Compilare: main.cpp: In function 'void prepare()': main.cpp:45:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(i < expression.length()) { ~~^~~~~~~~~~~~~~~~~~~~~ main.cpp:55:69: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(expression[i] >= '0' && expression[i] <= '9' && i < expression.length()) { ~~^~~~~~~~~~~~~~~~~~~~~