Compilare: main.cpp: In function 'int calc(int, int)': main.cpp:33:56: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0 ; '0' <= exp[i] && exp[i] <= '9' && i < exp.size() ; a = a * 10 + (exp[i] - '0'), ++ i) ; ~~^~~~~~~~~~~~ main.cpp:36:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 1 ; i < exp.size() ; ) { ~~^~~~~~~~~~~~