```
/* 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.

*/
```

 Send comments and mail to pollock@acm.org.