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/* FibFunc.c - Calculate the n-th Fibonacci number.
 * This program asks for the number of months that a single
 * pair of rabbits and their progeny will breed for,
 * where each month every pair older than 1 month will
 * reproduce 1 new pair of rabbits.  This value is the
 * n-th Fibonacci number.  This version uses a function.
 * Written by Wayne Pollock, 2001.
 */

#include <stdio.h>

unsigned long fib ( unsigned int n );

int main ( void )
{  int n;

   printf ( "For how many months do the rabbits breed: " );
   if ( scanf( "%i", &n ) != 1 )
   {  fprintf( stderr, "***ERROR: Bad input!\a\n" );
      return -1;
   }
   if ( n < 0 )
   {  fprintf( stderr, "***ERROR: Input must be a number >= zero!\a\n" );
      return -1;
   }
   printf ( "The number of pairs of rabbits after %i months is %lu.\n",
           n, fib(n) );
   return 0;
}


unsigned long fib ( unsigned int n )
{  unsigned int i;
   unsigned long prev = 0, result = 1, next;

   for ( i = 1; i <= n; ++i )
   {
      next = prev + result;
      prev = result;
      result = next;
   }
   return result;
}

/*
=====================Output of Fib1.c follows======================

For how many months do the rabbits breed: 12
The number of pairs of rabbits after 12 months is 233.

*/