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When I was taking CS in college (mid 80's), one of the ideas that was constantly repeated was to always write loops which test at the top (while...) rather than at the bottom (do ... while) of the loop. These notions were often backed up with references to studies which showed that loops which tested at the top were statistically much more likely to be correct than their bottom-testing counterparts.
As a result, I almost always write loops which test at the top. I don't do it if it introduces extra complexity in the code, but that case seems rare. I notice that some programmers tend to almost exclusively write loops that test at the bottom. When I see constructs like:
if (condition)
{
do
{
...
} while (same condition);
}
or the inverse (if inside the while), it makes me wonder if they actually wrote it that way or if they added the if statement when they realized the loop didn't handle the null case.
I've done some googling, but haven't been able to find any literature on this subject. How do you guys (and gals) write your loops?
I always follow the rule that if it should run zero or more times, test at the beginning, if it must run once or more, test at the end. I do not see any logical reason to use the code you listed in your example. It only adds complexity.
Use while loops when you want to test a condition before the first iteration of the loop.
Use do-while loops when you want to test a condition after running the first iteration of the loop.
For example, if you find yourself doing something like either of these snippets:
func();
while (condition) {
func();
}
//or:
while (true){
func();
if (!condition) break;
}
You should rewrite it as:
do{
func();
} while(condition);
Difference is that the do loop executes "do something" once and then checks the condition to see if it should repeat the "do something" while the while loop checks the condition before doing anything
Does avoiding do/while really help make my code more readable?
No.
If it makes more sense to use a do/while loop, then do so. If you need to execute the body of a loop once before testing the condition, then a do/while loop is probably the most straightforward implementation.
First one may not execute at all if condition is false. Other one will execute at least once, then check the conidition.
For the sake of readability it seems sensible to test at the top. The fact it is a loop is important; the person reading the code should be aware of the loop conditions before trying to comprehend the body of the loop.
Here's a good real-world example I came across recently. Suppose you have a number of processing tasks (like processing elements in an array) and you wish to split the work between one thread per CPU core present. There must be at least one core to be running the current code! So you can use a do... while something like:
do {
get_tasks_for_core();
launch_thread();
} while (cores_remaining());
It's almost negligable, but it might be worth considering the performance benefit: it could equally be written as a standard while loop, but that would always make an unnecessary initial comparison that would always evaluate true - and on single-core, the do-while condition branches more predictably (always false, versus alternating true/false for a standard while).
Yaa..its true.. do while will run atleast one time.
Thats the only difference. Nothing else to debate on this
The first tests the condition before performing so it's possible your code won't ever enter the code underneath. The second will perform the code within before testing the condition.
The while loop will check "condition" first; if it's false, it will never "do something." But the do...while loop will "do something" first, then check "condition".
Yes, just like using for instead of while, or foreach instead of for improves readability. That said some circumstances need do while and I agree you would be silly to force those situations into a while loop.
It's more helpful to think in terms of common usage. The vast majority of while loops work quite naturally with while, even if they could be made to work with do...while, so basically you should use it when the difference doesn't matter. I would thus use do...while for the rare scenarios where it provides a noticeable improvement in readability.
The use cases are different for the two. This isn't a "best practices" question.
If you want a loop to execute based on the condition exclusively than use
for or while
If you want to do something once regardless of the the condition and then continue doing it based the condition evaluation.
do..while
For anyone who can't think of a reason to have a one-or-more times loop:
try {
someOperation();
} catch (Exception e) {
do {
if (e instanceof ExceptionIHandleInAWierdWay) {
HandleWierdException((ExceptionIHandleInAWierdWay)e);
}
} while ((e = e.getInnerException())!= null);
}
The same could be used for any sort of hierarchical structure.
in class Node:
public Node findSelfOrParentWithText(string text) {
Node node = this;
do {
if(node.containsText(text)) {
break;
}
} while((node = node.getParent()) != null);
return node;
}
A while() checks the condition before each execution of the loop body and a do...while() checks the condition after each execution of the loop body.
Thus, **do...while()**s will always execute the loop body at least once.
Functionally, a while() is equivalent to
startOfLoop:
if (!condition)
goto endOfLoop;
//loop body goes here
goto startOfLoop;
endOfLoop:
and a do...while() is equivalent to
startOfLoop:
//loop body
//goes here
if (condition)
goto startOfLoop;
Note that the implementation is probably more efficient than this. However, a do...while() does involve one less comparison than a while() so it is slightly faster. Use a do...while() if:
you know that the condition will always be true the first time around, or
you want the loop to execute once even if the condition is false to begin with.
Here is the translation:
do { y; } while(x);
Same as
{ y; } while(x) { y; }
Note the extra set of braces are for the case you have variable definitions in y. The scope of those must be kept local like in the do-loop case. So, a do-while loop just executes its body at least once. Apart from that, the two loops are identical. So if we apply this rule to your code
do {
// do something
} while (condition is true);
The corresponding while loop for your do-loop looks like
{
// do something
}
while (condition is true) {
// do something
}
Yes, you see the corresponding while for your do loop differs from your while :)
As noted by Piemasons, the difference is whether the loop executes once before doing the test, or if the test is done first so that the body of the loop might never execute.
The key question is which makes sense for your application.
To take two simple examples:
Say you're looping through the elements of an array. If the array has no elements, you don't want to process number one of zero. So you should use WHILE.
You want to display a message, accept a response, and if the response is invalid, ask again until you get a valid response. So you always want to ask once. You can't test if the response is valid until you get a response, so you have to go through the body of the loop once before you can test the condition. You should use DO/WHILE.
I tend to prefer do-while loops, myself. If the condition will always be true at the start of the loop, I prefer to test it at the end. To my eye, the whole point of testing conditions (other than assertions) is that one doesn't know the result of the test. If I see a while loop with the condition test at the top, my inclination is to consider the case that the loop executes zero times. If that can never happen, why not code in a way that clearly shows that?
It's actually meant for a different things. In C, you can use do - while construct to achieve both scenario (runs at least once and runs while true). But PASCAL has repeat - until and while for each scenario, and if I remember correctly, ADA has another construct that lets you quit in the middle, but of course that's not what you're asking.
My answer to your question : I like my loop with testing on top.
Both conventions are correct if you know how to write the code correctly :)
Usually the use of second convention ( do {} while() ) is meant to avoid have a duplicated statement outside the loop. Consider the following (over simplified) example:
a++;
while (a < n) {
a++;
}
can be written more concisely using
do {
a++;
} while (a < n)
Of course, this particular example can be written in an even more concise way as (assuming C syntax)
while (++a < n) {}
But I think you can see the point here.
while( someConditionMayBeFalse ){
// this will never run...
}
// then the alternative
do{
// this will run once even if the condition is false
while( someConditionMayBeFalse );
The difference is obvious and allows you to have code run and then evaluate the result to see if you have to "Do it again" and the other method of while allows you to have a block of script ignored if the conditional is not met.
I write mine pretty much exclusively testing at the top. It's less code, so for me at least, it's less potential to screw something up (e.g., copy-pasting the condition makes two places you always have to update it)
It really depends there are situations when you want to test at the top, others when you want to test at the bottom, and still others when you want to test in the middle.
However the example given seems absurd. If you are going to test at the top, don't use an if statement and test at the bottom, just use a while statement, that's what it is made for.
You should first think of the test as part of the loop code. If the test logically belongs at the start of the loop processing, then it's a top-of-the-loop test. If the test logically belongs at the end of the loop (i.e. it decides if the loop should continue to run), then it's probably a bottom-of-the-loop test.
You will have to do something fancy if the test logically belongs in them middle. :-)
I guess some people test at the bottom because you could save one or a few machine cycles by doing that 30 years ago.
To write code that is correct, one basically needs to perform a mental, perhaps informal proof of correctness.
To prove a loop correct, the standard way is to choose a loop invariant, and an induction proof. But skip the complicated words: what you do, informally, is figure out something that is true of each iteration of the loop, and that when the loop is done, what you wanted accomplished is now true. The loop invariant is false at the end, for the loop to terminate.
If the loop conditions map fairly easily to the invariant, and the invariant is at the top of the loop, and one infers that the invariant is true at the next iteration of the loop by working through the code of the loop, then it is easy to figure out that the loop is correct.
However, if the invariant is at the bottom of the loop, then unless you have an assertion just prior to the loop (a good practice) then it becomes more difficult because you have to essentially infer what that invariant should be, and that any code that ran before the loop makes the loop invariant true (since there is no loop precondition, code will execute in the loop). It just becomes that more difficult to prove correct, even if it is an informal in-your-head proof.
This isn't really an answer but a reiteration of something one of my lecturers said and it interested me at the time.
The two types of loop while..do and do..while are actually instances of a third more generic loop, which has the test somewhere in the middle.
begin loop
<Code block A>
loop condition
<Code block B>
end loop
Code block A is executed at least once and B is executed zero or more times, but isn't run on the very last (failing) iteration. a while loop is when code block a is empty and a do..while is when code block b is empty. But if you're writing a compiler, you might be interested in generalizing both cases to a loop like this.
In a typical Discrete Structures class in computer science, it's an easy proof that there is an equivalence mapping between the two.
Stylistically, I prefer while (easy-expr) { } when easy-expr is known up front and ready to go, and the loop doesn't have a lot of repeated overhead/initialization. I prefer do { } while (somewhat-less-easy-expr); when there is more repeated overhead and the condition may not be quite so simple to set up ahead of time. If I write an infinite loop, I always use while (true) { }. I can't explain why, but I just don't like writing for (;;) { }.
I would say it is bad practice to write if..do..while loops, for the simple reason that this increases the size of the code and causes code duplications. Code duplications are error prone and should be avoided, as any change to one part must be performed on the duplicate as well, which isn't always the case. Also, bigger code means a harder time on the cpu cache. Finally, it handles null cases, and solves head aches.
Only when the first loop is fundamentally different should one use do..while, say, if the code that makes you pass the loop condition (like initialization) is performed in the loop. Otherwise, if it certain that loop will never fall on the first iteration, then yes, a do..while is appropriate.
From my limited knowledge of code generation I think it may be a good idea to write bottom test loops since they enable the compiler to perform loop optimizations better. For bottom test loops it is guaranteed that the loop executes at least once. This means loop invariant code "dominates" the exit node. And thus can be safely moved just before the loop starts.
This question is inspired by this question, which features the following code snippet.
int s;
if((s = foo()) == ERROR)
print_error();
I find this style hard to read and prone to error (as the original question demonstrates -- it was prompted by missing parentheses around the assignment). I would instead write the following, which is actually shorter in terms of characters.
int s = foo();
if(s == ERROR)
print_error();
This is not the first time I've seen this idiom though, and I'm guessing there are reasons (perhaps historical) for it being so often used. What are those reasons?
I think it's for hysterical reasons, that early compilers were not so smart at optimizing. By putting it on one line as a single expression, it gives the compiler a hint that the same value fetched from foo() can be tested rather than specifically loading the value from s.
I prefer the clarity of your second example, with the assignment and test done later. A modern compiler will have no trouble optimizing this into registers, avoiding unnecessary loads from memory store.
When you are writing a loop, it is sometimes desirable to use the first form, as in this famous example from K&R:
int c;
while ((c = getchar()) != EOF) {
/* stuff */
}
There is no elegant "second-form" way of writing this without a repetition:
int c = getchar();
while (c != EOF) {
/* stuff */
c = getchar();
}
Or:
int c;
for (c = getchar(); c != EOF; c = getchar()) {
/* stuff */
}
Now that the assignment to c is repeated, the code is more error-prone, because one has to keep both the statements in sync.
So one has to be able to learn to read and write the first form easily. And given that, it seems logical to use the same form in if conditions as well.
I tend to use the first form mostly because I find it easy to read—as someone else said, it couples the function call and the return value test much more closely.
I make a conscious attempt at combining the two whenever possible. The "penalty" in size isn't enough to overcome the advantage in clarity, IMO.
The advantage in clarity comes from one fact: for a function like this, you should always think of calling the function and testing the return value as a single action that cannot be broken into two parts ("atomic", if you will). You should never call such a function without immediately testing its return value.
Separating the two (at all) leads to a much greater likelihood that you'll sometimes skip checking the return value completely. Other times, you'll accidentally insert some code between the call and the test of the return value that actually depends on that function having succeeded. If you always combine it all into a single statement, it (nearly) eliminates any possibility of falling into these traps.
I would always go for the second. It's easier to read, there's no danger of omitting the parentheses around the assignment and it is easier to step through with a debugger.
I often find the separation of the assignment out into a different line makes debugger watch or "locals" windows behave better vis-a-vis the presence and correct value of "s", at least in non-optimized builds.
It also allows the use of step-over separately on the assignment and test lines (again, in non-optimized builds), which can be helpful if you don't want to go mucking around in disassembly or mixed view.
YMMV per compiler and debugger and for optimized builds, of course.
I personally prefer for assignments and tests to be on different lines. It is less syntactically complicated, less error prone, and more easily understood. It also allows the compiler to give you more precise error/warning locations and often makes debugging easier.
It also allows me to more easily do things like:
int rc = function();
DEBUG_PRINT(rc);
if (rc == ERROR) {
recover_from_error();
} else {
keep_on_going(rc);
}
I prefer this style so much that in the case of loops I would rather:
while (1) {
int rc = function();
if (rc == ERROR) {
break;
}
keep_on_going(rc);
}
than do the assignment in the while conditional. I really don't like for my tests to have side-effects.
I often prefer the first form. I couldn't say exactly why, but it has something to do with the semantic involved.
The second style feels to me more like 2 separate operations. Call the function and then do something with the result, 2 different things. In the first style it's one logical unit. Call the function, save the temprary result and eventually handle the error case.
I know it's pretty vague and far from being completely rational, so I will use one or the other depending on the importance of the saved variable or the test case.
I believe that clarity should always prime over optimizations or "simplifications" based only on the amount of characters typed. This belief has stopped me from making many silly mistakes.
Separating the assignement and the comparison makes both clearer and so less error-prone, even if the duplication of the comparison might introduce a mistake once in a while. Among other things, parentheses become quickly hard to distinguish and keeping everything on one line introduces more parentheses. Also, splitting it up limits statements to doing only one of either fetching a value or assigning one.
However, if you expect people who will read your code to be more comfortable using the one-line idiom, then it is wide-spread enough not to cause any problems for most programmers. C programmers will definately be aware of it, even those that might find it awkward.
Looks like
while( condition ) {
//do stuff
}
is completely equivalent to
for( ; condition; ) {
//do stuff
}
Is there any reason to use the latter instead of the former?
There's no good reason as far as I know. You're intentionally misleading people by using a for-loop that doesn't increment anything.
Update:
Based on the OP's comment to the question, I can speculate on how you might see such a construct in real code. I've seen (and used) this before:
lots::of::namespaces::container::iterator iter = foo.begin();
for (; iter != foo.end(); ++iter)
{
// do stuff
}
But that's as far as I'll go with leaving things out of a for-loop. Perhaps your project had a loop that looked like that at one time. If you add code that removes elements of a container in the middle of the loop, you likely have to control carefully how iter is incremented. That could lead to code that looks like this:
for (; iter != foo.end(); )
{
// do stuff
if (condition)
{
iter = foo.erase(iter);
}
else
{
++iter;
}
}
However, that's no excuse for not taking the five seconds needed to change it into a while-loop.
Some compilers warn about constant loop conditions:
while (true) { /* ... */ } /* Warning! */
for (;;) { /* ... */ } /* No warning */
In the specific case of an infinite loop, I might choose a for loop over a while loop for that reason. But if the condition is not empty, I don't really see any benefit. My guess as to why it appeared in the mentioned project is that the code somehow evolved through maintenance, but was written in a more conventional way originally.
No. No. No.
Even if there were a microscopic performance difference, you'd have to be an end-stage Jedi performance tuner to have it matter enough to care.
Is there any reason to use the latter
instead of the former?
A misguided effort to impress your colleagues that you know that those two forms are equivalent.
A foolish maneuver to ensure "job security" by making your code as confusing as possible so that no one will ever want to change it.
The "w" key on your keyboard is broken.
It started life as a for loop with initializers and incrementing condition, and when the logic changed, the developer was too busy to change it.
It's possible to compile
for(INIT; CONDITION; UPDATE)
{
BODY
}
into
{
INIT
while(CONDITION)
{
BODY
UPDATE
}
}
UPDATE: The seemingly redundant extra scope is to cage any variable definitions in INIT, i.e. from for(int i = 0; ...). Thanks!
It's basically just a reordering of the expressions. So there's no reason to prefer one over the other, for performance reasons. I would recommend while() if possible, since it's simpler. If a simpler construct expresses what you want to do, I think that's the one to use.
As far as I know the two statements are optimized by the compiler into the same assember code anyway.. so no, there's no reason to do so - just personal preference.
I think "while" and "for" loops are meant for different idioms. The idiom of using "while" is "do something, while certain conditions are true". The idiom for "for" is "iterate over a certain range of elements"...
Whenever I read a code, I expect these idioms (and I think I am not alone). When I see "for" I understand, that someone is iterating over the certain range and I do not go into details. When I see the for cycle, used for another idiom (not the one, I expect), I get confused and have to go into details.
Anyway, it is very subjective...
In this case, I personally prefer the first loop as it is easier to write and read.
But if I have a loop that needs to some post statement, I'd use for loop like this:
for (; i < 10; i += 2)
There might be small compiler-dependent differences on the assembly level, but ideally both should behave exactly the same, and the former is more readable. So no, no reson to use the latter version other than nonconformism.
Compile both and check the resulting disassembly, if they are the same (which they probably are). Choose the one you find most readable.
if you want to do something a limited amount of times, then "for" let's you specify the constraint without jumbling it in with the logic inside your loop.
Keeping readability aside for a small while, there is usually no performance difference between the different loops. At least there is no significant difference.
For desktop applications you can chose based on Readability criteria. Refer to the other posts - e.g. looking at for loop someone thinks the incrementor is declared within the loop.
It seems for web applications e.g. client side scripting there might be a difference.
Check this site: http://www.websiteoptimization.com/speed/10/10-2.html
Run your own experiments and go by the results else stick by readability rules.
I can see 2 reasons, none of which I'd consider:
Only have 1 loop construct, but then Kristo's objection stands
write "for (; EVER;)", but then prefer a LOOP_FOREVER macro if really want this.
There really is no difference in C-ish languages between a for (;cond;) loop and a while loop. Generally what I do in C-ish languages is start off writing the loop as a "for" and change it into a "while" if I end up with that form. It is kinda rare though, as you are always iterating through something, and C lets you put any code you want in that last area.
It would be different if C had real (pre-computed iteration) for loops.
You might want to use a do-while loop instead of a for loop so the code is processed at least once before conditions are checked and met (or not).
I used to write some pretty cryptic C/C++ code. Looking back, I would probably do this in a while loop:
ifstream f("file.txt");
char c;
for(f.get(c); !f.eof(); f.get(c)) {
// ...
}
I guess my point is that for loops are usually shorter but less readable, if they're not used in the traditional sense of looping over a range.
This question has been answered - the language has a more natural construct for expressing what you want - you should use it. For example, I can certainly write this:
for (bool b = condition(); b; b = !b) {
/* more code */
}
or:
while (condition()) {
/* more code */
break;
}
instead of the more conventional:
if (condition()) {
/* more code */
}
But why? C (and all languages) have idioms and most of them make rational sense in terms of expressivity and expectation of meaning. When you dick with the idiom, your mess with the sensibilities of the person who has to read your code.
So I have some C++ code for back-tracking nodes in a BFS algorithm. It looks a little like this:
typedef std::map<int> MapType;
bool IsValuePresent(const MapType& myMap, int beginVal, int searchVal)
{
int current_val = beginVal;
while (true)
{
if (current_val == searchVal)
return true;
MapType::iterator it = myMap.find(current_val);
assert(current_val != myMap.end());
if (current_val == it->second) // end of the line
return false;
current_val = it->second;
}
}
However, the while (true) seems... suspicious to me. I know this code works, and logically I know it should work. However, I can't shake the feeling that there should be some condition in the while, but really the only possible one is to use a bool variable just to say if it's done. Should I stop worrying? Or is this really bad form.
EDIT: Thanks to all for noticing that there is a way to get around this. However, I would still like to know if there are other valid cases.
I believe that there are cases where it's fine for seemingly infinite loops to exist. However this does not appear to be one of them. It seems like you could just as easily write the code as follows
while (current_val != searchVal ) {
MapType::iterator it = myMap.find(current_val);
assert(current_val != myMap.end());
if (current_val == it->second) // end of the line
return false;
current_val = it->second
}
return true;
This seems to express the true intent of the loop better
My two cents is: code should be self-documenting. That is, when given a piece of code, I'd rather be able to look and tell the programmer's intent then have to read comments or trudge through the surrounding code. When I read:
while(true)
That tells me the programmer wanted an infinite loop; that the end condition couldn't be specified. This is the programmers intent in some circumstances; a server loop for instance, and that is when it should be used.
In the above code, the loop isn't meant to be forever, it has a clear end condition, and in order to be semantically clear, as others have pointed out:
while (currentVal != searchVal)
works, so the while(true) is clearly inferior and should be avoided in this instance.
There are times and places for infinite loops - I am not convinced this is one of them. On the other hand, it is far from being an egregious problem here.
while (currentVal != searchVal)
{
...
}
return true;
One place to use them is when the process is truly indefinite - a daemon process with a monitor loop that won't terminate.
There are situations where a construct like this makes sense:
The break condition is computed within the loop
There are more breaking conditions and they are all equally important
You really want an endless loop ;) ..
I agree with the other answers that there's no need for an infinite loop in this case.
However, another point might be that when you do have an infinite loop, for(;;) might be a better way to express it. Some compilers generate warnings for while(true) (condition always evaluates to false), and your intent is less clear because it looks like any other loop. Perhaps it used to say while (x == true), and you accidentally removed the x instead of the true. for(;;) says pretty clearly that this is intended to be an infinite loop. Or perhaps you intended to write something like while(t), but Intellisense in your IDE kicked in and decided to autocomplete to true.
for(;;) on the other hand, isn't something you'd ever type accidentally. (and it's easier to search for. while(true) could also be written as while(1))
Neither version is wrong, but for(;;) might be more intuitive because there is no loop condition.
while(true) is used in games for the main game loop - games continually read player input, process interactions between objects and paint your screen, then repeat. This loop continues infinitely until some other action breaks out of that loop (quitting the game, finishing the level).
I tried to quickly find this main loop in the Quake 1 source code for you, but there were at least 50 occurrences of 'while(1)', as well as some written as 'for(;;)', and I wasn't immediately sure which one was the main game loop.
Although I've done them before, I'd vote for always trying to go for the clearer solution by using something readable, which would generally include a valid expression in the while loop--otherwise you're scanning code to look for the break.
I'm not really terrified of them or anything, but I know some people are.
Well, a comment saying that it is not really an infinite loop would help:
while (true) // Not really an infinite loop! Guaranteed to return.
I do agree that it should have a condition, but this is okay in some situations (and it's not always possible or easy to make a condition).
Stop worrying. This is not bad form if it helps to simplify the logic of the code and improve maintainability and readability. Worthwhile though to document in comments on the expected exit conditions and on why the algorithm will not slip into an infinite loop.
Well, yes, but the two pages of code you have to write if you don't want your main loop to be something like while(true) is even worse form.
It is not uncommon to find infinite loops in embedded systems code - often surrounding finite state machines, checking peripheral chips and devices, etc.
I love infinite loops as the outside control structure of a finite state machine. It's effectively a structured goto:
for (;;) {
int c = ReadInput();
if (c == EOF)
return kEOF;
switch (state) {
case inNumber: state = HandleNumber(c); break;
case inToken: state = HandleToken(c); break;
case inWhiteSpace: state = HandleWhiteSpace(c);
default:
state = inError;
break;
}
if (state == inError) ThrowError();
}
Myself and a colleague have a dispute about which of the following is more elegant. I won't say who's who, so it is impartial. Which is more elegant?
public function set hitZone(target:DisplayObject):void
{
if(_hitZone != target)
{
_hitZone.removeEventListener(MouseEvent.ROLL_OVER, onBtOver);
_hitZone.removeEventListener(MouseEvent.ROLL_OUT, onBtOut);
_hitZone.removeEventListener(MouseEvent.MOUSE_DOWN, onBtDown);
_hitZone = target;
_hitZone.addEventListener(MouseEvent.ROLL_OVER, onBtOver, false, 0, true);
_hitZone.addEventListener(MouseEvent.ROLL_OUT, onBtOut, false, 0, true);
_hitZone.addEventListener(MouseEvent.MOUSE_DOWN, onBtDown, false, 0, true);
}
}
...or...
public function set hitZone(target:DisplayObject):void
{
if(_hitZone == target)return;
_hitZone.removeEventListener(MouseEvent.ROLL_OVER, onBtOver);
_hitZone.removeEventListener(MouseEvent.ROLL_OUT, onBtOut);
_hitZone.removeEventListener(MouseEvent.MOUSE_DOWN, onBtDown);
_hitZone = target;
_hitZone.addEventListener(MouseEvent.ROLL_OVER, onBtOver, false, 0, true);
_hitZone.addEventListener(MouseEvent.ROLL_OUT, onBtOut, false, 0, true);
_hitZone.addEventListener(MouseEvent.MOUSE_DOWN, onBtDown, false, 0, true);
}
In most cases, returning early reduces the complexity and makes the code more readable.
It's also one of the techniques applied in Spartan programming:
Minimal use of Control
Minimizing the use of conditionals by using specialized
constructs such ternarization,
inheritance, and classes such as Class
Defaults, Class Once and Class
Separator
Simplifying conditionals with early return.
Minimizing the use of looping constructs, by using action applicator
classes such as Class Separate and
Class FileSystemVisitor.
Simplifying logic of iteration with early exits (via return,
continue and break statements).
In your example, I would choose option 2, as it makes the code more readable. I use the same technique when checking function parameters.
This is one of those cases where it's ok to break the rules (i.e. best practices). In general you want to have as few return points in a function as possible. The practical reason for this is that it simplifies your reading of the code, since you can just always assume that each and every function will take its arguments, do its logic, and return its result. Putting in extra returns for various cases tends to complicate the logic and increase the amount of time necessary to read and fully grok the code. Once your code reaches the maintenance stage then multiple returns can have a huge impact on the productivity of new programmers as they try to decipher the logic (its especially bad when comments are sparse and the code unclear). The problem grows exponentially with respect to the length of the function.
So then why in this case does everyone prefer option 2? It's because you're are setting up a contract that the function enforces through validating incoming data, or other invariants that might need to be checked. The prettiest syntax for constructing the validation is the check each condition, returning immediately if the condition fails validity. That way you don't have to maintain some kind of isValid boolean through all of your checks.
To sum things up: we're really looking at how to write validation code and not general logic; option 2 is better for validation code.
As long as the early returns are organized as a block at the top of the function/method body, then I think they're much more readable than adding another layer of nesting.
I try to avoid early returns in the middle of the body. Sometimes they're the best way, but most of the time I think they complicate.
Also, as a general rule I try to minimize nesting control structures. Obviously you can take this one too far, so you have to use some discretion. Converting nested if's to a single switch/case is much clearer to me, even if the predicates repeat some sub-expressions (and assuming this isn't a performance critical loop in a language too dumb to do subexpression elimination). Particularly I dislike the combination of nested ifs in long function/method bodies, since if you jump into the middle of the code for some reason you end up scrolling up and down to mentally reconstruct the context of a given line.
In my experience, the issue with using early returns in a project is that if others on the project aren't used to them, they won't look for them. So early returns or not - if there are multiple programmers involved, make sure everyone's at least aware of their presence.
I personally write code to return as soon as it can, as delaying a return often introduces extra complexity eg trying to safely exit a bunch of nested loops and conditions.
So when I look at an unfamiliar function, the very first thing I do is look for all the returns. What really helps there is to set up your syntax colouring to give return a different colour from anything else. (I go for red.) That way, the returns become a useful tool for determining what the function does, rather than hidden stumbling blocks for the unwary.
Ah the guardian.
Imho, yes - the logic of it is clearer because the return is explicit and right next to the condition, and it can be nicely grouped with similar structures. This is even more applicable where "return" is replaced with "throw new Exception".
As said before, early return is more readable, specially if the body of a function is long, you may find that deleting a } by mistake in a 3 page function (wich in itself is not very elegant) and trying to compile it can take several minutes of non-automatable debugging.
It also makes the code more declarative, because that's the way you would describe it to another human, so probably a developer is close enough to one to understand it.
If the complexity of the function increases later, and you have good tests, you can simply wrap each alternative in a new function, and call them in case branches, that way you mantain the declarative style.
In this case (one test, no else clause) I like the test-and-return. It makes it clear that in that case, there's nothing to do, without having to read the rest of the function.
However, this is splitting the finest of hairs. I'm sure you must have bigger issues to worry about :)
option 2 is more readable, but the manageability of the code fails when a else may be required to be added.
So if you are sure, there is no else go for option 2, but if there could be scope for an else condition then i would prefer option 1
Option 1 is better, because you should have a minimal number of return points in procedure.
There are exceptions like
if (a) {
return x;
}
return y;
because of the way a language works, but in general it's better to have as few exit points as it is feasible.
I prefer to avoid an immediate return at the beginning of a function, and whenever possible put the qualifying logic to prevent entry to the method prior to it being called. Of course, this varies depending on the purpose of the method.
However, I do not mind returning in the middle of the method, provided the method is short and readable. In the event that the method is large, in my opinion, it is already not very readable, so it will either be refactored into multiple functions with inline returns, or I will explicitly break from the control structure with a single return at the end.
I am tempted to close it as exact duplicate, as I saw some similar threads already, including Invert “if” statement to reduce nesting which has good answers.
I will let it live for now... ^_^
To make that an answer, I am a believer that early return as guard clause is better than deeply nested ifs.
I have seen both types of codes and I prefer first one as it is looks easily readable and understandable for me but I have read many places that early exist is the better way to go.
There's at least one other alternative. Separate the details of the actual work from the decision about whether to perform the work. Something like the following:
public function setHitZone(target:DisplayObject):void
{
if(_hitZone != target)
setHitZoneUnconditionally(target);
}
public function setHitZoneUnconditionally(target:DisplayObject):void
{
_hitZone.removeEventListener(MouseEvent.ROLL_OVER, onBtOver);
_hitZone.removeEventListener(MouseEvent.ROLL_OUT, onBtOut);
_hitZone.removeEventListener(MouseEvent.MOUSE_DOWN, onBtDown);
_hitZone = target;
_hitZone.addEventListener(MouseEvent.ROLL_OVER, onBtOver, false, 0, true);
_hitZone.addEventListener(MouseEvent.ROLL_OUT, onBtOut, false, 0, true);
_hitZone.addEventListener(MouseEvent.MOUSE_DOWN, onBtDown, false, 0, true);
}
Any of these three (your two plus the third above) are reasonable for cases as small as this. However, it would be A Bad Thing to have a function hundreds of lines long with multiple "bail-out points" sprinkled throughout.
I've had this debate with my own code over the years. I started life favoring one return and slowly have lapsed.
In this case, I prefer option 2 (one return) simply because we're only talking about 7 lines of code wrapped by an if() with no other complexity. It's far more readable and function-like. It flows top to bottom. You know you start at the top and end at the bottom.
That being said, as others have said, if there were more guards at the beginning or more complexity or if the function grows, then I would prefer option 1: return immediately at the beginning for a simple validation.