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My friend showed me this and I have no idea how it works and what it's called. Can someone explain to me how it loops the way it does? For example:
for(;;){
cout << "loop" << endl;
}
It will just keep looping the string forever. This kind of loop can be used for anything. How does this work?
According tho the language specification, empty condition in for iteration statament is equivalent to true condition.
6.5.3 The for statement
1 The for statement
for ( for-init-statement conditionopt; expressionopt) statement
is equivalent to
{
for-init-statement
while ( condition ) {
statement
expression ;
}
}
...
2 Either or both of the condition and the expression can be omitted. A missing condition makes the implied while clause equivalent to while(true).
So, the loop loops forever. That's all there is to it.
It loops infinitely as no initialization, conditional and increment values are passed in the parameters of the loop. A typical for loop takes parameters as follows: (<initialization>;<conditional>;<increment>)
This post explains it quite well in my opinion. See the answer by spex:
Why can the condition of a for-loop be left empty?
With the structure of the for loop being for(clause; expression-2; expression-3){}, when expression-2 is left out it is replaced with a nonzero constant. This is the part of the loop that determines whether it should keep looping or not. As a nonzero constant evaluates to true, it becomes an infinite loop.
That for loop essentially says the following three things (each separated by the semicolons in your for loop "header?"):
Don't initialize anything.
Don't break from the loop.
Perform no afterthoughts for each loop iteration.
Wikipedia's for loop page actually has a section about this.
As many have pointed out, it is equivalent to while (1).
When is it useful? Wherever you need an infinite loop such as:
A game loop - would be kinda useful to have the game, loop indefinitely until the user decides to quit the game.
OS scheduler - The scheduler needs to loop indefinitely, scheduling processes according to some algorithm until the OS stops
An intepreter - If you have ever programmed in python, you may have come across the interpreter which lets you type some command and then executes it. This is also implemented using a similar infinite loop
In all those examples, the common factor that leads to using an infinite loop is that the terminating condition is not known or the terminating condition is complex (game loops for example)
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Whenever I need to break out from a for(unsigned int i=0;i<bound;++i) expression in C++, I simply set the index variable i=bound, the same way as described in this answer. I tend to avoid the break statement because, honestly, I have no good understanding of what it actually does.
Compare the two instructions:
for(unsigned int i=0;i<bound;++i) {
if (I need a break) {
break;
}
}
and
for(unsigned int i=0;i<bound;++i) {
if (I need a break) {
i=bound;
}
}
I speculate that the second method does one extra variable set and then one extra comparison between i and bound, so it looks more expensive, from performance point of view. The question is then is it cheaper to call break, then doing these two tests? Are the compiled binaries any different? Is there any instance, where the second method breaks, or can I safely choose either of these two alternatives?
Related: Does `break` work only for `for`, `while`, `do-while`, `switch' and for `if` statements?
Breaking out of a loop without a break statement [C]
Using break will be more future proof and more logical.
Consider the following example,
for (i = 0; i < NUM_OF_ELEMENTS; i++)
{
if(data[i] == expected_item)
break;
}
printf("\n Element %d is at index %d\n", expected_item, i);
But the second method won't be useful here.
There are three main technical differences that come to mind:
as other have stated, if your index variable is not confined to the for scope break leaves it intact, while your method destroys its content; when you are searching e.g. an array with break the code is more concise (you don't have to keep an extra variable to write down where you stopped);
break quits the loop immediately; your method requires you to execute the rest of the body. Of course you can always write:
for(int i=0; i<n; ++i) {
if(...) {
i=n;
} else {
rest of the loop body
}
}
but it adds visual and logical clutter to your loop;
break is almost surely going to be translated to a simple jmp to the instruction just following the loop (although, if you have block-scoped variables with a destructor the situation may be more complicated); your solution is not necessarily recognized by the compiler as equivalent.
You can actually see it here that gcc goes all the way to generate the code that moves n into i, while in the second case it jumps straight out of the loop.
On the stylistic side:
I find "your way" to be overly complicated and not idiomatic - if I encountered it in real code I would ask myself "why didn't he just use a break?", and then check twice to make sure that it's not like I'm missing some side effect and that the intent was actually just to jump out of the loop;
as other said, there's some risk of your inner assignment to go out of sync with the actual loop condition;
it doesn't scale when the loop condition becomes more complicated than a simple range check, both on the logic side (if the loop condition is complicated then tricking it can become more complicated) and on the performance side (if the loop condition is expensive and you already know you want to exit you don't want to check it again); this too can be circumvented by adding an extra variable (which is typically done in languages that lack break), but that's again extra distractions from what your algorithm is actually doing;
it doesn't work at all with range-based loops.
I prefer break; because it leaves the loop variable intact.
I frequently use this form while searching for something:
int i;
for(i=0; i<list.size(); ++i)
{
if (list[i] == target) // I found what I'm looking for!
{
break; // Stop searching by ending the loop.
}
}
if (i == list.size() ) // I still haven't found what I'm looking for -U2
{
// Not found.
}
else
{
// Do work with list[i].
}
Are the compiled binaries different?
Almost certainly yes. (although an optimizer may recognize your pattern, and reduce them to nearly the same)
The break; statement will likely be an assembly "jump" statement to jump to the next instruction outside the list, while leaving the control variable unchanged.
Assigning the variable (in non-optimized code) will result in an assignment to the control variable, a test of that variable, and then a resulting jump to end the loop.
As others have mentioned, assigning the variable to its final value is less future-proof, in case your loop condition changes in the future.
In general, when you say:
"I have no good understanding of what it actually does. (so I use a workaround)",
I respond with:
"Take the time to learn what it does! A main aspect of your job as a programmer is to learn stuff."
Using break to do this is idiomatic and should be the default, unless for some reason the rather obfuscatory alternative serves to set the stage for logic below. Even then I'd prefer to do the variable setup after the loop exits, moving that setting closer to its usage for clarity.
I cannot conceive of a scenario where the performance matters enough to worry about it. Maybe a more convoluted example would demonstrate that. As noted the answer for that is almost always 'measure, then tune'.
In adition to the break statement to exit a for or [do] while loop, the use of goto is permitted to break out nested loops, e.g.:
for (i=0; i<k; i++) {
for (j=0; j<l; j++) {
if (someCondition) {
goto end_i;
}
}
}
end_i:
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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.