Calculating Fibonacci Numbers Recursively in C

Question

I'm trying to learn C by writing a simple program to output Fibonacci numbers. It isn't working.

fibonacci.h

unsigned int fibonacci_recursive(unsigned int n);

fibonacci.c

#include <stdio.h>
#include "fibonacci.h"

main() {
    unsigned int i;
    for (i = 0; i < 10; i++) {
        printf("%d\t%n", fibonacci_recursive(i));
    }
    getchar();
}

fibonacci_recursive.c

unsigned int fib_rec(unsigned int n);

main(unsigned int n) {
     return fib_rec(n);
}

unsigned int fib_rec(unsigned int n) {
    if (n == 0) {
        return 0;
     } 
     if (n == 1) {
           return 1;
     }
     return fib_rec(n - 1) + fib_rec(n - 2);
}

This is the error message VS 2010 gives me when I try to build the project:

1>ClCompile:
1>  fibonacci_recursive.c
1>fibonacci_recursive.obj : error LNK2005: _main already defined in fibonacci.obj
1>fibonacci.obj : error LNK2019: unresolved external symbol _fibonacci_recursive referenced in function _main
1>c:\users\odp\documents\visual studio 2010\Projects\Fibonacci\Debug\Fibonacci.exe : fatal error LNK1120: 1 unresolved externals
1>
1>Build FAILED.
1>

What am I doing wrong here? Thanks for helping someone new to C.

Answer 1

Your approach seems strange, you should have:

• a main file (example main.c) with the main method and that includes fibonacci.h
• a fibonacci.h with the prototype unsigned int fibonacci_recursive(unsigned int n);
• a fibonacci.c with the implementation of the method, and it should include fibonacci.h too

Actually you define main function twice too..

main.c

#include <stdio.h>
#include "fibonacci.h"

main()
{
    unsigned int i;
    for (i = 0; i < 10; i++)
    {
        printf("%d\t%n", fibonacci_recursive(i));
    }
    getchar();
}

fibonacci.h

unsigned int fibonacci_recursive(unsigned int n);

fibonacci.c

#include "fibonacci.h"
unsigned int fibonacci_recursive(unsigned int n)
{
    if (n == 0) 
    {
        return 0;
     } 
     if (n == 1) {
           return 1;
     }
     return fibonacci_recursive(n - 1) + fibonacci_recursive(n - 2);
}

Answer 2

You have the main() function defined twice in your project. This is the entry point of your program, and you only need one.

Answer 3

You need \n not %n for your printf. Also, you can simplify as:

#include "fibonacci.h"

unsigned int fibonacci_recursive(unsigned int n) {
if (n < 2) 
    return n;
else
    return fibonacci_recursive(n - 1) + fibonacci_recursive(n - 2);
}

Answer 4

You haven't created a fibonacci_recursive function that you declared in fibonacci.h.

Answer 5

You declared two main() functions, and the new line character is '\n'.

Answer 6

Well, I preface that recursive function is not an efficient method to calculate Fibonacci and it may be used for dev training/demonstrations purposes only, because every recursion is stored in stack, and it may also overflow for large fibonacci numbers. It is rather worth the effort to write down a more efficient Fibonacci function that uses a loop, like following code:

#include <stdio.h>

#define MAX_ITERS 20


int fibonacci(int);

int main(int argc, char *argv[])
{
    unsigned int iters;

    if(argc>1) {
        iters=atoi(argv[1]);
    } else
        iters=MAX_ITERS;

    fibonacci(iters);

    return 0;
}

int fibonacci(int iterations)
{
   unsigned register int i;
   double first=0.0, second = 1.0, lastsum;
   printf("First %d iterations of Fibonacci series are :\n",iterations);

   for ( i = 0 ; i < iterations ; i++ )
   {
      if ( i <= 1 )
         lastsum = (double)i;
      else
      {
         lastsum = first + second;
         first = second;
         second = lastsum;
      }
      printf("%.0f\n",lastsum);
   }

}

Try to compare by your own, running ./fibonacci 50 with this method, for instance on a low cost processor (eg. on a Raspberry PI), and the one with the recursive functions and 50 first numbers as well, and see the difference! ,-)