foo.cc: In function 'int main()':
foo.cc:24:13: error: 'x' was not declared in this scope
   for(int i=x;i<=n;i++)
             ^
foo.cc:13:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a[i]);
   ~~~~~^~~~~~~~~~~~