Compilare: main.cpp: In function 'int main()': main.cpp:24:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 24 | for (j = 0; j < a[i].size(); j += 2) | ~~^~~~~~~~~~~~~ main.cpp:28:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 28 | for (i = 0; i < a[0].size(); i++) | ~~^~~~~~~~~~~~~