Compilare: main.cpp: In function 'int main()': main.cpp:72:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < expr.length(); ++i){ ~~^~~~~~~~~~~~~~~ main.cpp:76:57: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(expr[i] >= '0' && expr[i] <= '9' && i < expr.length()){ ~~^~~~~~~~~~~~~~~