foo.cc: In member function 'void expr::scan()':
foo.cc:62:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (i=0,j=0,k=0;i<strlen(ts);i++)
                  ~^~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:111:6: warning: ignoring return value of 'char* fgets(char*, int, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 fgets(ss,sizeof(ss),stdin);
 ~~~~~^~~~~~~~~~~~~~~~~~~~~
foo.cc: In member function 'void expr::scan()':
foo.cc:61:6: warning: ignoring return value of 'char* fgets(char*, int, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 fgets(ts,sizeof(ts),stdin);
 ~~~~~^~~~~~~~~~~~~~~~~~~~~

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