/in/foo.cc: In function 'int main()':
/in/foo.cc:14:8: warning: iteration 17 invokes undefined behavior [-Waggressive-loop-optimizations]
    b[i]=s;
    ~~~~^~
/in/foo.cc:11:16: note: within this loop
   for(int i=3;i<=20;i++)
               ~^~~~