Syntax error:

Hi, anyone knows why the highlighted lines(red color) cause syntax errors?
Please advise.

Code:

#include <stdio.h>
#include <ctype.h>
#include <string.h>
//declare structure
struct result
{
        int num_a=0, num_b=0;
};
//declare function prototype 
struct result count( char str[], int idx );

int main()
{
        char str[] = "acdefAbbA";
        struct result res = count( str, 0 );
        /*char *pa=res.num_a, *pb=res.num_b;*/
        
        printf( "num a = %d and num b = %d\n", res.num_a, res.num_b );
        // should output : "num_a = 1 and num_b = 2"
        
        
        return 0;
}

struct result count( char str[], int idx )
{
        switch( str[idx] )
        {
                struct result res;
                case 'a':
                                (res.num_a)++;
                                idx++;
                                return count( str[idx], idx);
                case 'b':
                                (res.num_b)++;
                                idx++;
                                return count( str[idx], idx);
                case '\0':
                                return res;
        }
}

Listen to your compiler, it will tell you what is wrong. Post the error messages you don't understand.

int num_a=0, num_b=0;

In structure you have declare above variable with initialization . That is problem.
This one causes error because C does not allow in-struct initialization .
You should define the struct without initializations.

return count( str[idx], idx);

In above statement you are calling the function with character argument and integer argument.
Actually you have defined the function with accepting the character array integer.
You should call the function like this. count ( str, idx ) ;
It will work fine.

struct result count( char str[], int idx )
accepts a character array and an integer.
But you are passing a character ... str[idx].