while loop for fibonacci series

Hi

Which version of the two do you prefer? I prefer the "1" because the while condition seems more understandable to me (don't really know if it's correct or not). Any suggestions.

EDIT: There is some problem with CODE 1. It just keeps running.

And I'm also getting this warning: warning: this decimal constant is unsigned only in ISO C90|

What does this warning mean? Could you please tell me? Thanks.

CODE 1

Code:

// fibo.cpp
// demonstrates WHILE loops using fibonacci series

#include <iostream>
#include <cstdlib>

using namespace std;

int main()

{
   const unsigned long limit = 4294967295; //largest unsigned long
   unsigned long next_to_last=0;       //next-to-last term
   unsigned long last=1;       //last term

   while( (next_to_last + last) < limit )   //don't let results get too big
      {

      cout << last << "  ";    //display last term
      long new_last = next_to_last + last;  //add last two terms
      next_to_last = last;             //variables move forward
      last = new_last;              //   in the series

      }

   cout << endl << endl;

   system("pause");
   return 0;
}

CODE 2

Code:

// fibo.cpp
// demonstrates WHILE loops using fibonacci series

#include <iostream>
#include <cstdlib>

using namespace std;

int main()

{
   const unsigned long limit = 4294967295; //largest unsigned long
   unsigned long next_to_last=0;       //next-to-last term
   unsigned long last=1;       //last term

   while( next_to_last < limit / 2 )   //don't let results get too big
      {

      cout << last << "  ";    //display last term
      long new_last = next_to_last + last;  //add last two terms
      next_to_last = last;             //variables move forward
      last = new_last;              //   in the series

      }

   cout << endl << endl;

   system("pause");
   return 0;
}

You may prefer the test in #1, but as you have noted it doesn't work. When you overflow an unsigned variable, you start back over again at 0; so if next_to_last + last would (if done in "real math") go over the limit, the computer will be unable to detect that since next_to_last + last would in fact be 17 or whatever.

Originally Posted by tabstop

You may prefer the test in #1, but as you have noted it doesn't work. When you overflow an unsigned variable, you start back over again at 0; so if next_to_last + last would (if done in "real math") go over the limit, the computer will be unable to detect that since next_to_last + last would in fact be 17 or whatever.

Thanks a lot, tabstop. I didn't know that. What does happen in case of signed variable? Where does it start? At 0 or maximum negative value allowed? Please let me now. Thank you.

Originally Posted by jackson6612

Thanks a lot, tabstop. I didn't know that. What does happen in case of signed variable? Where does it start? At 0 or maximum negative value allowed? Please let me now. Thank you.

No one knows. Or more precisely, you have to know how your system does signed integers to know what it does when you overflow.

Okay. Anyway, thanks. But what does that warning mean? Would you please tell me? Thank you.