Compilare: main.cpp: In function 'void buildPi(std::string&)': main.cpp:16:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 16 | for(i = 1; i < a.length(); ++i) | ~~^~~~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:43:13: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 43 | for(; i < a.length(); ++i) | ~~^~~~~~~~~~~~