foo.cc: In function 'int main()':
foo.cc:11:5: warning: unused variable 'n' [-Wunused-variable]
 int n;
     ^
foo.cc:14:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&arr[i].id);
 ~~~~~^~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务