Thread: 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;
}

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.

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