Hello! I was looking though a fellow developers code when I saw this ..
for (;;){
....
....
....
}
I have never seen ";;" used in a loop. What does this do exactly?
+21
A:
It loops forever. ';;' equates to no start value, no stop condition and no increment condition.
It is equivalent to
while (true)
{
...
}
There would usually be a conditional break; statement somewhere in the body of the loop unless it is something like a system idle loop or message pump.
Mitch Wheat
2010-04-29 13:15:08
Awesome! Thanks Mitch! Now that I view it as "loops forever because of no specified parameters" it makes perfect sense.
Ben Baraga
2010-04-29 13:24:35
The note on infinite loops in "Learning Perl" notes that although the for form is the idiomatic one in Perl is the while form (they recommend while(1) ).
HerbN
2010-04-29 14:02:48
@Ben: if you liked that, check out the `-->` operator, as in `while ($x --> 0) { ... }`.
Ether
2010-04-29 15:31:02
@Ether - It's not nice to play tricks on future programmers with the parser. ;) I had to play with the RHS of the condition for a sec to make sure I understood what that was doing, as I don't think I'd seen that before in perl5. +1 for making me think.
kbenson
2010-04-29 19:15:27
+2
A:
Infinite Loop (until you break out of it).
It's often used in place of:
while(true) { // Do something }
Justin Niessner
2010-04-29 13:15:59
Sounds like a bad idea to use in instead of `while(true) { ... }`. It only confuses noobs.
Hamish Grubijan
2010-04-29 13:18:50
For those of us who come from C++ using VS, while(true) throws up a warning when compiling but for(;;) does not. Projects that run 0 compiler warnings are forced to use for(;;) and it carries over when you switch languages.
tloach
2010-04-29 13:28:09
+10
A:
All 3 parts are optional. An empty loop initialization and update is a noop. An empty terminating condition is an implicit true. It's essentially the same as
while (true) {
//...
}
Note that you it doesn't have to be all-or-nothing; you can have some part and not others.
for (init; cond; ) {
//...
}
for (; cond; update) {
//...
}
for (init; ; update) {
//...
}
polygenelubricants
2010-04-29 13:16:35
+4
A:
Infinite loop. Alot of the time it will be used in a thread to do some work.
boolean exit = false;
for(;;) {
if(exit) {
break;
}
// do some work
}
BrennaSoft
2010-04-29 13:18:42
+2
A:
It's the same as
while(true) {
...
}
It loops forever.
You don't need to specify all of the parts of a for loop. For example the following loop (which contains no body, or update token) will perform a linear search of myArray
for($index = -1; $myArray[++$index] != $target;);
Geoff
2010-04-29 13:19:03
+4
A:
Just like in C, the for loop has three sections:
- a pre-loop section, which executes before the loop starts.
- a continuing condition section which, while true, will keep the loop going.
- a post-iteration section which is executed after each iteration of the loop body.
For example:
for (i = 1, acc = 0; i <= 10; i++)
acc += i;
will add up the numbers from 1 to 10 inclusive (in C and, assuming you use Perl syntax like $i and braces, in Perl as well).
However, nothing requires that the sections actually contain anything and, if the condition is missing, it's assumed to be true.
So the for(;;) loop basically just means: don't do any loop setup, loop forever (breaks notwithstanding) and don't do any iteration-specific processing. In other words, it's an infinite loop.
paxdiablo
2010-04-29 13:22:35