while (true) and for (; ; ) [modified]

[jsc42]

CPallini wrote:

you may know in the middle of the loop that you've to exit and while you may skip the following statements with an if and then use the condition, I prefer an immediate break.

What you have described is known as an n-and-a-half times loop. The Pascal P-Code compiler uses the following ungainly way of doing it (excuse the non-PASCAL syntax ... my Pascal is extremely rusty, but the principle is clear)

DO
(* bits before the test *)
test = exit condition;
IF (NOT test)
(* bits after the test condition *)
UNTIL (test)

It always struck me as bad - the compiler writers having to use an idiom to do something that their language did not support cleanly - surely, they should be able to adjust the language when they saw its inherent deficiencies. Languages that I was involved with in the late 1970s always 'magically' got extensions that supported n-and-a-half times loops. For example, my Rationalised FORTRAN (which included block structured constructs long before FORTRAN 77 became available) had:

LOOP
bits before the test
EXITIF condition // You could have multiple EXITIF sections
bits after the test
ENDLOOP

(minor later variant was an EXITIFNOT statement as an alternative to EXITIF) A WHILE loop then simply becomes

LOOP
EXITIFNOT condition
body of the loop
ENDLOOP

An UNTIL loop then simply becomes

LOOP
body of the loop
EXITIF condition
ENDLOOP

A normal FOR/NEXT or DO/CONTINUE loop becomes

controlvar = startvalue
LOOP
EXITIF controlvar > endvalue
body of the loop
controlvar = controlvar + step
ENDLOOP

A 'must do at least once' FOR/NEXT or DO/CONTINUE loop becomes

controlvar = startvalue
LOOP
body of the loop
controlvar = controlvar + step
EXITIF controlvar > endvalue
ENDLOOP

Example: To see if an item is in an array

i = 1 // Lower bound of subscript
LOOP
EXITIF i > noofelementsinthearray
EXITIF array(i) = valuetofind // Previous EXITIF ensure that array subscript overflow will not occur
i = i + 1
ENDLOOP

or

i = 1 // Lower bound of subscript
LOOP
EXITIF i > noofelementsinthearray
IF (array(i) = valuetofind)
do whatever you want with the found value
EXITIF true // I never got around to creating a simple unconditional EXIT statement!
END IF
i = i + 1
ENDLOOP

[ness2u2]

Andrew Glow wrote:

while(true)
{
lock(syncObject)
{
if (checkSomeCriticalCondition())
break;
}

doSomeOtherStuff();
}

If possible, I'd move the lock{ someCriticalCondition } out to a function returning boolean. Then toss the result of that as the while condition. while(CheckCondition()){ doSomeOtherStuff(); } The semantics are identical, unless there is code before the lock... In my opinion, this displays the intent of code/loop better than having an internal break.

[Andrew Glowacki]

True. I guess having it just like this would be useless unless maybe doSomeOtherStuff actually used the same data in the critical section that other threads were using as well. In this case I would do something like this:

while (true)
{
lock(syncObject)
{
if (checkSomeCriticalCondition())
break;
doSomeOtherStuff();
}
Sleep(someAmountOfTime);
}

... I can see a lot more holes in this design now but I still think that having a critical section check could be used somehow.

[ness2u2]

Depending on the language and framework's your using, there is probably a timer you could use to avoid the loop all together? If the bulk of the code in the loop is critical enough to require a lock, I wonder if it is something that would actually benefit from running in a multithreaded environment? I work mostly with C# and have seen very few scenarios where while(true) or for(;;) is the best choice. Just make sure its commented well!

[Andrew Glowacki]

Very good point, for some reason I always assume people are using .NET. I did mean C# anyway. However, nothing comes to mind for a concrete example of where this would be useful at the moment.

[Gary Huck]

Opinion: goofy. There's no reason you can't have a while (!done) construct. Better yet, esp for the next guy, put in something meaningful. Yeah, for kicks one can do a loop-infinitely construct but you'd better comment why.

[Mark AJA]

Sometimes if I want to tempory remove some code I use; IF false THEN Code to tempory remove. ENDIF

[Mikel Taylor]

I hate to sound like a politician, but it depends. However, when there is a clear way to use "either," I personally prefer "for" looping. If working with object arrays, data rows, etc, I prefer to either check the count in an "if" block (or the UBOUND in the case of an array >-1) prior to the for loop. This approach allows you to predetermine iterations (if any are needed) at runtime- with your max value value being a variable. I usually reserve the use of "while" loops for conditions that are not numeric in nature... i.e. Fuzzier routines. In these cases, I usually place a boolean variable AND a counter out of the scope of the procedure and wrap a MAX If on loop iterations. I usually only go down this road if I am having to enumerate using self-calling recursion or something complex like that.

I float like a butterfinger & stank like a bee.

[Pcube]

That's incorrect. This code will only hang a single core machine if there are no interrupts to force context switching (which is the case on the vast majority of machines that run an OS, anyway). Also, this code is useful to spawn a thread that continuously polls hardware. If you have a piece of hardware that can change its output at any given time, but you're not sure when, you can spawn the equivalent to a listener thread to capture (sample) changes at the output. In fact, this code is useful in many "thread listener" type scenarios, such as when a server is idling and listening for clients...

[skunkmeister]

DirectX programming relies on while(true) for the primary loop. As long as a graceful exit exists (catching exceptions is not graceful), this is a valid programming code piece.

[Macotti]

I never got to use it in production but i used it once in my undergraduate thesis. I was working with artificial neural networks simulations and I couldn't find any way on how to determine how long it will take a particular ANN to learn. So I kept the loop going until the error was lower than that of the maximum allowed. I had no choice, the iteration can go from a few thousands to millions.

[RineezTVM]

I have used while(true) but only in some kind of Listener threads.

[PICguy]

Good thoughts: pacemaker or long running app where shutting down the computer is how to exit. But what about running under a multitasking OS like this: while(true)

{
osWaitMilliseconds(x);
doStuff();
}

No need to spawn a task every x milliseconds when one is waiting to be resumed. Or how about PIC code that looks like this:

mainLoop:
call sub1
call sub2
...
call subN
goto mainLoop

[Asday]

Call it bad practice but I'm a hobbyist, and I use that sorta stuff in my Python all the time, things like displaying a menu, giving the user a choice, and breaking (the function, not the trainwreck), when the user's asking to quit.