foo.cc: In function 'int main()':
foo.cc:16:12: warning: unused variable 'index' [-Wunused-variable]
  int n,i,j,index,k,flag;
            ^~~~~
foo.cc:16:18: warning: unused variable 'k' [-Wunused-variable]
  int n,i,j,index,k,flag;
                  ^
foo.cc:16:20: warning: unused variable 'flag' [-Wunused-variable]
  int n,i,j,index,k,flag;
                    ^~~~
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);
   ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:17:20: warning: 'n' is used uninitialized in this function [-Wuninitialized]
  struct student s[n],t; 
                    ^

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