foo.cc: In function 'void output(std::vector<std::vector<int> >&)':
foo.cc:6:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |  for (int i = 0;i < m.size();i++)
      |                 ~~^~~~~~~~~~
foo.cc:8:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    8 |   for (int j = 0;j < m[i].size();j++)
      |                  ~~^~~~~~~~~~~~~

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