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Suppose you come across an answer that is very close to being right, but still incorrect. The error in the answer is not as trivial as a missing semicolon, an unclosed brace, or a typo in a variable's name - for example, it is a missing pair of parentheses that leads to a logical error.

Is it better to down-vote the answer, explain the nature of the problem in a comment, and wait for the author to produce an edit before reversing your vote, or to edit the answer yourself? Is the strategy the same for recent (<24 hr) answers vs. not-that-recent answers?

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Copy, correct it and post as your own answer :P (NO. But I have seen this, actually...) – H2CO3 Aug 14 '12 at 18:07
up vote 3 down vote accepted

If it's a minor mistake that's likely due to inattention, edit. If it's a fundamental mistake that completely invalidates the answer or a large part of it, downvote, comment to explain what is wrong and provide a better answer. If there's something suboptimal but not fundamentally broken, it's generally better to comment. If you can tell that there's something wrong in a side point but you aren't sure what to correct it to, comment.

For example, suppose a question asks how to test whether a file is a directory on Linux and an answer suggests the stat function and provides this sample code:

stat(path, &st);
if (st.st_mode & S_IFMT == S_IFDIR) …

then edit the answer to add the missing pair of parentheses around st.st_mode & S_IFMT. The main point of the answer is to point the asker to stat and the st_mode field, and the mistake in the sample code is likely due to inattention. On the other hand, if you wanted to recommend using the S_ISDIR macro, this belongs in a comment (because using the S_IFMT mask is not actually wrong), and you might want to post that as a separate answer (or just leave it as a comment). If the question was about operator precedence in C, the code above would warrant a downvote.

There is nothing wrong with editing the code of an answer, as long as what you are changing is a secondary point that does not change the intended meaning of the answer, and you are sure that your correction is valid. Be sure to explain your edit in the edit summary (“added parentheses that are necessary due to operator precedence”).

(On the other hand, never edit code in a question, except to fix badly lacking indentation in a language that isn't sensitive to indentation.)

If the code as posted is potentially dangerous, for example because of the risk of SQL injection, the threshold for editing is lower. If the structure of the code is basically correct but some additional quoting needs to be done, edit the answer to add the necessary quoting. If the code works on good input but cannot be easily fixed to cope with arbitrary input, and nothing indicates that the input is assumed to be pre-sanitized, downvote and explain the problem in a comment. If the question states that the input is already sanitized, but the answer doesn't mention this, it may be worth commenting on the answer to remind readers that the answer assumes sanitized input.

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Edit and/or comment.

For simple things like missing parentheses, I'll either edit it directly, or leave a comment. The comment alone should be enough to either (a) nudge the original answerer to fix it, and/or (b) guide people reading the answer to the complete solution.

Downvoting for something so trivial seems punitive and IMO doesn't contribute anything.

Depending on the answer I might answer it myself if I thought I could significantly improve on it.

But... "better" in what sense, and who's the arbiter?

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You should be wary of editing code for non-trivial changes in an answer. If it's a logical error you should do some combination of the following: comment, downvote, upvote a competing correct answer, provide a competing correct answer. Editing is more for spelling/formatting/grammar/English errors/etc. or for updating a post based on changes since the time of an answer (i.e. updating a link or noting changes in the framework). – Servy Aug 14 '12 at 15:01
@Servy Agreed; I don't think I would make a logical change unless it was something as simple as precedence, and I'd leave a comment to make the change explicit. But missing parens fall into my "it was likely a careless mistake", which I wouldn't necessarily downvote. – Dave Newton Aug 14 '12 at 15:22

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