foo.cc: In function 'int main()':
foo.cc:35:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < matrix.size(); ++i)
                     ~~^~~~~~~~~~~~~~~
foo.cc:39:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < matrix.size(); ++i)
                     ~~^~~~~~~~~~~~~~~
foo.cc:41:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = 0; j < matrix[0].size(); ++j)
                         ~~^~~~~~~~~~~~~~~~~~
foo.cc:62:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (x >= 0 && x < matrix.size() && y >= 0 && y < matrix[0].size())
                           ~~^~~~~~~~~~~~~~~
foo.cc:62:60: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (x >= 0 && x < matrix.size() && y >= 0 && y < matrix[0].size())
                                                          ~~^~~~~~~~~~~~~~~~~~

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