foo.cc: In function 'int main()':
foo.cc:16:21: error: 'i' was not declared in this scope
   16 |   double now=a[r]/a[i]-M;
      |                     ^
foo.cc:8:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    8 |  scanf("%d",&n);
      |  ~~~~~^~~~~~~~~
foo.cc:10:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   10 |   scanf("%d",&a[i]);
      |   ~~~~~^~~~~~~~~~~~

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