How to detect overflows within a for loop

Hi, everyone. I have the following loop:

Code:

unsigned int i;
const unsigned int max = 10;
bool ignoremax = // true or false
for (i=1;i<max || ignoremax; i++)
{
  printf ("This is loop number %d\n", i);
}

Now, what I want is to detect when i has overflowed and prevent it from wrapping around and giving misleading output within the loop. What's the most elegant way to do this? Is this acceptable or hacky?

Code:

unsigned int i;
const unsigned int max = 10;
bool ignoremax = // true or false
for (i=1;i<max || ignoremax; if (i) i++)
{
  if (i) printf ("This is loop number %d\n", i);
  else
    printf ("We've lost count\n");
}

It's usually an error to start i at 1, in C.

Code:

for(i=0;i<MAX;i++) {
  if(i>=BadNumber)
     break;
  array[i] = (2x + y ) * 2; //normal processing
  printf("\nThis is loop number %3d", i+1); //i starts at zero, but here, for human "consumption", it's i+1

}

Since both 'i' and 'max' are the same type and the condition is:

Code:

i < max

how can 'i' become larger than max?

The test is i<max || ignoremax. My idea is that the loop goes a certain number of times, unless the user desires to have it loop infinitely. That's why i can overflow.

Richard

If an unsigned variable becomes zero after incrementing, overflow has occurred. Wrapping (probably better described as confirming to modulo rules) is a basic property of unsigned types.

Well, if you're starting at 1, then you could always make your loop condition (i && (i<max || ignoremax)), then once you've lost count, and looping is unnecessary anyway, you could break from the loop.

Really, it depends on what processing you're doing within the loop.

Originally Posted by Richardcavell

Hi, everyone. I have the following loop:
Now, what I want is to detect when i has overflowed and prevent it from wrapping around and giving misleading output within the loop. What's the most elegant way to do this? Is this acceptable or hacky?

Now the bad news... You can't do that in C or any of it's cousins.

The C language does not include range checking. It is up to you as a programmer to avoid situations where an integer overflow might occur.