foo.c: In function 'main':
foo.c:25:6: error: 'i' undeclared (first use in this function)
  for(i=0;i<n;i++)
      ^
foo.c:25:6: note: each undeclared identifier is reported only once for each function it appears in
foo.c:17:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ^~~~~~~~~~~~~~
foo.c:19:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a[j]);
  ^~~~~~~~~~~~~~~~~
foo.c:20:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&k);
  ^~~~~~~~~~~~~~

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