITE would destroy every previous entry.
- A program produces a fresh report each night, replacing yesterday's. WRITE: creating or overwriting is exactly what is wanted.
- A program reads a configuration file at start-up. READ.
- The mode is chosen by what must happen to the existing contents: kept and added to, destroyed, or left alone.
Marks that slip away
- The update pattern ends by replacing the original with the temporary file. Copying without that final step changes nothing.
- A search loop needs both conditions,
NOT EOFandNOT found, and something to test afterwards. - WRITE overwrites; APPEND adds. Choosing wrongly destroys the file at the moment of opening.
- Always
CLOSEFILE. Say why: buffered data may be lost otherwise.
You've got it
OPENFILE … FOR READ | WRITE | APPEND, thenREADFILEorWRITEFILE, thenCLOSEFILE;EOFis tested before each read- WRITE creates or overwrites, APPEND keeps the contents and adds at the end, READ starts at the beginning
- search with
WHILE NOT EOF AND NOT found, so the loop stops early and "not found" can be reported - update by copying through a temporary file, writing each line changed or unchanged, skipping a line to delete or adding one to insert, then replacing the original