Compilare: user.cpp: In function ‘int level_two_block()’: user.cpp:22:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (idx<exp.length() && exp[idx]>='0' && exp[idx]<='9') r=r*10 + exp[idx++] - '0'; ~~~^~~~~~~~~~~~~ user.cpp: In function ‘int level_one_block()’: user.cpp:32:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (idx<exp.length() && (exp[idx]=='*' || exp[idx]=='/')) { ~~~^~~~~~~~~~~~~ user.cpp: In function ‘int eval()’: user.cpp:46:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (idx<exp.length() && (exp[idx]=='+' || exp[idx]=='-')) { ~~~^~~~~~~~~~~~~