foo.cc: In function 'int main()':
foo.cc:12:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(j=0;j<=sf.size();j++)
             ~^~~~~~~~~~~
foo.cc:14:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(j=0;j<=s.size();j++)
             ~^~~~~~~~~~

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