Re: File Merge
- From: Eric Sosman <esosman@xxxxxxxxxxxxxxxxxxx>
- Date: Fri, 05 Aug 2005 22:38:16 -0400
Michael R. Copeland wrote:
while ((!feof(wf3)) || (!feof(wf1)))
This is the wrong way to check for end-of-file. Please see Question 12.2 in the comp.lang.c Frequently Asked Questions (FAQ) list at
Indeed, but that's why I wanted suggestions how I can improve this kludgey logic. Regardless of the normal method to read a file, I couldn't work out a clean and simple way to loop through both files while performing the merge. I guess I'd need to see how the conventional logic applies to my particular problem (and I couldn't find anything on google on this...).
Hmmm. The FAQ seems pretty clear -- but then again, I have the advantage of already knowing the answer. "It's elementary," as Sherlock always says *after* explaining how he figured it out.
Okay: End-of-input is only detected by attempting a read and having it fail. There's no way to ask "Is there any input left?" without actually attempting an input operation. This may seem a silly restriction in connection with disk files, but consider the situation when input is coming from a keyboard or a network socket or something of the kind: There is no way to predict that the user is about to strike ^D or ^Z or whatever the local end-of-input key sequence is, nor is there any way to predict that the other end of your socket connection is about to hang up the phone on you. You cannot know what happened until it actually happens -- so the only way to know that you have reached end-of-input is to try to read something and get a failure.
Now, end-of-file is only one of the reasons an input attempt might fail: for example, input from a disk could fail in the event of a head crash, or input from a keyboard could fail if you spilled Coke Classic into the mechanism and shorted it out with caramelized sugar. Most of C's input functions report a kind of "generalized failure" no matter what the cause -- and the *only* reason feof() exists is to let you figure out that cause. If the function could read no more input because it detected end-of-file, feof() will be true; if feof() is false, the failure was something like a bad disk sector (and ferror() will be true).
Putting this all together, you should write code that looks something like
if (fgets(fgets(WDBRec, sizeof(WDBRec), wf1) == NULL) {
/* Woops! Couldn't get any more input. Why not? */
if (feof(wf1)){
/* Aha! We've reached end-of-file, and should
* remember the fact and not try to read any more.
*/
}
else {
/* Oh, woe! Oh, woe! The Nazgul have eaten
* the disk controller, and are even now taking
* the Token Ring back to Sauron! If I were to
* test ferror(wf1) at this point it would return
* true, confirming my worst fears.
*/
}
}
else {
/* Whoopee! I got some input data! */
}I hope you understand by now that it is pointless to call feof() or ferror() *before* an input operation; they only make sense after an input operation has already failed.
>> You haven't shown us what copy() is. I'm going to assume >> that it copies the second through sixth characters (that is, >> array elements [1] through [5]) into a six-char array somewhere >> and appends a '\0'. Whether this works depends a lot on the >> location and nature of that intermediate six-char array; see >> Question 7.5 for a description of one all-too-frequent error. > > No, it is a "substr" function that copies character 1-5 of > the input. I didn't include some stuff I felt was extraneous > to the logic, sorry...
Um, er, that's exactly how I described it, is it not? And all the things I said about it (including the Q7.5 reference and the stuff about repeating the final record indefinitely after EOF) still hold.
-- Eric Sosman esosman@xxxxxxxxxxxxxxxxxxx .
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- From: Michael R. Copeland
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