Nick Heiner Nick Heiner - 3 months ago 11
C Question

Calculating Fibonacci Numbers Recursively in C

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

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);
}
Comments