foo.c: In function 'output':
foo.c:6:16: error: 'a' undeclared (first use in this function)
    6 |   printf("%d ",a[j]);
      |                ^
foo.c:6:16: note: each undeclared identifier is reported only once for each function it appears in
foo.c: In function 'main':
foo.c:13:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   13 |  scanf("%d",&n);
      |  ^~~~~~~~~~~~~~
foo.c:16:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   16 |   scanf("%d",&a[i]);
      |   ^~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务