Compilare: main.cpp: In function 'int main()': main.cpp:25:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 2; i < s.size(); ++i) { ~~^~~~~~~~~~ main.cpp:27:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j = i; j < s.size(); j += i) ans[j] = '1'; ~~^~~~~~~~~~ main.cpp:31:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 2; i < s.size(); ++i) { ~~^~~~~~~~~~ main.cpp:34:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j = i+i; j < s.size(); j += i) if(s[j] == '1') ans[i] = '0'; ~~^~~~~~~~~~ main.cpp:35:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(ans[i] == '1') for(int j = i+i; j < s.size(); j += i) ans[j] = '1'; ~~^~~~~~~~~~ main.cpp:40:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 2; i < s.size(); ++i) cout << ans[i]; ~~^~~~~~~~~~ main.cpp:9:37: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("ciurulet.in", "r", stdin); ^ main.cpp:10:39: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result] freopen("ciurulet.out", "w", stdout); ^