foo.cc: In function 'int gcd(int, int)':
foo.cc:5:11: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
  while (r = m % n)
         ~~^~~~~~~
foo.cc: In function 'int main()':
foo.cc:15:24: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  while (scanf("%d", &n), n > 0)
         ~~~~~~~~~~~~~~~^~~~~~~
foo.cc:17:31: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   for (i = 0; i < n; i++)scanf("%d", &a[i]);
                          ~~~~~^~~~~~~~~~~~~

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