foo.c: In function 'main':
foo.c:7:8: error: 'o' undeclared (first use in this function)
  for(i=o;i<10;i++)
        ^
foo.c:7:8: note: each undeclared identifier is reported only once for each function it appears in
foo.c:8:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a[i]);
  ^~~~~~~~~~~~~~~~~

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