foo.cc: In function 'int main()':
foo.cc:13:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < a.length(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:22:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < str.length(); i++)
                     ~~^~~~~~~~~~~~~~
foo.cc:31:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < a.length(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:42:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < str.length(); i++)
                     ~~^~~~~~~~~~~~~~
foo.cc:11:15: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
         system("pause");
         ~~~~~~^~~~~~~~~
foo.cc:18:19: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
             system("pause");
             ~~~~~~^~~~~~~~~
foo.cc:27:19: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
             system("pause");
             ~~~~~~^~~~~~~~~
foo.cc:36:19: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
             system("pause");
             ~~~~~~^~~~~~~~~
foo.cc:46:11: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
     system("pause");
     ~~~~~~^~~~~~~~~

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