/in/foo.cc: In function 'std::__cxx11::string convert(std::__cxx11::string, int)':
/in/foo.cc:14:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i*k<s.length(); i++){
                  ~~~^~~~~~~~~~~
/in/foo.cc:18:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0; i*k<s.length(); i++){
                      ~~~^~~~~~~~~~~
/in/foo.cc:19:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if(i*k+j<s.length())
                ~~~~~^~~~~~~~~~~
/in/foo.cc:21:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if((i+1)*k-j>=0&&(i+1)*k-j<s.length())
                              ~~~~~~~~~^~~~~~~~~~~
/in/foo.cc:25:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i*k+numRows-1<s.length(); i++){
                  ~~~~~~~~~~~~~^~~~~~~~~~~