while () not exiting even when exit condition is met

aatwell · 08-28-2008, 04:34 PM

Here's a bizzard one. I have an while(running) loop and during the loop execution when I certain condition is met I set running = 0; But the loop continues to infinity. Within the loop I tell it to give me constant feedback and I can watch the value of running change from 1 to 0... but the loop continues..... Any ideas?

rocket357 · 08-28-2008, 04:42 PM

Quote:

Originally Posted by aatwell

Here's a bizzard one. I have an while(running) loop and during the loop execution when I certain condition is met I set running = 0; But the loop continues to infinity. Within the loop I tell it to give me constant feedback and I can watch the value of running change from 1 to 0... but the loop continues..... Any ideas?

Why not do this:

Code:

while (running > 0) {
    //do stuff
    if (end_condition)
        running = 0;
}

To answer the question you've asked, that's heavily language-dependent. Some languages, such as Python, associate "0" with "false", so the above loop would end. Other languages, however, do not make that association, and a "while (variable)" would simply check that the variable *exists*, so regardless of the variable's value, the loop would continue infinitely.

aatwell · 08-28-2008, 04:48 PM

I tried an if statement to change the variable. I have tried these.

Code:

while(running) {
  if (mycondition) running=0;
  };
}


while(running) {
  if (mycondition) return(EXIT_SUCCESS);
  };
}


while(running) {
  if (mycondition) exit(running);
  };
}


while(running) {
  if (mycondition) running=0;};
  if (running==0) {exit(running)};
  if (running==0) return(EXIT_SUCCESS);
}

none of them work.

matthewg42 · 08-28-2008, 04:59 PM

None of them are legal C, so I don't think you have really tried them - they will not compile (too many } characters).

Please post your actual code.

rocket357 · 08-28-2008, 05:14 PM

Quote:

Originally Posted by aatwell

none of them work.

You missed my point. while (running > 0) is what I was getting at

aatwell · 08-28-2008, 06:22 PM

Thanks so much....but I did get the point. I tried while(running > 0) and that didn't resolve the issue.

Cheers !

pinniped · 08-28-2008, 06:40 PM

This is an easy one - 'mycondition' is never true. Have a close look at your code (we can't help because you didn't post it).

rocket357 · 08-28-2008, 06:56 PM

Quote:

Originally Posted by aatwell

I tell it to give me constant feedback and I can watch the value of running change from 1 to 0

Quote:

Originally Posted by pinniped

This is an easy one - 'mycondition' is never true. Have a close look at your code (we can't help because you didn't post it).

mycondition does become true at some point, or running wouldn't change from 1 to 0.

ta0kira · 08-28-2008, 06:57 PM

You never know, maybe it has a blocking system call in it.
ta0kira

pinniped · 08-28-2008, 07:09 PM

Do you have nested loops and the condition is being reset?
Have you tried compiling with a different "optimization" level (check for compiler bug).

ta0kira · 08-28-2008, 07:34 PM

Maybe there is no code and OP is waiting for a program to exit that doesn't exist?
ta0kira

rocket357 · 08-28-2008, 07:36 PM

Quote:

Originally Posted by pinniped

Do you have nested loops and the condition is being reset?
Have you tried compiling with a different "optimization" level (check for compiler bug).

Also the OP could check to make sure he/she isn't shadowing variables. Declaring "running" inside the while loop and then setting running = 0 will not change the value of the "running" variable that is outside the loop.

This is the best I can do to reproduce the problem:

Code:

int main(){
    int running = 1;
    int count = 0;
    while (running) {
        int running = 1;
        if (count / 1000 > 0) {
            running = 0;
        }
        printf("running = %i", running);
        count++;
    }
    return 0;
}

But this is broken intentionally!

ta0kira · 08-28-2008, 08:36 PM

Quote:

Originally Posted by aatwell

I tried an if statement to change the variable. I have tried these.

Code:

while(running && !mycondition) {
  //if (mycondition) running=0;
  //}; ???
}

If that doesn't work then your condition is faulty.
ta0kira

aatwell · 08-29-2008, 01:12 PM

Code:

#include <stdio.h>
int main(){
    int running = 1;
    int count = 0;
    while (running) {
        int running = 1;
        if (count / 1000 > 0) {
            running = 0;
        }
        printf("running = %i", running);
        count++;
    }
    return 0;
}

I think you have something here. I am completely amazed when I compile and run this code that even when loop begins again, it doesn't check the value of running, find it false and exit the loop. I think you have found my issue. But now what's the solution? Can anyone explain this?

Thanks.

rocket357 · 08-29-2008, 02:00 PM

In the code I posted, the variable "running" is being shadowed.

When you declare int running = 1, a *new* memory location is allocated. The outer "running" variable (above the loop) is NOT the same variable as the inner "running" variable (declared inside the loop). You can increment/decrement/assign values to/etc... the *inner* "running" variable and it won't change the *outer* "running" variable. Problem is, the while condition is checking the outer variable, whereas within the loop you're modifying the inner variable.

Check it out:

Code:

int main() {
   int running = 1;
   int count = 4;
   printf("The value of (outer) running is %i", running); // will print 1

   while (running++ < count) {
      int running = 0;
      printf("The value of (inner) running is %i", running); // will print 0
   }

   printf("The value of (outer) running is %i", running); // will print 5
}

[rocket357@FreeBSD ~]$ ./a.out
The value of (outer) running is 1
The value of (inner) running is 0
The value of (inner) running is 0
The value of (inner) running is 0
The value of (outer) running is 5
[rocket357@FreeBSD ~]$

The reason this prints values 1, 0, 0, 0, 5 is because within the loop, "running" is a different variable than "running" that the while condition checks (the outer "running" variable). (Note that the while condition increments the *outer* running variable as evidenced by the last printf).

Now without the shadowed variable:

Code:

int main() {
   int running = 1;
   int count = 4;
   printf("The value of (outer) running is %i", running);

   while (running++ < count) {
      printf("The value of (inner) running is %i", running);
   }

   printf("The value of (outer) running is %i", running);
}

[rocket357@FreeBSD ~]$ ./a.out
The value of (outer) running is 1
The value of (inner) running is 2
The value of (inner) running is 3
The value of (inner) running is 4
The value of (outer) running is 5
[rocket357@FreeBSD ~]$

Without the shadowed variable, the "outer" variable is the only one available, so it is used in the inner printf calls.

The fix is simple: check to make sure you aren't shadowing "running".