foo.c: In function 'main':
foo.c:13:6: error: 'i' undeclared (first use in this function)
  for(i=0;i<n+1;i++)
      ^
foo.c:13:6: note: each undeclared identifier is reported only once for each function it appears in
foo.c:4:2: warning: ignoring return value of 'freopen', declared with attribute warn_unused_result [-Wunused-result]
 {freopen("test.txt","w",stdout);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:6:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ^~~~~~~~~~~~~~
foo.c:9:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a[i]);
   ^~~~~~~~~~~~~~~~~
foo.c:11:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&x);
  ^~~~~~~~~~~~~~

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