Compilare: main.cpp: In function 'bool is_letter(char)': main.cpp:13:22: warning: suggest parentheses around '&&' within '||' [-Wparentheses] return ( 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' ); ~~~~~~~~~^~~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:24:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( int i = 0; i < s.size(); ++i ) ~~^~~~~~~~~~ main.cpp:30:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while( i < s.size() && is_letter( s[i] ) ) ~~^~~~~~~~~~