/in/foo.c: In function 'main':
/in/foo.c:8:13: error: 'f' undeclared (first use in this function)
    8 |             f[i]=f[i-1]+f[i-2];
      |             ^
/in/foo.c:8:13: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:14:20: warning: implicit declaration of function 'f' [-Wimplicit-function-declaration]
   14 |                 if(f(i)%3==0)
      |                    ^