In file included from /usr/include/c++/6/cstdio:42:0,
                 from /usr/include/c++/6/ext/string_conversions.h:43,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/in/foo.cc: In function 'int main()':
/in/foo.cc:6:25: error: expected ')' before 'stdin'
  freopen("date.txt","r" stdin);
                         ^
/in/foo.cc:6:30: error: too few arguments to function 'FILE* freopen(const char*, const char*, FILE*)'
  freopen("date.txt","r" stdin);
                              ^
In file included from /usr/include/c++/6/cstdio:42:0,
                 from /usr/include/c++/6/ext/string_conversions.h:43,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/stdio.h:280:14: note: declared here
 extern FILE *freopen (const char *__restrict __filename,
              ^~~~~~~
/in/foo.cc:11:13: error: 'gap' was not declared in this scope
  int maxgap=gap;
             ^~~
/in/foo.cc:13:2: error: 'intgap' was not declared in this scope
  intgap=abs(x2-x1);
  ^~~~~~