斐波那契函数的递归实现

Question

I have written code to find both the Fibonacci number & the number of times the call is made to find the same for the recursive version in C. I am unable to remove compilation errors. Please help.

The code is given below:

#include <stdio.h>

int main(int fib) {
  int n,m, count=0; //'count' counts #times function is called
  printf("enter n");
  scanf("%d",&n);
  return fib_rec(n, &count);
}

int fib_rec(int n, int *count)
{
  int b=0,c=1;
  *count = *count +1;
  if(n<=1)
  { 
    return n;
  }
  else
  {
    printf (count);
    return fib_rec(n-1, *count)+ fib_rec(n-2, *count);
  }
}

The compilation errors are given below upon running on reptl.it site, as shown.

main.c: In function 'main':
main.c:7:10: warning: implicit declaration of function 'fib_rec' [- 
Wimplicit-function-declaration]
   return fib_rec(n, &count);
      ^~~~~~~
main.c: In function 'fib_rec':
main.c:20:13: warning: passing argument 1 of 'printf' from incompatible 
pointer type [-Wincompatible-pointer-types]
   printf (count);
         ^~~~~
In file included from main.c:1:
/usr/include/stdio.h:364:43: note: expected 'const char * restrict' but 
argument is of type 'int *'
   extern int printf (const char *__restrict __format, ...);
                ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
main.c:21:25: warning: passing argument 2 of 'fib_rec' makes pointer from 
integer without a cast [-Wint-conversion]
   return fib_rec(n-1, *count)+ fib_rec(n-2, *count);
                     ^~~~~~
main.c:10:25: note: expected 'int *' but argument is of type 'int'
   int fib_rec(int n, int *count)
                ~~~~~^~~~~
main.c:21:47: warning: passing argument 2 of 'fib_rec' makes pointer from 
integer without a cast [-Wint-conversion]
   return fib_rec(n-1, *count)+ fib_rec(n-2, *count);
                                           ^~~~~~
main.c:10:25: note: expected 'int *' but argument is of type 'int'
  int fib_rec(int n, int *count)
                ~~~~~^~~~~
enter n 10
exit status -1
>

Answer 1

I pasted your code on https://tio.run/#c-gcc , and here's the result:

main prototype

.code.tio.c:3:7: warning: ‘main’ takes only zero or two arguments [-Wmain]
   int main(int fib) {
       ^~~~

the main function prototype is int main(void) or int main(int argc, char *argv[])

Since you want user to input a number, you can choose the first form.

count type

.code.tio.c: In function ‘main’:
.code.tio.c:7:5: error: ‘count’ undeclared (first use in this function)
     count = 0;  // counts the number of times the function is called
     ^~~~~

You must give a type to count variable, like

int count = 0;

fib_rec declaration

.code.tio.c:8:12: warning: implicit declaration of function ‘fib_rec’ [-Wimplicit-function-declaration]
     return fib_rec(n, &count);
            ^~~~~~~

You have not declared the function before using it.

You can declare it this way: int fib_rec(int n, int *count) for instance before the main definition.

printf usage

.code.tio.c: In function ‘fib_rec’:
.code.tio.c:21:15: warning: passing argument 1 of ‘printf’ from incompatible pointer type [-Wincompatible-pointer-types]
       printf (count);
               ^~~~~

The printf function ask for some formatting. If you want to display a integer value, use %d :

printf("count value is: %d\n", count);

incompatible pointer type

.code.tio.c:22:27: warning: passing argument 2 of ‘fib_rec’ makes pointer from integer without a cast [-Wint-conversion]
       return fib_rec(n-1, *count)+ fib_rec(n-2, *count);
                           ^~~~~~

Here count is already a pointer on integer, * is unnecessary:

return fib_rec(n-1, count)+ fib_rec(n-2, count);

display of computed value

Your code return the computed value, but doesn't display it. To do so, replace return fib_rec(n, &count); with

printf("fib_rec(%d) = %d\n", n, fib_rec(n, &count));
return 0;

---

So the corrected code could be:

#include <stdio.h>

int fib_rec(int n, int *count);

int main(void) {
    int n;
    printf("enter n\n");
    scanf("%d",&n);
    int count = 0;  // counts the number of times the function is called
    printf("fib_rec(%d) = %d\n", n, fib_rec(n, &count));
    return 0;

}

int fib_rec(int n, int *count)
{
    int b=0,c=1;
    *count = *count +1;
    if(n<=1)
    { 
       return n;
    }
    else
    {
      printf ("count: %d\n", *count);
      return fib_rec(n-1, count)+ fib_rec(n-2, count);
    }
}

Answer 2

in main

1. main has a prototype of int main(void) or int main(int argc, int argc *[]) . fib is not a valid input for main .

2. main will typically return 0 . So the line fib_rec(n, &count); should be put before the return.

3. variable count was not declared. Variable m is unused.

4. Before the main function the declaration for fib_rec should be there. int fib_rec(int n, int *count);

In fib_rec

5. Function for printf is not correct. - It should be %d with the *count

6. The recursive call is not correct, Since count in fib_rec is a pointer, it can be directly passed to the function again without taking the value. Like this - return (fib_rec(n-1, count)+ fib_rec(n-2, count));

7. Unused variable b and c

This solves the compilation issues. Code is below.

int fib_rec(int n, int *count);

int main(void) {

  int n;
  int count;
  printf("enter n"); 
  scanf("%d",&n);
  count = 0;  // counts the number of times the function is called
  fib_rec(n, &count);
  return 0;

}

int fib_rec(int n, int *count)
{
   *count = *count +1;

   if ((n<=1) )
   { 
      return 1;
   }
   else
   {
     printf ("%d ", n);
     return (fib_rec(n-1, count)+ fib_rec(n-2, count));
   }
}