foo.cc: In function 'int main()':
foo.cc:33:53: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                 (string(ptr->d_name).rfind(".out",l)==l-4||
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
foo.cc:34:53: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                  string(ptr->d_name).rfind(".ans",l)==l-4)) {
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
foo.cc:38:13: warning: ignoring return value of 'char* realpath(const char*, char*)', declared with attribute warn_unused_result [-Wunused-result]
     realpath(".",s);
     ~~~~~~~~^~~~~~~
foo.cc:56:11: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
     system(("cp -f ../../data/"+f+'/'+v[n].second+" "+f+".out").c_str());
     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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