/in/foo.c: In function 'Prime':
/in/foo.c:6:12: error: 'n' undeclared (first use in this function)
  for(i=2;i<n;i++)
            ^
/in/foo.c:6:12: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:13:2: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
  if(i=num)
  ^~