Compilare: main.cpp: In function 'int main()': main.cpp:18:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < ec.length(); i++) ~~^~~~~~~~~~~~~ main.cpp:30:121: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(ec[i] != 'x' && ec[i] != 'y' && ec[i] != 'z' && ec[i] != 't' && ec[i] != '+' && ec[i] != '-' && i < ec.length()) ~~^~~~~~~~~~~~~