foo.cc: In function 'int main()':
foo.cc:12:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.length();i++)
                 ~^~~~~~~~~~~
foo.cc:16:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<b.length();i++)
               ~^~~~~~~~~~~
foo.cc:31:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while(n != string::npos)
         ~~^~~~~~~~~

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