foo.cc: In function 'int main()':
foo.cc:16:8: warning: unused variable 'j' [-Wunused-variable]
  int n,j,i;
        ^
foo.cc:16:10: warning: unused variable 'i' [-Wunused-variable]
  int n,j,i;
          ^
foo.cc:21:8: 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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