foo.cc: In function 'int main()':
foo.cc:36:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(j=0;j<strlen(str)-1;j++)
           ~^~~~~~~~~~~~~~
foo.cc:28:17: warning: variable 'tmp' set but not used [-Wunused-but-set-variable]
  char str[1000],tmp;
                 ^~~
foo.cc: In function 'double operation(double, double, char)':
foo.cc:23:1: warning: control reaches end of non-void function [-Wreturn-type]
 } 
 ^
foo.cc: In function 'int main()':
foo.cc:35:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%s",str);
   ~~~~~^~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。