/in/foo.cc: In function 'int main()':
/in/foo.cc:9:16: warning: unused variable 'n' [-Wunused-variable]
  double a[N],c,n;
                ^
/in/foo.cc:11:6: warning: iteration 279 invokes undefined behavior [-Waggressive-loop-optimizations]
  a[i]=a[i-1]+1.00/(i+1);
  ~~~~^~~~~~~~~~~~~~~~~~
/in/foo.cc:10:16: note: within this loop
  for (int i=1;i<=N;i++)
               ~^~~