Nick Heiner - 1 year ago 101
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.

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);
}
``````
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