/in/foo.cc: In function 'int main()':
/in/foo.cc:14:14: warning: operation on 'i' may be undefined [-Wsequence-point]
    c=a[i]-a[i--];
             ~^~
/in/foo.cc:15:9: warning: operation on 'i' may be undefined [-Wsequence-point]
    v=a[i++]-a[i];
        ~^~