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Stalafin

Member

From: Berlin, Germany

Registered: 2007-10-26

Posts: 617

C programming language - syntax question

I have the following (loop) construction in some online C source code:

for (;;) {

/* here follows some random code */

}

I didn't, however, find any particular code within the sequence embraced by the for, that actually refers to this strange loop.

If you wanna have a look at the complete source code, please refer to http://omlc.ogi.edu/software/mc/tiny_mc.c

EDIT: Oopsie! Typing the URL I forgot what I wanted to ask! So, what does this 'for' do? What does that (;;) evaluate?

Last edited by Stalafin (2008-06-05 14:36:54)

chicha

Member

From: France

Registered: 2007-04-20

Posts: 271

Re: C programming language - syntax question

This is exactly the same as :

while (true) {
}

ie : infinit loop

Allan

Pacman

From: Brisbane, AU

Registered: 2007-06-09

Posts: 11,485

Website

Re: C programming language - syntax question

As chicha pointed out, this loop just keeps going until it hit a break statement.

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Stalafin

Member

From: Berlin, Germany

Registered: 2007-10-26

Posts: 617

Re: C programming language - syntax question

Ah, okay... thanks a lot.

So, why would one use that whing instead of a simple while loop? Wouldnt a while be more efficient?

chicha

Member

From: France

Registered: 2007-04-20

Posts: 271

Re: C programming language - syntax question

Technically it does not make a difference using the while(true) or for(;; ) : the compiler should be smart enough and make those loops as fast as each other...

Practically both loops are ugly : you should always make sure that your loop end at one time. You cannot warranty that the algorithm inside the loop will reach a break instruction at one time, in particular in complex loop.

Sometimes some developers prefer to write a short line of ugly code instead of 3 lines of safe code ...
However I hardly recommand not to use such a loop (and avoid 'while' loop as much as possible for the same reason).
Make sure your loop will always finish in any situation.

I hope it helped !

dr.cranium

Member

Registered: 2008-01-25

Posts: 65

Re: C programming language - syntax question

I'm not that great with programming yet but isn't that a fork bomb?

Cerebral

Forum Fellow

From: Waterloo, ON, CA

Registered: 2005-04-08

Posts: 3,108

Website

Re: C programming language - syntax question

I'm not that great with programming yet but isn't that a fork bomb?

infinite loop != forkbomb.  Infinite loop spins a single process forever, while a fork bomb spawns exponentially many processes.  Very different things.

There's really nothing wrong with a while(true) loop - you just need to do some verification that the loop will eventually exit.  While tougher than a "standard" for loop, it's doable.

rson451

Member

From: Annapolis, MD USA

Registered: 2007-04-15

Posts: 1,233

Website

Re: C programming language - syntax question

for (;;) {

/* here follows some random code */

}

i dunno about you guys but i've always referred to these loops as "for-ever" loops.  i personally think it reads better than while(true) when reading source.  it's all personal preference i suppose.

---

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xaw

Member

From: Chapel Hill

Registered: 2007-08-09

Posts: 177

Re: C programming language - syntax question

@rson451: My CS instructors always used the term "for-ever" loops whenever they encountered one, so you wouldn't be the only one to use that terminology.

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The water never asked for a channel, and the channel never asked for water.

dr.cranium

Member

Registered: 2008-01-25

Posts: 65

Re: C programming language - syntax question

Thanks Cerebral for the clarification.

shining

Pacman Developer

Registered: 2006-05-10

Posts: 2,043

Re: C programming language - syntax question

Practically both loops are ugly : you should always make sure that your loop end at one time. You cannot warranty that the algorithm inside the loop will reach a break instruction at one time, in particular in complex loop.

You can generalize that statement to any C loops.
You can easily imagine a standard for loop "for(int i = 0; i < n; i++)" where i is modified in complex ways inside the loop.
Same for a while loop.

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underpenguin

Member

Registered: 2007-02-01

Posts: 116

Re: C programming language - syntax question

Well, at least in this example, it is a horrible way to do it. here is the complete loop:

for(;;) {
    xi1=2.0*rand()/RAND_MAX - 1.0; 
    xi2=2.0*rand()/RAND_MAX - 1.0; 
    if ((t=xi1*xi1+xi2*xi2)<=1) break;
}

It should be, by all reasonable standards at least:

do{
    xi1=2.0*rand()/RAND_MAX - 1.0; 
    xi2=2.0*rand()/RAND_MAX - 1.0; 
} while ((t=xi1*xi1+xi2*xi2)>1);

(re-organizing the algorithm could lead to a nicer while loop i'm sure).

I feel loops with a termination condition like this should always clearly state that condition 'out in the open', instead of hiding it within the body of the loop.

melrom

Member

From: Bayville, NJ / Medford, MA

Registered: 2008-06-06

Posts: 9

Re: C programming language - syntax question

I feel loops with a termination condition like this should always clearly state that condition 'out in the open', instead of hiding it within the body of the loop.

this is true. i was always brought up to hate breaks.

underpenguin

Member

Registered: 2007-02-01

Posts: 116

Re: C programming language - syntax question

Well, I don't mean to say that one should never use breaks. Some algorithms are too complex to realistically forbid the use of breaks. (multiple return-s are another story, however )