Compilare: user.cpp: In function ‘void read()’: user.cpp:17:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<a.size(); i++) ^ user.cpp: In function ‘void write()’: user.cpp:27:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=1; i<x.size(); i++) ^ user.cpp:34:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<x[m].size(); i++) ^ user.cpp: At global scope: user.cpp:38:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^ user.cpp: In function ‘int main()’: user.cpp:41:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=1; i<a.size(); i++) ^ user.cpp:43:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j=0; j<x.size(); j++) ^