/in/foo.cc: In function 'void f(int, int, int, int)':
/in/foo.cc:10:6: error: 'a' was not declared in this scope
   if(a[l]=='0')
      ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:39:2: error: 'string' was not declared in this scope
  string a;
  ^~~~~~
/in/foo.cc:40:2: error: 'cin' was not declared in this scope
  cin>>a;
  ^~~
/in/foo.cc:40:7: error: 'a' was not declared in this scope
  cin>>a;
       ^