How to Write Comments
Denis Krukovsky writes "Should I write comments? What is a good comment? Is it possible to comment a class in 5 minutes? See "
Everybody knows that good code is self documenting- which is why my prof in college demanded we write in Ada. I instead suggest commenting in haiku.
Just like good documents are self coding...
If I could, I'd destroy you all.
It's like C++ with some thought to design...
Who is Denis Krukovsky, and why should I (or anyone) care what he says?
I did RTFA: it was poorly written and makes some statements that most people would STRONGLY disagree with. For instance:
The point to start writing comments is... when the code is ready to be presented for others.
A comment should tell you why something is in place rather than what the code is doing:
// Check tarriff is null
...
// 30-11-2005 Fixes a null reference exception that occurs later on if no reference is available.
...
A trival example:
Don't do this:
public bool CheckSmsValue(Account smsAccount)
{
if (Account.Tarrif == null)
return;
}
Do do this:
public bool CheckSmsValue(Account smsAccount)
{
if (Account.Tarrif == null)
return;
}
Simon.
The code can tell me what it is doing, but it can't tell me what it is supposed to be doing. The comment should tell me why the code is doing what it is doing. Then I can look at the comment and code together and tell whether the code is right. (And the comment won't have to change as I modify the code: It either stays because the why still exists, or it is removed because it doesn't.)
'Sensible' is a curse word.
When there aren't comments, it is hard to figure out what parts of what do which.
What parts do what should be clear from the names of function calls and variables, but whenever a function becomes longer than something really short, yes, it needs comments describing what happens where. If a function does something complicated, it's worth starting with a comment describing pre- and post conditions.
That said, before you add a comment, first check if you can make the various identifiers any clearer. And then still add the comment, unless it's suddenly become really stupid.
Good comments are written first, before the code, describing what the following code does. It is gramatically correct, punctuated, easy for a stranger to read. It says what the following code does in terms of the real world, not just in terms of other code, unless the sole purpose of the code is to connect other code without relation to anything expressible in real world terms. I prefer my comments to be in the present tense, as if they could be directly compiled themselves. I put comments inside practically every block, like function definitions, loops, conditionals. I often put comment labels after block closers, especially complex conditional sets, embedded loops and functions. That labeling makes it easier to keep track of context within which variables, their scope and the "current task" are in operation. I'd rather spend a few more seconds typing up front, and save a lot of scrolling and delimiter-matching later (not to mention reducing confusion and mistakes).
Code gets shuffled around in different order, read by strangers, and reread much later by yourself, often after you've changed by experience (either in programming or in the task being programmed). Writing the code first is a good way to outline the program, and to detect flaws in your approach. It also gets a little bit of the program done, on screen where you can see it. Often coding to support the comments is more like a cleanup task than starting from scratch.
--
make install -not war
Possibly the best advice I ever read/heard (I can't remember the origin), is to assume that the guy reading your code is perfectly familiar with the language. (Sadly this is usually inaccurate, but moving on.) So he can see what, mechanically, your code is doing. The idea of a comment is to explain how and why you are doing something. What is usually clear from the function name and accompanying documentation (be it doxygen/javadoc style or MSDN style or something else). I.e. if you have some jacked up mega-compound for-loop, a good comment explains why that loop is the way it is, and how it's achieving its goal (and possibly what precisely it's doing). A bad comment would be "this loop increments i, j, k, theta, and cheez_it until the cheez_it is failing to exceed the sum of i, j and the product of i, j, and k". That kind of information is right there in the code.
In short, comments convey concepts and explanations, not mechanical descriptions.
He's used two stupid examples of commenting, examples that are popular jokes, rarely appearing in real life and usually the result of sarcastic nudge-nudging from experienced programmers, and pretended that's what we're talking about when we talk about commenting. When he finally admits they may have a use, the description is so vague it's hard to see what he means - which, if he comments the same way, is probably as true of his code as it is his prose.
It doesn't take much, or add any clutter to code, to put a brief, one or two line, comment before each paragraph of code, that describes the intended functionality of the code block. It makes a massive difference when you revisit your code three years, or even three months, later, or worse have a collegue look at it.
Nor is it a massive imposition to have more obscure decisions you've made be explained in a comment block before the code itself.
Code is not self-documenting. It becomes intensely verbose when you try to make it self-documenting, and it's rare that anyone, no matter how well skilled, can produce something that transmits the intended functionality of the written code in the implemented functionality. This is especially true if you're using an optimal algorithm. Reasonable, non-excessive, use of comments, describing functionality rather than function, are extremely important.
You are not alone. This is not normal. None of this is normal.
Comments can be good,
Avoid 'magic numbers' too,
You've heard of constants?
Seriously, this is not good code: if (u & 0xFF1234) - what the hell is u? Is it the start of the file? What if your file structure changes, you want to grep for every instance of 0xFF1234 and see if it needs to be changed? What if you changed your definition of what a good file is?
Why not: if isValid(fileStart) - or if all you're doing is printing, just put it in the print statment? You do have to comment to explain why you're doing something, but the clearer the code is the easier it is to read and maintain.
"Procedure names should reflect what they do; function names should reflect what they return"
This is one of the most effective methods of producing self-commenting code and I wish everyone writing programs would do this.
As the OP says, code will only ever tell you the "how" not the "why". As in this snippet:
i++ ; increment counter
while trivial, it tells you nothing about why you wish to increment the counter. Ada,C(++) or any other high-level language is always limited to this.
The best comments are a summary at the start of a block of code that describes the autors intent. It should have correct spelling and grammar. if the coder can't even get the coment right - the code is probably wrong, too.
politicians are like babies' nappies: they should both be changed regularly and for the same reasons
It's not uncommon for my code to do something really non-obvious to accomplish a task in a more efficient way. Processing sensor readings, for example - on a PC it'd be a simple floating point math operation. On the chips I use, the floating point library would itself fill the entire available memory. Instead, I wind up with a bit of hard-to-read code that accomplishes the same thing using the shift and multiply operations the CPU is good at. For my own sanity I leave very specific comments about what's going on and what the equivalent calculation is.
The code is the 'How'. What the reader needs to know is 'Why' you are taking these steps. What larger goal are you accomplishing? What is the purpose of this code? What is its justification for existance?
Fill in this blank: "If were weren't running this code right here right now, we wouldn't be able to do _____. We could have done it this other way, but we chose this method because of X, Y, and Z.
In a real world example, code is like "Turn left, Go to High Street, turn right, continue on to 1122 High St, pull into the driveway, and park the vehicle." Those are the steps taken, but the goal you are acommplishing is "We want to return the library books, so we are going to drive the books to the library using the car."
OK, so why are we taking the books to the library? Ultimately all comments will filter up to the goals of the application. They are all nested subgoals of the design specs.
Computers are useless. They can only give you answers.
-- Pablo Picasso
I've always thought that you should be able to tell what is happening from the code... the comments are supoosed to tell you *why* it's doing what it's doing.
The World Wide Web is dying. Soon, we shall have only the Internet.
Sure, well written code should read clearly and be clear about what is happening at every step. But in any larger scale project, no matter how well you make your data structures or how cleanly you encapsulate, eventually you'll code things where the motivation isn't clear.
Good comments don't talk about the code itself, they talk about why the code is doing what it's doing. What the code is doing should be obvious if it's well written, but I've never written a code file that couldn't benefit from a little english exposition.
Cheers.
Boom, you're fired. If you have to ask that you're clearly incompetent.
As to what a good comment is, it's something that gives context to a section of code. Comments aren't supposed to "explain" every step of an algorithm but rather explain why they're there...
e.g.
// for loop from 1 to 5
for (i = 0; i < 5; i++)
// strcmp for "key"
if (!strcmp(strings[i], "key")) dowork();
Could be written better as
// we are going to look for the string "key" in the array
for (i = 0; i < 5; i++)
if (!strcmp(strings[i], "keys")) dowork();
(better yet is to replace '5' with some constant or other label).
In cryptographic tasks I assume the reader has the RFC [or other spec] handy and I just explain what parts of the standard I'm fulfilling, e.g.
// step 3c, xor key with 0x5e
for (i = 0; i < keylen; i++) key[i] ^= 0x5e
That way the reader can follow my code against the spec quicker.
If you're not capable of these sorts of comments it's because you don't think like a developer. You're slinging one line of code against another instead of properly breaking your task down into many smaller more modular tasks which can then be easily expressed on their own.
Tom
Someday, I'll have a real sig.
If you are programming anything non-trivial, you are going to have sections of code that are obscure, and when you have to go back and fix a bug, or add functionality, you won't have any idea what the hell you were doing.
For example, I've written code that had to run on displays with 256 color palettes in windows. It involved saving the current palette when the window gained focus, and then restoring it when the window lost focus. But I couldn't even tell you how I did that now. If I had to go back and look at that code today, I'd have no idea what I was thinking. I do recall that is wasn't actually very many lines of code.
Back before UML was a common thing, I used to 'write' my code in comments and stubs, as a design. After I could read through the code as a narrative of what my app/service/dll did, I would actually fill in the stubs to make it work. This ended up saving me a lot of time in the long run, as I didn't really have much refactoring work to do while coding.
Since when did operating systems become a religion?
"Good code" will be easy to read AND have good comments. For example, my company has some old SQL Reports that need to be maintained on rare occasion. I can kind-of-sort-a make out what the code is doing, but I am not used to that syntax and I havn't been able to find any tutorials for the stuff on the web anywhere. It sure is nice to have comments along with the code that I can read in english what the code is doing. "This will print the subtotals","This pulls the exchange rate", etc. Does that make it a bad programming language? Probably, but it still has to be maintained. Life sure would be alot more difficult for me if the original programmer of those old reports had thought that comments were a waste of his time.
There's no place like ~/
Pretty much my philosophy is to comment any section of code that IS NOT OBVIOUS what it's intent is.
//Loading X with the value 5
//Initialize Y to be false
//Initialize Z to 10.2 because it is important
I mean, any reasonably skilled programmer should be able to look at a block of code and understand what is going on without an excessive description of what the original programmer intended to do. But there are always those cases, especially if the original programmer got crafty and found ways to streamline or optimize the code for performance, where anybody not involved in the original development would just scratch their head and wonder what the heck is going on.
Comments can be very detrimental in many cases. If I get some code that is heavily commented, to the point where the actual code is separated by long blocks of commented code, I just nuke the comments and condense the file. I have actually found files that are thousands of lines long be reduced to only a few hundred lines be removing superfluous comments, and the actual code is easier to understand without the unecessary comments.
NOBODY should ever write a comment like
int x = 5;
I mean, this is a very obvious and exagerated case, but often this happens. It is very obvious what the code is doing, anyone with at least 1 day of programming lessons can understand it easily.
Usually, its more like
bool y = false;
Why should y be initialized to false. I many cases, false is just an arbritrary initial value, but in some cases, the initial condition is important, this importance should be commented and highlighted.
For the most part, comments end up being inaccurate.
double Z = 6.1;
So what do you do here? If your reviewing the code, is 10.2 still the important value, or has a bug been fixed by changing the inital value to 6.1. Is a bug occuring because Z is not 10.2?
As a programmer, one should never blindly read the comments and not review the code. Learning to understand the code makes more sense then deciphering the comments. In most cases, the comments are either superfluous, meaningless, or just wrong. The best skill a programmer can learn is to ignore comments and read the code.
Ultimatly, I comment a block of code to give a general sense of what I am trying to do. I don't go into particulars within a section such as why I am deleting a pointer or loading a value (it should be obvioius what your doing), its the end result that is important, not all the minutia involved in getting there.
Also, I CAN'T stand notation that lists the history of file changes. I mean, the CURRENT code is what your interested in, not what someone did 6 years ago. Knowing that person X modified Line Y in 1992 is of no benefit to my ability to read, understand, fix, or update code in 2005. Often, these modifications refer to code changes that no longer exist in the file. Someone made a fix to code in 2001, but someone in 2003 rewrote the whole code, the 2001 fix is irrelavent. Serious programmers invest in a source control product, like Visual Source Safe, CVS, or SubVersion. These programs STORE the history of a file, there is no need to write a header that can be hundreds of lines long telling you about all the bug fixes and file changes. If you need to review old code, simply go into your source control and compare the file between 2005 and 1992 to find out what is different and changed. Often, most of the people involved in the file's history no longer work at the company.
Lastely, one of the MOST important commenting tricks is to insert nothing at all! A blank line can speak volumes. It can separate functional sections in code, allowing you to understand the flow of the code and realize when certain results are accomplished. I am an object oriented programmer, so seeing blocks of functional units where a blank line separates some operation or result just makes sense (even more if you turn the code section into a class
I haven't thought of anything clever to put here, but then again most of you haven't either.
Everybody knows that good code is self documenting
Sigh. This rubbish again. That a lecturer is promoting this nonesense is even worse.
The "self documentation" you refer to means that you cannot check the documents against the code in order to detect errors. It also means you cannot check the tests against the documents to detect errors - you can only check the tests against the code, which is self defeating.
Good code is commented. The best code in the world that has no comments is
a maintenance nightmare - how can you tell why a particular part of the code
is written this way rather than that way and why is that special case there?
Those that don't believe this either haven't been writing software long enough or have yet to work on a sufficiently large and complex product to
realise the error of their ways.
I'm currently working on a project that is 11,000,000 lines of C++ and assembly including comments. About 7,000,000 without comments and without whitespace. It would be a nightmare without the comments (i.e. "self documenting" - pah).
an distract from actual comprehension of what is going on while reading the code, particularly when there are bits of orphaned antique leftover comments
:)
I know this is a religous topic, but I personally would say that old, left-over comments are simply bad practice. Well-maintained comments and well maintained code are the ideal solution. I don't think there's any excuse for not updating a comment which is right there, in the code you're about to change.
I've suffered from antique comments, and also no comments; IMHO, they are both as bad.
Feel free to flame me now
'No rational religion claims "supernatural" exists, that's an atheist slander.' - seen on slashdot.
What a great example of lousty code and worse comments. How about the folowing... (Slashdot appears to offer a couple different ways to mis-format code; I've chosen incorrect indenting over comments can't start a line)
// reason "blah, blah, blah", which I can not imagine existing,
// certainly deserves a comment, but it applies to the decision
// to say (falsely) "file not found", so it belongs here.
// the only thing a comment here should explain is why you use & and not ==
if (uFileHeader & VALID_FILE_HEADER)
{
printf("File shares at least one set bit with a valid file!\n");
}
else
{
printf("File not found.\n");
}
I'm sure you're not the only one who does it that way, but I'm one of the people who does it the exact opposite way. When I write [new] code, I make a bunch of comments in plain English summarizing what I need my future code to accomplish. Then I group my comments into logical sections and put the actual code underneath each section.
One nice thing about doing it this way is that I don't usually need to re-write my comments even if I change my implementation (unless my original comments weren't abstract enough). A downfall is that someone hunting down a bug in my code might rely too much on my comments and not see the bug. But then again, at least the maintenance programmer will know what I was thinking and what I was trying to accomplish, and will hopefully have a better idea of how to fix it.
I guess my brain just works best if I write the comments first, rather than last. It helps me keep my focus and see the bigger picture as I go...
Code should tell you what
And the code should tell you how
Comment tells you why
As per the subject line, the author of this article is on crack. I'm not going to argue the why's and wherefores of his text, but I have a major objection to his "when". He states that the best time to comment code is once it's all done, and you're just about to submit it. WRONG!
Has he ever worked on a major project? One that cannot be held in one brain in its entirety at one point in time? START with the comments. Start with the program architecture. Decide what each part will do. Write out how each part will accomplish its goals. Then, copy/paste that into your editor, and write the code to match the comments.
Believe me, if you can plan out how everything will work in the first place, and then just follow your plan, the whole project will be much easier. An added bonus is that the code comments just come straight from your design document. Of course, from the tone of the article, I'd guess that this guy's response would be "What design document?"
It may look like I'm doing nothing, but I'm actively waiting for my problems to go away.
--Scott Adams
That's probably useful advice in a general sense, but I can envision examples where commenting a single line might be useful. Consider the case of a complex regular expression: not everyone can glance at one of those monsters and immediately understand exactly what it does. A comment explaining what the thing does and why would be useful to most folks.
I am an old school coder, and I see a lot of this stuff these days:
// Bleah
// Bleah
// Bleah
// Bleah
// Bleah // Bleah // Bleah // Bleah
if (foo) {
}
Why do people put the opening bracket on the same line as the conditional? where the hell did this come from? I see it a lot in JS, and more modern C/C++ code. I always though you were supposed to use carrage returns and tabs to make it easy to see the body of a conditional:
(underscores for whitespace; damn you slashcode!)
if (foo)
{
_____
_____
_____
_____
}
Did I miss something? Are all the 'cool' coders doing this now, and I'm just old?
HA! I just wasted some of your bandwidth with a frivolous sig!
basic programming classes seem to push overcommenting.
good comments should imo cover
1: the why (why am i doing it this way)
2: the why not (why am i not doing this the obvious way)
3: the high level what (though to some extent this can be pointed out through method signatures etc)
4: the low level what in cases where it wouldn't be obvious to someone reasonablly skilled in the language.
However you don't get many of those in trivial programming excercises but the teachers are still supposed to encourage people to use comments. So naturally comments that point out trivialites are the result.
note: i'm known as plugwash most places but i screwd up registering that here somehow in the past and now can't register
If it is obvious from the code, your project is too simple.
(Flippant, but not totally false. I work on research code that does...significantly complicated things. It can be hard enough for me to keep track of the interactions of the algorithms even when I'm designing them on paper; translate them into code, and the result is not at all trivial.
What my code does can be hard to understand when I've made a serious effort to clearly explain what it does in prose; even then, I expect understanding what it's doing to require effort from other researchers in my subfield. To expect any of them---much less a more junior researcher---to understand what is going on from the code alone is simply nonsensical. They would dismiss it as a waste of their time, and rightly so.
If code were that easy to read and understand, it would be found in most computer science research papers; that such papers avoid it like the plague suggests that's not the case, even for less-complicated problems.)