foo.cc: In function 'int main()':
foo.cc:37:81: warning: ' ' flag used with '%c' gnu_printf format [-Wformat=]
   printf("%-8d %-8s% -8c%-8.0f\n",s[i].num,s[i].name,s[i].sex?'M':'F',s[i].score);
                                                                                 ^
foo.cc:21:11: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
      scanf("%d %s %d %f",&s[i].num,s[i].name,&s[i].sex,&s[i].score);
      ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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